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 distinct organic cation:H+ exchanges and a separate carrier-mediated system driven by the inside-negative membrane potential. In this study, the driving force(s) for the uptake of the endogenous quaternary amine, choline, were determined in rat liver basolateral plasma membrane (blLPM) vesicles. Choline uptake into an osmotically sensitive space was not stimulated by an outwardly directed H+ or inwardly directed Na+ gradient. Instead, an inside-negative K+ diffusion potential stimulated choline uptake, suggesting the presence of a conductive pathway for choline uptake in bILPM vesicles. Conductive choline uptake was confirmed by inducing variable changes in the transmembrane potential with anions of different membrane permeability. Choline uptake in blLPM vesicles exhibitied 1) temperature dependency; 2) trans-stimulation; and 3) saturability, with an approximate Michaelis constant (Km) of 0.34 mmol/L and maximum velocity (Vmax) 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 vesicles, including thiamine, tetraethylammonium (TEA), tri-n-butyl-methylammonium (TBuMA), and N'-methylnicotinamide (NMN). These findings demonstrate an electrogenic pathway on the sinusoidal membrane for the uptake of this essential nutrient and support the existence of multiple pathways for the sinusoidal uptake of endogenous and exogenous organic cations.