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Spring 2008 Renal Physiology
Questions and Answers
I. David Weiner, M.D.
Professor of Medicine and Physiology
University of Florida and NF/SGVHS
If you have a question, please e-mail it to me at
David.Weiner@medicine.ufl.edu.
I'll post questions (without any identifying information as to
who submitted the question) and my answers as they come in.
In general, I will try to post questions and answers in reverse
order (most recent, first), so that you only have to read the most recent
questions and answers to stay up-to-date for new questions that have been posed.
May 11, 2008
Do tomatoes make your blood pressure go up or down? Medical literature
appears to indicate tomatoes decrease blood pressure. However, increasing
extracellular K+ via diet would stimulate aldosterone release to secrete more K+
and therefore increase Na reabsorption. Wouldn't BP go up?
Thanks,
As in most cases, the direct effects predominate. Increasing
K intake decreases the BP.
Your mother was right - eating fruits and vegetables is good
for you!
I. David Weiner, M.D.
May 10, 2008
For question 2 from you clinical correlation lecture, I don't understand why
the patient's K is high. I know that a decrease in serum K will cause an
increase in ammonium metabolism (increase bicarbonate reabsorption) in an
attempt to increase K reabsorption (to a normal level). How does that fit into
the patient having a low bicarbonate value which then causes a high K+?
Thank you for your time and help,
-
The important aspect of this question is that the patient is
being treated with two medicines, one that lowers serum potassium, the
thiazide diuretic, and one that can raise the serum potassium, amiloride, a
sodium channel blocker, and also known as a potassium-sparing diuretic.
To determine which way the potassium changed, you want to take into account
the other information that is given. In this case, the patient's bicarbonate
concentration is decreased. So the question you would want to ask yourself
is which potassium disorder causes metabolic acidosis, hypokalemia or
hyperkalemia.
Hypokalemia stimulates renal ammonia metabolism, leading to metabolic
alkalosis. Hyperkalemia inhibits renal ammonia metabolism, leading to
metabolic acidosis.
Since this patient had metabolic acidosis, the net effect of the to
diuretics was to increase the serum potassium, which led to the development
of metabolic acidosis.
I. David Weiner, M.D.
May 10, 2008
Hi Dr. Weiner,
I had another question I wanted to ask you:
Why does hyperglycemia increase urine output? Under conditions of high blood
glucose (and increased osmolality), wouldn't the osmoreceptor cells in our
hypothalamus shrink and stimulate ADH secretion which would reabsorb more water?
- The major effect of hyperglycemia is not mediated through changes in
plasma osmolality, but from the excretion of high mounts of glucose in the
urine. With significant hyperglycemia, maximum renal tubular reabsorption of
glucose (in the proximal tubule) is exceeded. As a result there is
substantial glucose delivery to more distal segments, specifically the
collecting duct. There, glucose is of such high concentration that it is
substantially increases luminal osmolality. Since ADH-stimulated water
movement involves water movement from areas of low osmolality (collecting
duct lumen) to areas of higher osmolality, glucose induced increase in
luminal osmolality decreases water reabsorption.
I. David Weiner, M.D.
May 10, 2008
Dear Dr. Weiner,
I have accumulated some questions during studying the past few days that I
was hoping you could answer for me:
1. Why does low serum potassium cause increased glucose levels??? In
addition, wouldn't insulin stimulated by the high glucose levels make serum
potassium levels even lower because insulin promotes the activity of the Na/K
ATPase???
- Hypokalemia leads to both decreased rates of insulin secretion in
response to hyperglycemia and decreased end-organ sensitivity to insulin.
The combination of these two leads to increased glucose intolerance and
worsened glucose metabolism.
2. Does low serum potassium cause increased sodium retention because low
potassium stimulates an increase in K+/H+ ATPase in the collecting duct and
sodium can substitute for potassium???
- Yes, that appears to make major mechanism. With hypokalemia there is
increased expression of H-K-ATPase in collecting duct. There is also
decreased potassium delivery to the collecting duct. Thus, even though
sodium is a poor substrate for reabsorption by the H-K-ATPase, the
combination of increased H-K-ATPase expression and decreased luminal
potassium there is increased sodium absorption by the apical H-K-ATPase.
3. I might have written this wrong in your lecture: I wrote that hypokalemia
could be treated with insulin and glucose - Should it be hyperkalemia rather
than hypokalemia and if this is not true (if I had it correct the way I wrote it
in class) why is this???
- I think your latter prediction is correct. It is hyperkalemia that
is treated with insulin and glucose, not hypokalemia.
4. In your acid-base lectures I wrote that CO2 entry into cells is
transport-mediated? Is this correct and if so why doesn't Co2 simply diffuse
across the lipid cellular membrane???
- I did not want to make a big deal about this. Yes, about 50% of
CO2 movement in the proximal tubule across the apical membrane in the
process of bicarbonate reabsorption appears to be transporter-mediated,
specifically involving AQP-1. The other 50% is diffusive. Why is
it not all diffusive? Probably because the rate of CO2 movement is too
fast to be mediated solely by CO2 diffusion; the cell needs a
transporter-mediated mechanism to accelerate CO2 transport.
