The effect of dibutyryl cyclic AMP on the uptake of taurocholic acid by isolated rat liver cells.

The effect of dibutyryl cyclic AMP on the uptake of taurocholic acid by isolated rat hepatocytes was studied. In the presence of low levels (10-100 microM) of the cyclic nucleotide the initial rate of uptake was increased significantly, with a peak occurring at about 20 microM. In contrast, concentrations of dibutyryl cyclic AMP between 200 microM and 1 mM caused a significant decrease in the initial rate of uptake of the bile acid by the cells. Sodium-dependent transport of taurocholic acid was found to be enhanced by 20 microM dibutyryl cyclic AMP, but sodium-independent uptake appeared to be unaffected. Inhibition by 1 mM dibutyryl cyclic AMP, however, was found to occur in both the sodium-dependent and -independent components of the transport system. The initial rate of taurocholic acid uptake in hepatocytes incubated with 1.2 mM extracellular calcium was increased compared to that in calcium-depleted cells, and this increase was entirely due to enhanced sodium-dependent transport. 1.2 mM calcium and 20 microM dibutyryl cyclic AMP together did not stimulate the uptake rate to a greater extent than either treatment alone. It is concluded that calcium and low levels of dibutyryl cyclic AMP alter the rate of taurocholic acid uptake by changing the flux of sodium in the hepatocytes. The inhibitory effect of 1 mM dibutyryl cyclic AMP was not relieved by the presence of 1.2 mM calcium in the cell incubation medium. The results show that dibutyryl cyclic AMP can affect the rate of transport of bile acid into liver cells, and suggest a possible regulatory role for cyclic AMP in this process.