Renal Handling of Organic Solutes
When plasma glucose reaches such high levels that substantial amounts of glucose appear in the urine (glycosuria)
a. glucose is leaking back into the tubule through tight junctions.
b. there is not enough luminal sodium to move in symport with glucose.
c. all the glucose transporters are working at their maximum rate.
d. the glucose transporters are being inhibited by the high levels of glucose.
(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.
Useful small organic metabolites that should not be excreted are
a. generally not filtered.
b. reabsorbed paracellularly.
c. taken up by endocytosis and degraded.
d. reabsorbed transcellularly.
(d) Small useful organic solutes are freely filtered. They are reabsorbed transcellularly by a Tm system. The normal filtered load is below the Tm.
a. involves a step of active influx across the basolateral membrane.
b. is passive and paracellular.
c. occurs via simple diffusion through the tubular membranes.
d. utilizes the same nonspecific transporters as organic cation secretion.
(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.
a. low excretion of drugs that are weak acids.
b. active reabsorption of drugs that are weak bases.
c. low excretion of drugs that are weak bases.
d. high passive permeability of drugs that are weak acids.
(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.
The tubular concentration of urea
a. exceeds the plasma concentration at the hairpin turn of the loop of Henle.