Hepatic transport of a fluorescent stearate derivative: electrochemical driving forces in intact rat liver

We determined the effect of varying the transmembrane Na⁺ electrochemical gradient on extraction of a fluorescent derivative of stearate, 12-N-methyl-7-nitrobenzo-2-oxa-1,3,-diazol-amino stearate (NBD-stearate), by the isolated perfused rat liver. Membrane potential difference (PD) of individual hepatocytes and extraction of NBD-stearate were measured simultaneously under basal conditions and during changes in PD induced by perfusate ion substitutions. Under basal conditions, PD averaged -30 ± 1 mV, and extraction of 10 μM NBD-stearate from 1% albumin solutions averaged 0.54 ± 0.03. Fluorescence microscopy indicated that uptake exhibited a declining portal-to-central gradient in the presence but not absence of Na⁺. Substitution of nitrate for Cl^- hyperpolarized PD to -59 mV and increased extraction to 131% of control values. Withdrawal of nitrate and substitution of gluconate for Cl^- depolarized PD to -3 and -15 mV, respectively, and decreased extraction to 63 and 73% of control values. Substitution of choline for Na⁺ eliminated the out-to-in Na⁺ gradient, depolarized PD to -16 mV, and decreased extraction to 27% of control values, an effect greater than expected for membrane depolarization alone. Uptake of NBD-stearate was saturable and caused Na⁺-dependent membrane depolarization at higher concentrations (300 μM). These studies indicate that uptake of NBD-stearate occurs in large part by an efficient Na⁺-dependent mechanism compatible with electrogenic Na⁺-fatty acid cotransport.