5. Does metabolism of carbohydrates and lipids always produce just CO2 as an
acid and not a H+ species (i.e. a strong acid)? Doesn't this CO2 generated by
lipid/carb metabolism produce a H+ too via CO2 + H20 = H2CO3 = H+ + HCO3- or is
it because it produces an H+ with a bicarbonate that it is essentially like no
net gain of acid? I'd appreciate it if you could explain this to me.
- Carbohydrate and lipid metabolism produce CO2 as a metabolic byproduct.
CO2 is an acid because of reaction with water to form carbonic acid with
subsequent dissociation to H+ and HCO3-. Because this reaction
produces both a proton and a HCO3 molecule, this has led generations of
medical students to wonder whether in fact CO2 is not an acid. In fact, it
is a very a strong acid. This is because of the relative concentrations of
proton and bicarbonate and body fluids. Under normal circumstances, proton
concentration is 40 nM and bicarbonate concentration is 24 mM. Thus, the
bicarbonate concentration is 600,000 fold higher than the proton
concentration. Consequently, a doubling of the CO2 level that leads to a
doubling of the proton concentration involves an increase in the proton
concentration from 40 to 80 nmol/L. Since the production of proton
bicarbonate is equivalent to the amount of protons produced, this results in
a 40 nmol/L increase in the bicarbonate concentration. This amount of an
increase would not be detectable or physiologically relevant.
I. David Weiner, M.D.
May 9, 2008
Hi Dr. Weiner,
Just a few questions from some of your lectures:
1. In figure 5 of in the sodium transport in the kidney lecture, where does
the chloride go that enters trancellularly via the apical anion (formate)/chloride
exchanger? Does it diffuse through a basolateral chloride channel? Also in
figure of the acid-base lecture, where is the chloride ultimately going once it
enters the cell via the basolateral chloride/bicarbonate exchanger?
- Intracellular Cl- in the proximal tubule exits via a basolateral Cl-
channel. In intercalated cells chloride exits the cell also via a
basolateral Cl- channel.
2. At the beginning of the acid-base lecture, you said carbohydrate and lipid
metabolism generate about 15,000 mmol acid/day, while proteins generate 50-60 of
acid mmol/day. However, the next day, you were really emphasizing protein
degradation and its roles in acid generation, while not really mentioning the
effects of CHO and lipids. Did I miss something about protein metabolism that
causes an effective increase in acid generation compared to CHO and lipids?
- Lipid and carbohydrate metabolism generates "acid equivalents" in the
form of CO2. This CO2 is then rapidly excreted through respiration. As long
as respiration is normal, this is not a problem. Of course, if someone slows
down their breathing for prolonged periods then impaired CO2 excretion leads
to "respiratory acidosis."
3. How does hyperkalemia inhibit the formation of NH3? Does the increase in
plasma potassium cause a shift of potassium into the cell and movement of
protons out of the cell and into the blood (less protons secreted)? Wouldn't
this decrease the pH of blood and you would want more "new bicarbonate"?
- The exact mechanism through which changes in extracellular potassium
regulate proximal tubule ammonia production are not completely known. The
current belief is that changes in extracellular potassium alter proximal
tubule membrane voltage and intracellular electronegativity. With
hyperkalemia there is membrane depolarization and "less intracellular
electronegativity." This decreases bicarbonate exit by the basolateral
sodium-bicarbonate cotransporter, which transports a net charge of -2. Less
bicarbonate exit raises intracellular pH. The cell "senses" this, "thinks"
that it means that systemic pH is higher and that less ammoniagenesis and
new bicarbonate formation is needed. Thus, it decreases ammoniagenesis.
4. This is probably not the type of question you want to hear, but I have to
ask. Should we be focusing a lot of time on learning concentrations of ions,
percentages, pH values, etc.?
- My general policy is that the more emphasis a lecturer puts on a point,
the more important the lecturer thinks that point is, and vice-versa.
Thank you and I have enjoyed your lectures.
- I'm glad you've enjoyed the lectures. Good luck!
I. David Weiner, M.D.
May 9, 2008
Hi Dr. Weiner,
Regarding the collecting duct, can proton flow from anywhere else in the
nephron contribute to the proton concentration there? Also, it appears that the
cell in figure 5 is an intercalated cell. Is it safe to assume that when you
mention "cells in the collecting duct" that secrete protons, that you are only
referring to intercalated cells? Thanks again.
- With regard to "proton flow," yes proton secretion in more proximal
segments can contribute to luminal pH and H+ concentration.
With regard to Figure 5, you can assume that intercalated cells secrete
protons in the collecting duct. There is some controversy in the research
community as to whether principal cells secrete protons, but you don't need
to worry about that.
Good luck.
I. David Weiner, M.D.
May 8, 2008
Dr. Weiner,
I'm sorry if you've already answered this question. But in the notes, you
have it printed that formic acid is involved in Na reabsorption in the late
proximal tubule. Then in the lecture, one of the drawings you made, says that it
happens in the early proximal tubule. I'm not sure which may be right and I
recall there was a question on the website involving the figure I am referring
to and you said that you would try to clear up confusion about it. Have you been
able to find a way to clear up whatever discrepancy there was.
- My notes and the textbook give differing answers as to the relative
roles of the different Cl transport mechanisms in the early and late
proximal tubule. I spent several hours researching this, and have been
unable to date to identify clear data that says which is correct. As a
result, I do not expect you know which Cl transport mechanism is operative
in the early versus late proximal tubule. You should know the mechanisms
that are used in the proximal tubule, just not whether they are primarily
active in the early or late proximal tubule.
