Carbonic Anhydrase and Hormonal control questions

[sv3]

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So I have two questions as the post says. I think I'm missing something easy regarding both of them:

1) For hormone feedback systems, how can you tell when the variable (ex. cortisol) feeds back to the gland that secreted it (Adrenal cortex), or the tropic glands above it (hypo, Ant Pit)? Everything I've seen would indicate that the hormone doesn't actually inhibit the gland that secreted it, but inhibits the tropic glands. Basically, I've seen numerous examples regarding cortisol and all of them do not have cortisol inhibiting the adrenal cortex, but rather the hypo and ant pit. I am just wondering why it doesn't inhibit the cortex? (I know it doesn't have to since the tropics play here, but just looking for a better reason). Is it perhaps the things that cortisol controls (blood glucose) that would inhibit the adrenal cortex from secreting more? Just lost...........obviously

2) For the carbonic anhydrase reaction, I had a question ask what would happen if an inhibitor stopped the enzyme from doing its thing. The answer was that blood pH would decrease since the kidney could not excrete H+ (it was a GI passage). I am just wondering why that is? If the enzyme is inhibited, then don't you have H+ as well as HCO3- being trapped in the blood, thus an acid and a base. So I am not seeing why the drop on pH occurs since you can't excrete the base either.

thanks in advance
steve

 

[rocuronium]

One effect of carbonic anhydrase inhibitors is that they stimulate bicarbonate excretion by the kidneys. The excretion of bicarbonate in high quantities leads to bicarbonate wasting and a subsequent acidosis.

Here's a question for you to ponder: why (or when) would it be beneficial for someone to take a carbonic anhydrase inhibitor?

 

[sv3]

One effect of carbonic anhydrase inhibitors is that they stimulate bicarbonate excretion by the kidneys. The excretion of bicarbonate in high quantities leads to bicarbonate wasting and a subsequent acidosis.

Here's a question for you to ponder: why (or when) would it be beneficial for someone to take a carbonic anhydrase inhibitor?

So this is where I can't understand the action of the enzyme. You say that they stimulate excretion of bicarbonate - how? If you can't excrete H+ when the inhibitor is present how can you excrete HCO3-? Why the difference? They enzyme produces of these products together from carbonic acid (via CO2 and H20) right?

judging by what you've said i would think if your pH is too high you'd want that inhibitor.

 

[rocuronium]

I did a little bit of searching and managed to find a website which explains it fairly well. It even has a diagram.

http://www.mc.uky.edu/pharmacology/instruction/pha824dr/PHA824dr.html

Scroll down a few pages to the "carbonic anhydrase inhibitor" section.

 

[sv3]

I did a little bit of searching and managed to find a website which explains it fairly well. It even has a diagram.

http://www.mc.uky.edu/pharmacology/instruction/pha824dr/PHA824dr.html

Scroll down a few pages to the "carbonic anhydrase inhibitor" section.

Thanks for that link. I got a couple of questions about it as I saw some things i wasn't aware of.

So firstly, within the blood, When CA (enzyme) converts CO2 and H2O into HCO3- and H+, these latter two molecules actually go in different directions? One into the tissue, one into the prox tubule?

Secondly, it appears as if H+ keeps getting re-used to draw HCO3- in, but that HCO3- is not re-used as it gets transported into the tissue....is that right?

And finally the important one, so if CA is inhibited, why does H+ secretion stop? CA does not help transport H+ out of the blood, so why can't it still leave like it does when CA works? This is the answer that is really bugging me as it would explain the pH effects.

thanks very much, appreciated indeed.
steve

 

[rocuronium]

And finally the important one, so if CA is inhibited, why does H+ secretion stop? CA does not help transport H+ out of the blood, so why can't it still leave like it does when CA works? This is the answer that is really bugging me as it would explain the pH effects.

I'm not sure if there is a change in H+ secretion, and I can't find any evidence to suggest that there is. The drop in pH, however, can be attributed to the loss of blood bicarbonate (i.e. the blood has lost its buffering capacity which makes the remaining H+ more significant).

So firstly, within the blood, When CA (enzyme) converts CO2 and H2O into HCO3- and H+, these latter two molecules actually go in different directions? One into the tissue, one into the prox tubule?

Carbonic anhydrase in the red blood cells is used to convert carbon dioxide into an appropriate form for transport (i.e. bicarbonate). The H+ produced attaches to hemoglobin. Outside of the red blood cells (in the plasma) there is no carbonic anhydrase and, therefore, few hydrogen and bicarbonate ions are produced. Those that are are buffered by plasma proteins.

Secondly, it appears as if H+ keeps getting re-used to draw HCO3- in, but that HCO3- is not re-used as it gets transported into the tissue....is that right?

If I'm understanding you correctly, then yes. The purpose of the mechanism that we have been discussing is to get bicarbonate back into the blood. The bicarbonate goes into the blood, and the H+ is transported back into the lumen.

I hope that helps!

 

[sv3]

I'm not sure if there is a change in H+ secretion, and I can't find any evidence to suggest that there is. The drop in pH, however, can be attributed to the loss of blood bicarbonate (i.e. the blood has lost its buffering capacity which makes the remaining H+ more significant).



Carbonic anhydrase in the red blood cells is used to convert carbon dioxide into an appropriate form for transport (i.e. bicarbonate). The H+ produced attaches to hemoglobin. Outside of the red blood cells (in the plasma) there is no carbonic anhydrase and, therefore, few hydrogen and bicarbonate ions are produced. Those that are are buffered by plasma proteins.



If I'm understanding you correctly, then yes. The purpose of the mechanism that we have been discussing is to get bicarbonate back into the blood. The bicarbonate goes into the blood, and the H+ is transported back into the lumen.

I hope that helps!

Hey thanks very much. It's helping for sure. I am using TPR and they don't go over this reaction that much -its mentioned a fair bit but not reviewed in any sort of depth. I was focusing on where H+ went to understand the pH change and the whole time, as you pointed out, the blood is losing bicarbonate so the pH inevitably will go down. K, things are clicking better now. I'll read on this some more but your replies really helped.

thanks alot

PS (Calgary? I'm in Mississauga)