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 average -30 +/- 1 mV, and extraction of 10 microM 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 microM). 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.
Fitz
J G
JG
Department of Medicine, University of California, San Francisco 94143.
Bass
N M
NM
Weisiger
R A
RA
D009327:4-Chloro-7-nitrobenzofurazan / Q000031:analogs & derivatives* / Q000378:metabolism; D000818:Animals; D001693:Biological Transport, Active; D005227:Fatty Acids / Q000378:metabolism; D008099:Liver / Q000378:metabolism* / Q000502:physiology; D008297:Male; D008564:Membrane Potentials; D008856:Microscopy, Fluorescence; D010477:Perfusion; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism; D013228:Stearates / Q000378:metabolism*
D005227:Fatty Acids; D013228:Stearates; C060027:12-N-methyl-7-nitrobenzo-2-oxa-1,3-diazolamino stearate; D012964:Sodium; D009327:4-Chloro-7-nitrobenzofurazan