I also have another question. I get the impression that because aldosterone
increases Na reabsorption it will also decrease plasma K. Tonight, I read in a
review book about how hyperkalemia induces aldosterone synthesis. What is the
mechanism this happens by?
- Isn’t it fun when physiology makes sense! Wouldn’t you want a hormone
that increases K excretion to be stimulated when plasma/serum K is elevated?
Hyperkalemia appears to stimulate adrenal cortical aldosterone secretion
by depolarizing the plasma membrane, which increases intracellular calcium
and increases aldosterone production.
Thank you.
I. David Weiner, M.D.
May 8, 2008
Hi Dr. Weiner,
I wanted to make sure that I had an idea correct. In the PT, decreased plasma
K will cause an increase in NH3 metabolism. This NH3 will then tell the CD to
conserve K. Could you explain the last part?
- Ammonia has two effects on K transport in the collecting duct. First, it
inhibits sodium reabsorption, by inhibiting ENaC, which decreases principal
cell-mediated potassium secretion. Second, it has direct effects to
stimulate H-K-ATPase. These two actions decrease potassium secretion and
increase potassium reabsorption, leading to net potassium conservation.
How does increased Ca cause hypotension? Is it from the inhibiting urine
concentration idea via inhibiting ADH's ability to stimulate H2O reabsorption in
CD? Thus you have dilute high vol urine and low IV volume.
How does increasing PG cause an increase in K secretion? Does this occur in
the TAL by inhibiting the Na/K/2Cl like loop diuretics? We learned that PG
decrease Na reabsorption in the TAL... Could you compare the PG inc K secretion
mechanism to loop diuretic mechanism?
- Prostaglandins have two major ways that they alter renal K excretion.
First, prostaglandins, particularly PGE2 and PGI2, stimulate renin, and
thereby aldosterone, production. Aldosterone, of course, has multiple
effects on renal potassium homeostasis.
Second, prostaglandins increase the activity of the apical K channel in
the collecting duct principal cell.
Thus, in susceptible individuals, inhibiting PG production with either
NSAIDs or Cox-2 inhibitors, can lead decrease renal potassium excretion and
lead to hyperkalemia.
Is it correct that calcitonin will decrease Ca excretion in kidney? Seems to
be counterintuitive. I remember from Endocrine that it ???tones??? the Ca down
and gets it out of the blood. I know it puts it back in the bone and just
thought it should cause you to pee it out, too.
- Your prediction is correct. The best data is that calcitonin does
increases renal calcium excretion.
Also, how important are specific numbers and percentages for your tests. For
instance, should I know there are 3300 mEq K inside cells, there is 80% of this
or that absorbed in PT, etc. Or, for instance, should we just know that there is
a lot more K inside cells than in the ECF. Should we concentrate on general
trends or should we be memorizing specifics? Just wanted to know what was fair
game.
- My general policy is that the more emphasis a lecturer puts on a point,
the more important the lecturer thinks that point is, and vice-versa.
I am sorry there is so much in this email. I greatly appreciate your help!
Thank you!!!
I. David Weiner, M.D.
May 8, 2008
Could you please explain why an ECF contraction causes an increase in PO4 and
an increase in HCO3 reabsorption?
Is the P04 because more Na is being reabsorbed in the PT? Is the HCO3 because
of an increase in the Na/H exchanger in PT will increase activity to take in
more Na and secrete more H to promote HCO3 reabsorption?
I. David Weiner, M.D.
May 8, 2008
Thanks for the previous reply and reiterating the concept once more in class!
I have another question: Could you explain the hypokalemia in Renal Tubular
Acidosis Type I & II? The only thing I could come up with was the lack of HCO3-
reabsorption doesn't allow the osmolality of the tubular fluid to increase or
cause an positive-voltage buildup. So then K+ (which is driven by those two
forces) is not reabsorbed paracellularly. Am I correct?
- No, I think that the mechanism is different.
With Type I (Distal) RTA, the primary problem is an inability to
adequately secrete protons in the collecting duct. This has two
effects. First, the collecting duct is unable to completely reabsorb
filtered bicarbonate (HCO3-). This causes continued bicarbonate losses
in the urine, which contributes to generation of acidosis and also causes
the urine pH to be > 7, even when the person is acidotic. The
inability to acidify the urine also decreases titratable acid excretion, by
decreasing the number of secreted protons that can bind to the titratable
acids, and it decreases ammonia secretion in the collecting duct. Both
of these further lead to worsening metabolic acidosis.
The continued bicarbonate losses also require the presence of a
positively charged ion (cation) in the urine in order to keep the number of
positive and negative charges equal. The two major cations available
are sodium and potassium. Sodium losses cause reflexive stimulation of
ENaC sodium reabsorption in the collecting duct, which increases
potassium excretion, and thereby leading to the hypokalemia.
With Type II (Proximal) RTA, the primary problem is that during the phase
when there is ongoing bicarbonate losses (when the serum bicarbonate is
close to normal) the urinary bicarbonate has to have equal amounts of
cations. This causes increased potassium excretion through the same
mechanism as discussed for Type I (Distal) RTA.
I. David Weiner, M.D.
May 8, 2008
Concerning Table 3 of your acid-base handout, do ECV contraction and
aldosterone release act purely as signals to reabsorb bicarbonate, or do they
also stimulate bicarbonate generation?
