[TBR BIO] Glucose Transport at the Kidney

[Lunasly]

Confused about this question from TBR:

The reabsorption of sodium across the epithelial cells of the proximal convoluted tubule and into the peritubular capillary would be significantly reduced by:

A. reduced secretion of ADH from the posterior pituitary.

B. an increased rate of secretion of aldosterone from the adrenal cortex.

C. reduced plasma glucose concentrations.

D. an increase in the tubular colloid osmotic pressure.

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Answer: C is correct, reduced plasma glucose concentrations. Recall that in the proximal convoluted tubule the transport of sodium across the apical membrane (from the lumen of the proximal convoluted tubule to the cytoplasm of the lumenal epithelial cell) is mediated by proteins called symports (a cotransportcr). For example, these symports allow for the passage of both sodium and glucose together into the cell. Another example is the symport that allows for the passage of both sodium and various amino acids into the cell. If the concentration of glucose is decreased in the filtrate, the amount of sodium that can be reabsorbed at the level of the proximal convoluted tubule is reduced. The correct choice is C.
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I understand the explanation, but doesn't plasma glucose concentration refer to concentration of glucose in the blood and not the concentration of glucose in the filtrate?

Thanks for the help!

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[mcloaf]

Glucose is freely filtered by the kidney and so, before it is actively transported out of the filtrate, its concentration will be the same as in the plasma.

 

[Lunasly]

Thanks for the help, however I am not sure how that answers my question. I am well aware the concentration of glucose would be relatively the same in the blood and in the filtrate upon being filtered, however, transport of glucose back into the plasma or serum is accomplished via cotransport with sodium. If I recall correctly, this is an active form of transport, thus whether the concentration of glucose in the blood is high or low, glucose will be transported back into the blood. If anything, I would expect more glucose to be transported back into the blood (along with Na) if blood glucose was low since we want to conserve it.

I think this is more of a question of how the word "plasma" is interpreted. According to the answer, the author describes it as the filtrate.

 

[mcloaf]

I'm having trouble seeing the issue here. The question is not whether or more or less glucose is being taken up by the kidney (in non pathologic situations it should be pretty much 100% taken up), but how the plasma glucose levels will influence the uptake of sodium. Sodium and glucose are resorbed by secondary active transport symporters on the lumenal membrane using energy from Na/K ATPases on the PCT basal membrane. If plasma glucose and thus glucose in the lumen of the PCT is seriously decreased, there will be fewer glucose molecules taken back up by the kidney and so fewer sodium molecules will be taken up alongside the glucose. Glucose is the limiting reagent for the symporter, if you reduce glucose you reduce symport and so reduce sodium reuptake.

I think this is more of a question of how the word "plasma" is interpreted. According to the answer, the author describes it as the filtrate.

From the standpoint of the concentration of glucose pre-reuptake the two are functionally the same.

 

[IslandStyle808]

I think this is more of a question of how the word "plasma" is interpreted. According to the answer, the author describes it as the filtrate.

The author uses the term plasma and filtrate to describe the locations of glucose. Plasma glucose is used to describe glucose in the blood and filtrate glucose is used to describe glucose in the nephron. The glucose coming into the glomerulus is part of plasma and once it is filtered into the nephron that "same glucose" is now considered part of filtrate.

Mcloaf pretty much nailed the explanation of the process. Don't overthink the question, the main thing is to deduce which is the best answer.
A) is wrong because ADH only affects water reabsorption
B) is wrong because aldosterone mainly affects Na+ reabsorption in the DCT not the PCT
C) Makes the most sense out of all answers since glucose works in symport with Na+.
D) is wrong because this deals more with the movement of water

 

[Lunasly]

Thank you both. That actually makes a lot of sense. I was certainly over thinking it. The point you made before about the concentration of glucose in the filtrate in the PCT is essentially the concentration of glucose in the plasma upon filtration. Therefore, it makes sense that a lower glucose concentration (i.e., being the limiting reagent) will reduce the number of Na+ that is transported.

Thanks for the help 🙂