BASOLATERAL CHOLINE TRANSPORT IN ISOLATED RABBIT RENAL PROXIMAL TUBULES

Choline can undergo both net secretion and net reabsorption by renal p roximal tubules, but at physiological plasma levels net reabsorption o ccurs. During this process, choline enters the cells at the luminal si de down an electrochemical gradient via a specific transporter with a high affinity for choline. It appeared likely that choline was then tr ansported out of the cells against an electrochemical gradient at the basolateral membrane by countertransport for another organic cation. T his possibility was examined by studying net transepithelial reabsorpt ion and basolateral uptake and efflux of [C-14]choline in isolated S2 segments of rabbit renal proximal tubules. Basolateral uptake, which w as inhibited by other organic cations such as tetraethylammonium (TEA) , appeared to occur by the standard organic cation transport pathway. However, the addition of TEA to the bathing medium not only failed to trans-stimulate net transepithelial reabsorption and basolateral efflu x of [C-14]choline but it actually inhibited transepithelial reabsorpt ion by congruent to 60%. The results do not support the presence of a countertransport step for choline against an electrochemical gradient at the basolateral membrane. Instead, they suggest that choline crosse s this membrane by some form of carrier-mediated diffusion even during the reabsorptive process.