If aldosterone does stimulate bicarbonate generation then I am confused
because, aldosterone at the principle cells enhances the activity of eNac. Then
as more Na is reabsorbed (by eNac) more K+ is excreted into the urine. However
an increase in ammonia metabolism causes more K+ to be reabsorbed by the
intercalated cells. It seems like the effects of aldosterone has opposite
effects on K+ at these two collecting duct cell types. Could you please clarify
this?
Also, why does an ECV contraction stimulate an increase in filtered
bicarbonate?
Thank you again,
Aldosterone has multiple effects to stimulate
new bicarbonate generation. It probably has direct effects on the
proximal tubule to stimulate ammoniagenesis. It probably stimulates
TAL Na-K-2Cl cotransport activity, which increases NH4+ reabsorption.
It is clearly known to stimulate H+ secretion in the collecting duct, which
can increase titratable acid excretion and also increases ammonia secretion.
Aldosterone's major effect on potassium is to
increase potassium excretion and thereby lower serum potassium levels.
Aldosterone stimulates proton secretion in the collecting duct primarily
because of its effects on H-ATPase. It has relatively little effect on
H-K-ATPase.
In your last question, I presume that you mean
"why does ECV contraction stimulate an increase in filtered bicarbonate
reabsorption?" This occurs because ECV contraction stimulates Na+
reabsorption in the proximal tubule. Because the predominant component
of Na+ reabsorption is mediated by Na/H exchange, there is a parallel
increase in H+ secretion, which leads to increased bicarbonate reabsorption.
I. David Weiner, M.D.
May 7, 2008
Dr. Weiner, you mentioned today that HCO3 is lost in the urine
with either Na & K to balance the charges. You said that while the kidney can
compensate for the loss of Na, it cannot compensate for the loss of K. Why is
this?
Thank you!
-
I have really never heard a very good explanation as to why
that is. I imagine it's somewhat because the high sodium delivery to the
collecting duct results in stimulation of sodium absorption through the
epithelial sodium channel, and that this results in stimulation of potassium
secretion. Thus, mechanisms that help to minimize sodium losses actually
result in increased potassium losses.
I. David Weiner, M.D.
May 7, 2008
Can you explain to me the relationship btw K+, PGs, and NSAIDS.
I know that PGs will inhibit NA reabsorption in the TALH, but I am unclear as to
what kind of an impact PGs have on K handling. At the end of your lecture you
say that patients using inhibitors of PG formation like NSAIDS have increased
likihood of hyperkalemia, so I am guessing that somehow, PGs can lower both your
NA and K levels. Could you just expound on this a little? What's the mechanism
(unless that question makes the response 15 pages long!!)?
-
Prostaglandins probably are involved in this because they
help in regulating the apical potassium channel present in collecting duct
principal cells. When you block prostaglandin formation, with nonsteroidal
anti-inflammatory drugs, you decrease potassium movement through this apical
potassium channel, thereby decreasing the kidney's ability to excrete
potassium.
I. David Weiner, M.D.
May 7, 2008
I have a question about the last sentence on page 76 where you
say that a high extracellular osmolality will cause potassium ions to move out
of cells into the extracellular fluid compartment. Isn't that the opposite of
what should happen or do I have everything backwards?
-
The effects of hyperosmolarity on serum potassium are likely
due to hyperosmolarity causing cell shrinkage. Cell shrinkage occurs because
when extracellular osmolarity is increased, this results in a gradient for
water to move from areas of low osmolality, and higher water concentration,
to areas with higher osmolality. This causes cell shrinkage. The theory,
then, is that as a cell shrinks this concentrates intracellular potassium,
the cell notice is this and does not like it, and is then stimulates the
cell to extrude potassium. This redistribution of potassium from the
intracellular to extracellular compartment results in the hyperkalemia.
I. David Weiner, M.D.
May 7, 2008
In the middle of page two of this lecture, you are speculating
on the exit of Ca from the epithelial cell once it has been reabsorbed from the
lumen of the distal tubule through a channel.
You say that it probably gets out on the basolateral side of the
cell with a Ca ATPase and a Ca/NA exchanger. Which way would that NA be moving.
It sounds like it would be putting Na into the cell, but that seems
counterintuitive.
-
The primary purpose of the sodium calcium exchanger is not
related to sodium transport, but calcium transport. You are right, it would
result in sodium movement into the cell that would energize calcium exit. It
uses the low intracellular sodium concentration to drive this process. The
sodium that enters the cell is then pumped out of a cell by the basolateral
sodium potassium ATPase. This then becomes an way of indirectly using sodium
potassium ATPase to enable calcium movement out of the cell.
I. David Weiner, M.D.
May 7, 2008
Dr. Weiner,
Is there a mechanism to understand how hypokalemia causes
increased ammonia production?
-
A simple answer, and one that I personally believe is the
most correct, is that no one is totally certain at this time how hypokalemia
stimulates proximal tubule ammonia production.
Some people believe, and I admit there is some evidence to support, the
following theory. Low extracellular potassium results in hyperpolarization
of the proximal tubule membrane and increased intracellular
electronegativity. The basolateral sodium bicarbonate cotransporter
transports three bicarbonate molecules on one sodium molecule, and thereby
extrudes net negative charge out of the cell. The increased intracellular
electronegativity in hypokalemia might then increase the rate of HCO3 exit
through the sodium bicarbonate cotransporter. Increased bicarbonate exit
would stimulate generate intracellular acidification. This cell would
"sense" this intracellular acidification, take it is evidence of
extracellular acidosis, and use it is an indication that there is a need to
increased ammoniagenesis in order to generate more ammonia for net acid
excretion. It all makes sense, I might actually be correct!
