theonlytycrane

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The wording of the correct answer choice to the following question is confusing to me:

Na+K+ ATPase is found in epithelial cells that produce urine or other excretions that are hypertonic relative to tissues. Why?

a) To form a hypertonic solution, ions or molecules must be actively transported across their concentration gradients.
b) Cells similar to those found in the shark rectal gland occur in many other species that need to excrete salt (NaCl).
c) The sodium-potassium pump is directly responsible for moving sodium ions from blood of body tissues into the urine for salt excretion, against a concentration gradient.
d) They are usually located in the basolateral membrane of those epithelial cells, along with other channels and cotransporters.

The correct answer given is (a). The question is given in my bio book in relation to the kidney. I was thinking about the Na+K+ ATPase pumps in the proximal tubule of the nephron and their role in creating an ion gradient for reabsorption of nutrients and water back into the blood stream during filtration. The choice (a) is confusing to me because it seems to emphasize the formation of a hypertonic solution due to the sodium potassium pumps.

Am I thinking about this correctly?
 

65SpencerAve

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interesting.. in a hypertonic solution, the total molar concentration of dissolved solute particles (outside of the cell) is greater than the concentration of the cell (inside.) so the goal of the Na+K+ ATPase is to keep pumping these ions out (eventually against their concentration gradient), in order to keep the concentration greater on the outside of the cell than the inside. from the understanding of osmosis, water will diffuse from the inside (low concentration) of the cell to the outside (high concentration) of the cell, resulting in a shrinkage of the cell. i think the goal is to understand that the question is asking more about the formation of a hypertonic solution and less about the nephron. that is how i perceived the question. hopefully that helps.
 
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theonlytycrane

theonlytycrane

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The wording of the question is still a bit vague to me but I think you're right- I think the question is more general than I am making it out to be.

If epithelial cells produce urine or excretions that are hypertonic relative to tissues, the sodium potassium pump is needed to actively transport ions out of the cell into the hypertonic solution (against it's concentration gradient).
 
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theonlytycrane

theonlytycrane

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Actually, looking back I'm still not sure if this answer makes sense.

Na+ / K+ ATPase is found in the basolateral membrane of epithelial cells in the proximal tubule to create a gradient of Na+ from the lumen into the tubule membrane and then into the blood stream.

Any others thoughts on why (a) seems to emphasize Na+ being pumped into the filtrate instead of being reabsorbed?
 

ToldYouSo

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B and D are distractors that don't answer the question and C is wrong because the pump isn't *directly* used. The energy potential created by the pump is indirectly used because it provides the driving force for excretion of potassium and calcium (sodium potassium and sodium calcium exchangers as well as sodium gradient driven excretion of acid by a sodium hydrogen exchanger along with cytosolic and luminal side carbonic anhydrase).


A lot of the energy used to transport the cations and anyone into the lumen concentrating urine is due to the energy gradient from the sodium potassium pump (there are other transporters that directly use ATP however)
 
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theonlytycrane

theonlytycrane

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B and D are distractors that don't answer the question and C is wrong because the pump isn't *directly* used. The energy potential created by the pump is indirectly used because it provides the driving force for excretion of potassium and calcium (sodium potassium and sodium calcium exchangers as well as sodium gradient driven excretion of acid by a sodium hydrogen exchanger along with cytosolic and luminal side carbonic anhydrase).


A lot of the energy used to transport the cations and anyone into the lumen concentrating urine is due to the energy gradient from the sodium potassium pump (there are other transporters that directly use ATP however)
Thanks for the reply! One follow-up question:

After the sodium gradient is established by pumping sodium into the blood stream from the basolateral membrane, ions / glucose follow sodium into the lumen via cotransport and then passively flow into the bloodstream for reabsorption. Wouldn't this process make the urine less concentrated instead of hypertonic?

Edit: Unless the sodium-calcium and sodium-hydrogen exchangers that you mention are anti-porters on the apical face of the epithelial cell so that reabsorption of ions from the lumen is less than the calcium / hydrogen movement back into the lumen to keep the solution hypertonic?
 
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ToldYouSo

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Thanks for the reply! One follow-up question:

After the sodium gradient is established by pumping sodium into the blood stream from the basolateral membrane, ions / glucose follow sodium into the lumen via cotransport and then passively flow into the bloodstream for reabsorption. Wouldn't this process make the urine less concentrated instead of hypertonic?

Edit: Unless the sodium-calcium and sodium-hydrogen exchangers that you mention are anti-porters on the apical face of the epithelial cell so that reabsorption of ions from the lumen is less than the calcium / hydrogen movement back into the lumen to keep the solution hypertonic?
Yeah the sodium potassium pump is basolateral. Initial absorption of fluid *from the lumen to the cell* is isotonic. Most of sodium and all glucose and amino acid and other cations and anions are absorbed. It's later on in the thin descending loop of henle that water is absorbed making the urine hypertonic. In the ascending part that sodium gradient set up by the basolateral na/k pump is used to drive the transport of the remaining sodium potassium and calcium (NKCC channel) back into the cell. Without the na/k pump there would be no driving force for that to happen. It's get a little more complicated though as potassium has changed channels on the apical side (facing the lumen) that let it leak out, making the lumen positive and driving magnesium and calcium paracelluarly back into the blood (they want to move away from the K cations). The potassium that doesn't leak out goes back into the blood down its concentration gradient (there is high potassium concentration in the cell because of the na/k pump).

Urine gets less diluted in the early collecting tubule as more ions get absorbed with final concentration of the urine determined by aldosterone (absorb sodium, excrete potassium) alpha intercalated cell activity (proton pump excretion of h+ on the luminal side) and ADH depending on hydration/blood volume status.
 

ToldYouSo

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Rereading your question again just be aware that the the na/k pump isn't pumping sodium into the filtrate as its on the basolateral side (facing the interstitial fluid/blood).

The bottom line with the question is that it takes energy to move things against their gradient which is what you have to do to concentrate urine, and that energy largely comes from the sodium gradient that's set up (with some other pumps using ATP directly).
 
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Cell bio305

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I know this probably isn't the place to ask this, but I am not sure how to create a new forum. If the Na/K pump was poisoned, how many action potentials could a Neuron produce? I feel like this question is poorly written. Wouldn't the cell do one action potential and remain depolarized. Because Na would remain in high content in the cell. Potassium and Chloride wouldn't be able to counter it, with only the leak channels going?
Also it asks for equations that I would need to solve. I would think you would use the Goldman Hodge equation, but even then I am not sure what I am calculating or trying to show?
Any help or guidance here would be appreciated. The question doesn't specify whether the poison is fast acting or slow acting on the pump.