Cholic Acid Uptake into Isolated Rat Hepatocytes

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Abstract
Cholic acid uptake was studied in isolated rat hepatocytes using a centrifugal filtration technique to allow rapid sampling. Hepatocytes were found to absorb and transport cholic acid. The adsorption was characterized by a capacity of 24 nmol .times. mg cell protein-1 and an association constant of 0.59 .times. 103 M-1. Cholic acid uptake was linear with respect to concentration at or below 10.degree. C, suggesting a unsaturable uptake process which was considered to represent simple diffusion and is quantitated by a diffusion coefficient of 1.76 pmol cholic acid .times. min-1 .times. mg protein-1 .times. .mu.M-1. Above 10.degree. C the uptake curve was biphasic. After subtracting the unsaturable component from uptake rates at higher temperatures, a curve showing saturable kinetics resulted. The apparent Km and V values at 37.degree. C were calculated to be 61 .mu.M and 0.8 nmol .times. min-1 .times. mg protein-1, respectively. This saturable uptake process was temperature-dependent with an activation energy of 13 kcal .times. mol-1 (5.44 .times. 104 J .times. mol-1) and was inhibited by oligomycin and KCN. Countertransport was demonstrated with cholic, taurocholic and chenodeoxycholic acids. Cholic acid is apparently transported by an energy-dependent carrier-mediated process in addition to simple diffusion by hepatocytes, and the postulated carrier has affinity for other bile acids.