Regulation of plasma choline by the renal tubule: bidirectional transport of choline.

ACARA, MARGARET, AND BARBARA RENNICK. Rqylation of plasma choline by the renal tub&: bidirectional transport of choline. Am. J. Physiol. 225(5) : 1123-I 128. 1973-Choline is a biocation with a pK, of 13.5 which is actively transported by the organic cation transport system in the renal tubule. A radiochemical assay using choline kinase was adapted to measure free choline in plasma and urine without prior extraction. Renal clearance of choline was measured at endogenous levels in dogs and chickens. Only 1/30th of the choline filtered at the glomerulus was excreted in the urine, suggesting active reabsorption of filtered choline at the physiological plasma choline levels of 1 X lo-” M. When exogenous choline was infused into the renal artery of dogs, the choline renal clearance exceeded glomerular filtration rate indicating active tubular excretion of the cation choline. Homeostatic regulation of plasma choline by the renal tubule results from active reabsorption by the renal tubule of filtered choline at physiological levels of choline resulting in retention of choline by the body. FIowever at elevated plasma choline levels, the proximal renal tubule actively excretes choline resulting in a reduction of choline in the body. Increased tubular excretion begins when the endogenous plasma choline level is doubled. A transport maximum was demonstrated for the excretory transport. These results indicate a bidirectional tubular transport system for choline.