I hope this helps.
I. David Weiner, M.D.
May 7, 2008
Your handout says that hypokalemia stimulates ammonia production
and excretion. I understand that higher concentrations of K+ in the tubular
space will cause more pumping of protons out of the collecting duct cells. And
that the pumping of more protons in the luminal space will lead to more
production and excretion of the ammonium ion. I was wondering if this is the
only reason why hypokalemia stimulates ammonia metabolism and excretion or are
there other ones we should be aware of. Should we assume that the K+/proton pump
plays the more vital role in ammonia metabolism? Is my above reasoning about K+
correct?
Thank you.
-
Hypokalemia has multiple effects that probably interact with
each of the three major regions involved in ammonia metabolism, the proximal
tubule, thick ascending limb of the loop of Henle and the collecting duct.
In the question above, the mechanism, or lack of understanding of the exact
mechanism, though which hypokalemia stimulates proximal tubule
ammoniagenesis is discussed.
In the thick ascending limb of the loop of Henle, the likely mechanism is
somewhat understood. The most likely explanation relates to the fact that
potassium and NH4+ compete for binding on transport at the same binding site
of the apical sodium-potassium-to chloride cotransporter. With hypokalemia,
there is less potassium available to compete with the NH4+, resulting in
greater rates of NH4+ absorption by this transporter and thereby greater
rates of NH4+ transport by the thick ascending limb of the loop of Henle.
Hypokalemia also has effects on the collecting duct to stimulate both
mechanisms of proton secretion, H-K-ATPase and H-ATPase. Effects on
H-K-ATPase are logical to me. With potassium deficiency, as occurs with
hypokalemia, you would want to increase expression of a protein that
functions to reabsorb potassium out of the luminal fluid, thereby decreasing
urinary potassium losses. H-ATPase is also stimulated; the reason or the
mechanism through which this occurs is not known at present.
Good luck!
I. David Weiner, M.D.
May 7, 2008
Hi Dr. Weiner, I have a few questions from your lectures. The
potassium handout says that increased extracellular osmolarity due to compounds
that cannot cross the plasma membrane causes potassium to shift out of the cell
and result in hyperkalemia. Why does this happen? Also, did you mention in class
why high plasma calcium causes polyuria? Finally, where does aldosterone act to
increase HCO3- reabsorption?
-
The effects of hyperosmolarity on serum potassium are likely
due to hyperosmolarity causing cell shrinkage. Cell shrinkage occurs because
when extracellular osmolarity is increased, this results in a gradient for
water to move from areas of low osmolality, and higher water concentration,
to areas with higher osmolality. This causes cell shrinkage. The theory,
then, is that as a cell shrinks this concentrates intracellular potassium,
the cell notice is this and does not like it, and is then stimulates the
cell to extrude potassium. This redistribution of potassium from the
intracellular to extracellular compartment results in the hyperkalemia.
Next, you asked about hypercalcemia and polyuria. Hypercalcemia acts to
inhibit ADH's ability to stimulate water reabsorption in the collecting
duct, thereby impairing the ability to concentrate the urine, and resulting
in increased urine volume excretion.
Aldosterone acts in multiple areas to stimulate bicarbonate reabsorption. It
has direct effects on the proximal tubule to stimulate ammoniagenesis. It
also has direct effects in the collecting to stimulate intercalated cells to
increase acid secretion. A third mechanism through which it increases acid
secretion is a little bit more indirect. Aldosterone stimulates sodium
absorption by the principal cell through its effects on both basolateral
sodium-potassium-ATPase and on the apical sodium channel. Reabsorption of
sodium from the luminal fluid through this mechanism is partially, but not
completely balanced by potassium secretion. As a result, there is generation
of a net negative charge in the tubule lumen. This negative charge in the
tubule lumen makes it easier for the H-ATPase, which secretes a net positive
charge, to secrete protons into the luminal fluid.
I. David Weiner, M.D.
May 7, 2008
Hi Dr. Weiner
I have a few questions about NH4+ excretion from your Acid-Base
lecture. In figure 5 of your handout, you show NH4+ being converted to NH3 and
H+ in the cells of thick ascending loop of Henle, and then NH3 diffusing into
the collecting duct where it is then diffused out into the lumen (and it reacts
with H+ to form NH4+ which stays out in the lumen and is excreted).
My questions are:
1) Why can’t the NH4+ just stay in the TALH, and go into DCT,
and then CD and do its work?
2) What is separating the neighboring TALH and CD cells, that
NH3 is diffusing across?
3) What happens to the H+ that is derived from NH4+ in TALH?
Thanks!
-
These are excellent questions.
1. The current theory is that ammonia that stays in the lumen and reaches
the distal convoluted tubule and connecting segment in the cortex is likely
to diffuse out of these nephron segments. In the cortex with a very high
blood flow, there are numerous peritubular capillaries into which the
ammonia, once it has diffuses out of the tubule lumen, can enter, thereby
being returned to the systemic circulation through the renal veins. Ammonia
that returns to the systemic circulation is metabolized by the liver in a
HCO3-consuming reaction to form urea. Thus, ammonia that returns to the
systemic circulation results in no net bicarbonate generation in no
acid-base benefit.
