CHOLINE TRANSPORT IN RAT-LIVER BASOLATERAL PLASMA-MEMBRANE VESICLES

Previous studies have demonstrated several pathways for the sinusoidal uptake of endogenous and exogenous organic cations, including two dis tinct organic cation:H+ exchangers and a separate carrier-mediated sys tem driven by the inside-negative membrane potential, In this study, t he driving force(s) for the uptake of the endogenous quaternary amine, choline, were determined in rat liver basolateral plasma membrane (bl LPM) vesicles, Choline uptake into an osmotically sensitive space was not stimulated by an outwardly directed H+ or inwardly directed Na+ gr adient, Instead, an inside-negative K+ diffusion potential stimulated choline uptake, suggesting the presence of a conductive pathway for ch oline uptake in blLPM vesicles, Conductive choline uptake was confirme d by inducing variable changes in the transmembrane potential with ani ons of different membrane permeability, Choline uptake in blLPM vesicl es exhibited 1) temperature dependency; 2) trans-stimulation; and 3) s aturability, with an approximate Michaelis constant (K-m) of 0.34 mmol /L and maximum velocity (V-max) of 0.45 nmol/mg protein/15 s, Choline uptake in blLPM vesicles was cis-inhibited by the structurally similar derivative, hemicholinium-3 and acetylcholine, but not by substrates for other organic cation transport processes identified in blLPM vesic les, including thiamine, tetraethylammonium (TEA), tri-n-butyl-methyla mmonium (TBuMA), and N'-methylnicotinamide (NMN), These findings demon strate an electrogenic pathway on the sinusoidal membrane for the upta ke of this essential nutrient and support the existence of multiple pa thways for the sinusoidal uptake of endogenous and exogenous organic c ations.