AJP - Gastrointestinal and Liver Physiology, Vol 264, Issue 2 220-G230, Copyright © 1993 by American Physiological Society
Electroneutral uptake and electrogenic secretion of a fluorescent bile salt by rat hepatocyte couplets
S. A. Weinman, J. Graf, C. Veith and J. L. Boyer
Department of Medicine and Liver Center, Yale University School of Medicine, New Haven, Connecticut 06514.
The role of membrane voltage as a driving force for the hepatic uptake and
secretion of fluorescent bile salts has been examined in isolated
hepatocyte couplets. The present study demonstrates that the fluorescent
bile salt derivative (N-[7-(nitrobenz-2-oxa- 1,3-diazol-4-yl)]-7-amino-3
alpha, 12 alpha-dihydroxy-5-cholan-24-oyl)-2-aminoethanesulfonate (7
beta-NBD-NCT) is taken up into hepatocytes by a saturable process with a Kt
of 2.7 microM. Uptake rate was reduced by only 22% after total Na+
replacement and was independent of transmembrane potential difference over
a range of -135 to +25 mV. In contrast, secretion into the canalicular
space was strongly dependent on membrane voltage over the range from -34 to
0 mV in a manner consistent with electrodiffusion of an anion. Fitting the
secretion time course to that predicted by electrodiffusion demonstrated
that only approximately 50% of total secretion can result from
electrodiffusion. Studies in isolated perfused liver confirmed this
observation that depolarization caused a decrease in bile salt secretion
rate. These results demonstrate that 7 beta-NBD-NCT is transported by a
neutral uptake process at the sinusoidal membrane and is secreted across
the canalicular membrane in part by electrogenic transport. This suggests
that voltage changes could be a common pathway resulting in impaired
organic anion secretion in diverse cholestatic syndromes.