You might ask how does the ammonia diffuse out of the lumen in the distal
convoluted tubule and connecting segment. The answer is that ammonia is in
equilibrium in two molecular species, NH3 and NH4+. Although the minority
amount is present as NH3, NH3 has a relatively higher passive permeability
across plasma membranes, enabling diffusion out of the tubule lumen into the
interstitium, where the NH3 concentration is lower. In the cortex, where
there are numerous peritubular capillaries, this allows easy access of the
ammonia into peritubular capillaries and return to the systemic circulation.
In the medulla, where blood flow is much lower, resulting in substantially
fewer peritubular capillaries, the amount of ammonia that enters peritubular
capillaries and returns to the systemic circulation is much less. The
ammonia is instead available for transport across collecting duct cells into
the collecting duct lumen, where it can then exit the kidney into the final
urine.
2. The renal interstitium consists of fibrous tissue, interstitial cells,
and a water-based mixture of sodium chloride urea, ammonia and other
electrolytes.
3. The current model is a complicated one. Lets me see if I can explain it
in a relatively simple fashion. NH3 from the interstitium enters collecting
duct cells across the basolateral membrane and then can be transported
across the apical membrane into the tubule lumen. Luminal protons combine
with luminal NH3 to form NH4+, which is "trapped" and cannot move back into
the cells. Because NH3 is converted to NH4+, this decreases luminal NH3
concentration levels and enables continued movement of NH3 into the tubule
lumen down its concentration gradient. The proton that is secreted was
generated from, our old friend, reaction of intracellular water and carbon
dioxide to form carbonic acid which dissociates into proton and bicarbonate.
Bicarbonate is then transported across the basolateral membrane by
basolateral chloride/HCO3 exchanger. This bicarbonate then reacts with the
proton, the one you asked about, that was derived from NH4+ from the thick
ascending limb of the loop of Henle, to form carbonic acid, which
dissociates into water and CO2. The net to this process is movement of NH3
from the interstitium into the tubule lumen.
You might notice that this process does not result in new bicarbonate being
formed by the collecting duct in the process of ammonia secretion. That is a
correct interpretation. The new bicarbonate that is formed from ammonia
metabolism actually is generated in the proximal tubule from the metabolism
of glutamine to form equimolar amounts of ammonium (NH4+) and bicarbonate.
I am sorry this is so complicated!
I. David Weiner, M.D.
May 7, 2008
Hi Dr. Weiner, I had another question. Today in lecture you
talked about bicarb being made new in the collecting duct. I’m not sure I
understand why you did not consider bicarb as being ‘made new’ in the proximal
tubule?? As I understand, both cells take CO2 from the lumen and generate HCO3
in the cell using carbonic anhydrase. Can you clarify the difference?
Thanks
-
This is really a terminology question. If the bicarbonate
originally comes from the luminal fluid, then the process is referred to as
bicarbonate reabsorption. If the bicarbonate does not come from the lumen,
but is generated intracellular, and this is referred to as new bicarbonate
generation. I agree that it is somewhat of an artificial difference. The
critical point is that new bicarbonate generation comes from proton
secretion where the proton is bound to titratable acids or ammonia
metabolism.
Once again, if proton secretion binds luminal bicarbonate, forming carbonic
acid, which dissociates to water and carbon dioxide, and the carbon dioxide
moves into the cell, reforming with water to generate intercellular carbonic
acid which dissociates into intracellular protons and bicarbonate, and the
bicarbonate exits across the basolateral membrane, then this process is, by
definition, termed bicarbonate reabsorption.
If the proton that is secreted binds something other than bicarbonate,
either titratable acids or ammonia, then the process is, by definition,
termed new bicarbonate generation.
I. David Weiner, M.D.
May 7, 2008
I'm a little confused about HCO3- generation via K+ levels and
aldosterone. The notes indicate that aldosterone increases HCO3- reabsorption,
thus increasing plasma [HCO3-]. And elevated [K+] inhibits NH3 metabolism
tending to decrease HCO3- generation, which would decrease plasma [HCO3-]. So
these two substances have opposite effects. However, we also learned that
elevated levels of K+ increase aldosterone, and aldosterone would counteract the
effects of K+ on HCO3- generation. Is this supposed to happen to keep the system
in balance or am I missing something?
-
You are not confused, and this is a very good question. As
you correctly note, potassium's direct effect on ammonia metabolism differs
from its indirect effects which are mediated through aldosterone. The net
effect actually appears to depend on what animal you are talking about. If
you are talking about a dog, the effects of potassium on aldosterone "win
out," and the net result is that hypokalemia causes metabolic acidosis and
decreases renal ammonia metabolism. That is important information to know if
you are a veterinarian! If you want to take care of humans, the answer is
that the direct effects of potassium on ammonia metabolism outweigh its
indirect effects that are mediated through aldosterone. In humans,
hypokalemia leads to increased ammonia metabolism in the development of
metabolic alkalosis.
I am sorry that this is not simple!
