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Renal Handling of Organic Solutes

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When plasma glucose reaches such high levels that substantial amounts of glucose appear in the urine (glycosuria)

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a. glucose is leaking back into the tubule through tight junctions.

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b. there is not enough luminal sodium to move in symport with glucose.

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c. all the glucose transporters are working at their maximum rate.

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d. the glucose transporters are being inhibited by the high levels of glucose.

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(c) The large filtered load presents more glucose than the reabsorptive Tm-limited transporters can handle. Under all conditions, there is always far more filtered sodium than glucose and sodium is never rate-limiting.

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Useful small organic metabolites that should not be excreted are

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a. generally not filtered.

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b. reabsorbed paracellularly.

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c. taken up by endocytosis and degraded.

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d. reabsorbed transcellularly.

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(d) Small useful organic solutes are freely filtered. They are reabsorbed transcellularly by a Tm system. The normal filtered load is below the Tm.

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Organic anion secretion

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a. involves a step of active influx across the basolateral membrane.

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b. is passive and paracellular.

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c. occurs via simple diffusion through the tubular membranes.

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d. utilizes the same nonspecific transporters as organic cation secretion.

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(a) Anions that are secreted must enter the cell against a negative membrane potential, and usually against a concentration gradient as well; thus, they are actively transported.

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A high urinary pH favors

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a. low excretion of drugs that are weak acids.

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b. active reabsorption of drugs that are weak bases.

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c. low excretion of drugs that are weak bases.

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d. high passive permeability of drugs that are weak acids.

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(c) Drugs that are weak bases are usually neutral (unprotonated) at high pH. This favors their passive reabsorption by simple diffusion, and therefore low excretion.

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The tubular concentration of urea

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a. exceeds the plasma concentration at the hairpin turn of the loop of Henle.

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