AJP - Gastrointestinal and Liver Physiology, Vol 271, Issue 6 1084-G1095, Copyright © 1996 by American Physiological Society
Regulation of taurocholate and ursodeoxycholate uptake in hamster hepatocytes by Ca(2+)-mobilizing agents
B. Bouscarel, S. Reza, L. A. Dougherty, H. Fromm and R. Nussbaum
Department of Medicine, George Washington University Medical Center, Washington, District of Columbia 20037, USA.
In isolated hamster hepatocytes, the Ca2+ ionophore A-23187 immediately
decreased the uptake rate of taurocholic acid (TCA) by 60-70%, whereas it
slowly inhibited that of ursodeoxycholic acid (UDCA) by a maximum of
35-45%, with an inhibition constant (Ki) of 0.36 and 1.93 microM,
respectively. In contrast to ionomycin, which mimicked the effect of
A-23187, vasopressin inhibited the bile acid uptake rate by 40 and 45%,
respectively, only after a 5- to 10-min preincubation. The Na(+)-dependent
bile acid transport was exclusively inhibited by these agents, and this
inhibition was independent of extracellular Ca2+. However, intracellular
Ca2+ depletion with ethylene glycol-bis(beta-aminoethyl
ether)-N,N,N',N'-tetraacetic acid or chelation with
1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid resulted in 40-50%
inhibition of the uptake rate of both bile acids. The exogenous protein
kinase C activator, phorbol 12-myristate 13-acetate (PMA), but not the
nonactive 4 alpha-phorbol, significantly inhibited TCA uptake rate.
Although both A-23187 and ionomycin immediately increased and decreased the
cellular Na+ and K+ concentration, respectively, neither vasopressin nor
PMA had a significant effect on the cellular concentration of these
cations, even after a 10-min incubation. Furthermore, the effect of A-23187
and ionomycin on TCA uptake and Na+ flux, respectively, disappeared after a
40-min preincubation, and additional ionophore remained without effect.
However, after a 40-min incubation with A-23187, PMA was still able to
inhibit TCA uptake. Therefore, A-23187 and ionomycin transiently inhibited
Na(+)-dependent uptake of both TCA and UDCA, in part because of transient
alteration of the cellular Na+ and K+ concentration. Vasopressin and PMA
inhibited Na(+)-dependent bile acid uptake, at least in part, through
protein kinase C activation.