I. David Weiner, M.D.
May 6, 2008
Dr. Weiner,
You mentioned that in Gitelman's Syndrome and Gordon's syndrome
you have opposite effects on potassium. In Gitelman's, you mentioned you would
have decreased plasma K and in gordon's you have increased K in plasma. I am a
bit confused on how this occurs. If the NCC is working faster in Gordon's,
wouldn't K excretion be increased and thus K in plasma would fall? Or is it the
interplay of the potassium at the collecting duct which causes this increase in
plasma potassium? In that, you have less Na being reabsorbed in the CD so less K
is excreted. Any clarification would be greatly appreciated.
- Overactivity of NCC results in increased NaCl reabsorption in the DCT,
which results in decreased Na+ delivery to the collecting duct. This then
impairs the collecting duct principal cell ability to secrete potassium and
leads to hyperkalemia.
In Gitelman’s Syndrome the decreased NCC function results in decreased
DCT NaCl reabsorption, increased NaCl delivery to the collecting duct. This
then increases NaCl absorption in the collecting duct, which leads to
increased K secretion.
I. David Weiner, M.D.
May 6, 2008
Hello,
I have some more questions.
K Transport - When discussing the topic of serum K being a poor indicator of
actual K deficiency, you mentioned that if you reach a certain level of
hyperkalemia, the cell can no longer move potassium into the cell. Can you
please explain this concept.
- The problem has to do with osmolality. Normal plasma osmolality is ~300
mOsm/kg H20. Intracellular osmolality has to equal extracellular osmolality
under steady state conditions. Thus, intracellular osmolality is ~300 mOsm/kg
H2O, which means that intracellular K concentration can’t exceed 150 mmol/L.
Once it has reached that level, no more K can move into the cell.
I was confused by a line in your packet. ???Changes in extracellular
potassium, completely by itself, regulates renal potassium transport.??? How
does that statement relate to the release of aldosterone, insulin, and
beta-adrenergic agonist release which regulate the renal K+ reabsorption?
- Extracellular K can regulate K transport through mechanisms both
separate from and additive to the effects of insulin, aldosterone,
beta-adrenergic agonists and other regulatory mechanisms. The exact
mechanism is not known (sorry).
Calcium Transport ??? In regards to the loop and thiazide diuretics it seems
the effects on Ca2+ excretion can be explained largely by actions in the TAL
(thick ascending limb) and the effects on the K+ excretion can be explained by
actions in the Proximal convoluted tubule. Could you please explain how thiazide
diuretics increase K+ excretion? The thiazide diuretic causes a block of the NCC
channel (in the DCT) which causes more Na to excreted, resulting in a drop in
plasma volume. This then causes compensation by the TAL to increase Na
reabsorption, in the process the lumen becomes more positively charged which
explains why there is less Calcium loss to the urine (because it is reabsorbed
via paracellular transport). But since more Na is absorbed at the TAL then there
is less Na in the lumen when it reaches the Collecting duct. With less Na being
reabsorbed through the ENaC channel, isn't less K+ then excreted to the lumen
via the apical K+ channel? So overall how do thiazide diuretics increase K+
excretion?
Thank you again,
- Overall, the greatest effect of thiazide diuretics increase NaCl
delivery to the collecting duct by blocking DCT NaCl reabsorption. The
counter-regulatory mechanisms that you discuss are only partially adaptive.
Thus, the net effect is increased collecting duct K secretion. The effects
on calcium transport are different, because there is very little calcium
transport in the DCT and collecting duct.
I. David Weiner, M.D.
May 6, 2008
Dr. Weiner, in your notes it says that excessive extracellular K
leads to muscle weakness. On the board you drew an action potential and
mentioned that LOW K actually makes it harder to reach the threshold. Were you
talking about low extracellular or intracellular K? Can you clarify?
Thanks
I. David Weiner, M.D.
May 4, 2008
Hello,
I had a few questions from your last lecture.
I was reading another text which said that the thick ascending loop and
distal convoluted tubule were both impermeable to water (were both diluting
segments), but in the notes only the distal convoluted tubule was labeled
impermeable to water. Could you please clarify?
- The other text is correct, both the thick ascending limb of the loop of
Henle and the distal convoluted tubule are impermeable to water. In the
thick ascending limb, this is important for generation of the medullary
hypertonicity that is necessary for concentrating the urine in the
collecting duct.
With regards to the generation of dilute urine, only the distal
convoluted tubule plays a role. This is because the luminal fluid delivered
to the thick ascending limb is very hypertonic. This occurs because of water
movement, but not sodium or chloride movement, out of the thin descending
limb of the loop of Henle. In the thick ascending limb of the loop of Henle
sodium chloride, but not water transport, therefore only results in
decreasing luminal fluid osmotic back to serum levels, approximately 300
milliosmoles per kilogram water. Because the thick ascending limb does not
generate a urine luminal fluid that is dilute with respect to plasma, it is
generally not thought of as playing a major role in generation of a dilute
urine and recovery from hyponatremia.
The distal convoluted tubule, because it has a luminal fluid delivered to
it with a normal osmolality and is then able to reabsorb sodium and chloride
out of the luminal fluid, but does not transport water, then generates a
luminal fluid which is dilute (as a lower osmolality) as compared to plasma.
How does Liddle Syndrome effect the K+ amounts? If eNac is overactive in the
collecting duct then should more K+ be excreted? Would the effects be the
opposite of K+ sparing diuretics?
- Your prediction about Liddle syndrome and potassium transport is
correct. Overactivity of the epithelial sodium channel (ENaC) does result in
increased potassium secretion. This results in the development of
hypokalemia.
