AJP - Gastrointestinal and Liver Physiology, Vol 249, Issue 6 711-G718, Copyright © 1985 by American Physiological Society
Role of extracellular Ca2+ in hepatic bile formation and taurocholate transport
M. S. Anwer and L. M. Clayton
The role of extracellular Ca2+ in hepatic bile formation, biliary membrane
permeability, and taurocholate (TC) transport was studied in isolated
perfused rat livers and in isolated rat hepatocytes to determine the
functional importance of paracellular permeability in biliary bile acid
excretion. Each liver was perfused for 1 h with perfusate containing 1.3 mM
Ca2+ (control period) followed by another hour with 1.3, 0.5, 0.1, 0.05,
0.03, or 0.01 mM Ca2+ (experimental period). Basal bile flow and biliary
excretion of added TC declined significantly only at and below 0.05 mM
perfusate Ca2+ and was associated with an increase in bile-to-perfusate
concentration ratio of [3H]inulin (B/P inulin ratio). A twofold increase in
the diffusional permeability coefficient at 0.05 mM and a sixfold increase
at 0.03 and 0.01 mM perfusate Ca2+ could explain the increased in B/P
inulin ratios. Time-dependent increases in cell-to-medium concentration
ratios of inulin were less in the absence than in the presence of Ca2+.
Hepatic uptake rates of TC determined in isolated hepatocytes and from
perfusate disappearance of added TC and efflux rates of TC from preloaded
hepatocytes were not significantly affected by Ca2+ removal. It is possible
that the observed decline in biliary TC excretion at low perfusate Ca2+ is
due to regurgitation of secreted TC back into the perfusate followed by
reuptake. This was supported by an accumulation of perfusate radioactivity
when TC uptake inhibitors (furosemide and bumetanide) were added to the
perfusate (0.03 mM Ca2+) 60 min after the addition of [14C]TC.(ABSTRACT
TRUNCATED AT 250 WORDS)
