Cyclic amp stimulates fluorescent bile acid uptake into hepatocytes
by membrane hyperpolarization.
Gr[umlaut]une, Stefan, Xue-Jun Meng, and Steven A. Weinman.
Departments of Physiology & Biophysics, and Internal Medicine,
University of Texas Medical Branch, Galveston, Texas 77555
APStracts 2:0165G, 1995.
Elevation of intracellular cAMP hyperpolarizes hepatocytes and
increases the uptake rate of bile acids. The purpose of this study
was to determine to what extent these two phenomena are linked.
Fluorescent bile acid analogs (FBA) were used to probe bile acid
transport into whole-cell patch clamped hepatocytes. Na+-dependent
uptake of cholyl-NBD-lysine (C-NBD-L), an FBA with a net charge of
-1, was shown to be electrogenic, while uptake of
cholylglycylamidofluorescein (CGamF), an FBA with a net charge of -2,
was neutral. Incubation of hepatocytes with 8-Br-cAMP (100 [mu]M)
increased uptake rate of the electrogenicaly transported FBA by 25%
(p=0.002), but had no effect on uptake rate of the electroneutrally
transported FBA. Microelectrode impalements revealed that 8-Br-cAMP
or forskolin hyperpolarized hepatocytes by 6-8 mV. To determine if
hyperpolarization is responsible for the cAMP-induced increase in
uptake rate, cAMP was directly introduced into hepatocytes during
whole-cell patch clamp under voltage clamp conditions. As long as
voltage-clamp was maintained at -30 mV there was no stimulation of C
-NBD-L uptake. However, when voltage clamp was terminated by either
pipet removal or current-clamp, cAMP increased the uptake rate by 25
-34% (p<0.002). In both of these protocols, cAMP had no effect on
uptake of the electroneutrally transported FBA, CGamF. Finally, in
voltage-clamped hepatocytes in the absence of cAMP, a 10 mV
hyperpolarization increased uptake rate of C-NBD-L by 23%. We
therefore conclude that short-term cAMP-induced stimulation of
fluorescent bile acid uptake in hepatocytes is a direct consequence
of membrane hyperpolarization.
Received 7 June 1995; accepted in final form 5 August 1995.
APS Manuscript Number G243-5.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 14 August 1995.