Why do Loop Diuretics cause more Ca2+ loss but thiazide diuretics cause a
decrease in Ca2+ loss? Both increase the K+ excretion into the lumen and thereby
increase the positive luminal charge. How come both don't decrease the Ca2+
loss?
- Loop diuretics increase urinary calcium excretion because calcium
absorption in the thick ascending limb of the loop of Henle is regulated by
sodium reabsorption. We will talk about the details of this in the calcium
and phosphorus lecture on Monday.
Thiazide diuretics act in the distal convoluted tubule, a segment in
which there is much less calcium absorption. Accordingly, there is very
little effect on calcium transport in the distal convoluted tubule. Instead,
their effect on calcium excretion by the kidney is indirect and is related
to effects on calcium transport in the proximal tubule and the loop of
Henle. Thiazide diuretics, by increasing urinary sodium excretion, result in
mild degrees of intravascular volume depletion. This causes feedback
stimulation of sodium absorption in both proximal tubule and the thick
ascending limb of the loop of Henle. In both of these segments, calcium
absorption is linked to sodium absorption. Accordingly, there is increased
calcium absorption in these segments as a result of thiazide diuretic
administration.
Thank you for your help.
I. David Weiner, M.D.
April 30, 2008
Sorry for multiple emails, but I just came across another item that I could
use some clarification on.
On page 3 of your handout, near figure 5, there is the following statement:
"In the early proximal tubule more bicarbonate than chloride is reabsorbed,
resulting in the luminal chloride concentration gradually increasing along the
proximal tubule" and I was wondering if you wouldn't be able to say a bit more
about that, since bicarbonate isn't depicted in any of the figures.
From my own notes, I had that bicarbonate will combine with the H+ put out by
the Na+/H+ exchanger and H2O will move into the cell, causing Cl- concentration
in the lumen to rise, is this correct?
This topic was a little fuzzy for me, I appreciate the help! Thank you!
- The answer relates to the fact that the proximal tubule reabsorbs both
sodium chloride and sodium bicarbonate from the luminal fluid.
In the early proximal tubule, the relative rate of sodium bicarbonate
reabsorption is faster than the rate of sodium chloride absorption. Whereas
on the surface this would suggest that sodium, chloride and bicarbonate
concentration would decrease in the proximal tubule luminal fluid, it is a
little bit more complicated.
As you reabsorb sodium, along with either chloride or bicarbonate, there
is a decrease in luminal osmolality. This results in a parallel increase in
luminal fluid water concentration. This then enables water reabsorption by
the proximal tubule. The permeability to water is so high that there is
barely even a detectable difference in osmolality between the luminal and
peritubular fluids. Consequently, the luminal sodium concentration does not
change significantly, despite rapid rates of sodium absorption.
However, because the rate of bicarbonate absorption is relatively greater
than the rate of chloride absorption, the luminal bicarbonate concentration
decreases and the luminal chloride concentration increases.
We will discuss the specific mechanisms of bicarbonate absorption by the
proximal tubule in the acid-base lecture.
I hope this helps,
I. David Weiner, M.D.
April 30, 2008
Today at the start of your second lecture (Renal Sodium Transport II) you
redrew figure 7 on the board and said that there was an error in the drawing in
our handout. In looking back at my notes this afternoon, I couldn't recall the
inconsistency you had referred to and I was wondering if you could clarify that?
Thank you!
I. David Weiner, M.D.
April 30, 2008
Dr. Weiner,
I have a few questions regarding the sodium transport lecture:
1. I wrote down in class that paracellular Cl- transport occurs in the late
proximal tubule, but read in boron that it chiefly occurs in the early proximal
tubules. Which is correct?
2. Do the basolateral surfaces of the TAL cells have bicarb-chloride
transport?
3. Do the TAL cells have paracellular sodium transport, or is it only the
later proximal tubule cells that have paracellular sodium transport?
4. In the principal cells of the collecting duct, are there potassium
channels on the basolateral surface as well as the apical surface?
Thank you for your help.
- 1. I have found conflicting references regarding the data in Boron
textbook. I am contacting the authors to try to find out the evidence
supporting what is in the textbook. I will let you, and the rest of your
class, know as soon as I find out.
2. They probably
do, but this probably is not the mechanism of basolateral chloride exit.
Basolateral chloride-bicarbonate exchangers typically results in chloride
entry, because the intracellular chloride concentration, 20-40 millimolar,
is typically less than extracellular chloride concentration. Intra-cellular
electronegativity does not regulate culture chloride-HCO3 exchange
transport, because this protein exchanges a negative ion for negative ion.
Intracellular voltage does not alter transport since there is no net
movement of charge.
3. The paracellular pathway is substantially less
important in the thick ascending limb than in the late proximal tubule.
4. Yes, there are basolateral potassium channels.
They provide an element of regulation for potassium transport. They enable,
in states of potassium depletion, that potassium taken into the principal
cell by the basolateral sodium potassium ATPase can recycle across the
basolateral membrane through the basolateral potassium channel, thereby
minimizing potassium secretion into the urine.
I hope this helps,
I. David Weiner, M.D.
Please let me know any other suggestions you might have
regarding this website and ways that it may be more useful for you.
I. David Weiner, M.D.
Professor of Medicine and Physiology
University of Florida College of Medicine
Chief, Renal Section NF/SGVHS
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