The effect of dibutyryladenosine 3′,5′‐monophosphate on the synthesis of bile salts in isolated hepatocytes from rat

The effect of dibutyryladenosine 3′,5′‐monophosphate (Bt 2 cAMP) on the synthesis of conjugated cholic, chenodeoxycholic and β‐muricholic acids has been investigated. Hepatocytes were incubated with 1 mM Bt 2 cAMP for 3 h at 37°C. In cells from rats with a basal rate of bile salt synthesis (soft‐diet‐fed rats) production of conjugated cholic acid was increased about two fold, synthesis of conjugated chenodeoxycholic acid was increased 10–20‐fold but formation of its metabolite, conjugated β‐muricholic acid, was decreased by 30–50% in the presence of the cyclic nucleotide. The sum of the amounts of the three bile salts produced (total bile salt synthesis) was increased 30–50% by Bt 2 cAMP. When hepatocytes were prepared from rats in which bile salt synthesis had been stimulated by feeding the bile salt sequestrant, cholestyramine, Bt 2 cAMP had no effect on conjugated cholic acid synthesis, increased conjugated chenodeoxycholic acid production 3–5‐fold and decreased conjugated β‐muricholic acid synthesis by about 50 %. Total bile salt synthesis was unchanged. The ratio of the amount of conjugated cholic acid to conjugated chenodeoxycholic acid + conjugated β‐muricholic acid produced, an indication of the activity of 7 α‐hydroxycholest‐4‐en‐3‐one 12α‐hydroxylase, was raised by Bt 2 cAMP in hepatocytes from soft‐diet‐fed but not in those from cholestyramine‐fed rats. The effects of the cyclic nucleotide on the synthesis of the three bile salts in hepatocytes from soft‐diet‐fed rats were found to be saturable at a concentration of about 2 mM. Responses were half‐maximal at concentrations of Bt 2 cAMP varying between 0.5 and 1.5 mM. These results suggest that in hepatocytes from rats with a basal rate of bile salt synthesis Bt 2 cAMP has effects at three different stages in the pathway, at the level of cholesterol 7α‐hydroxylase, 7α‐hydroxycholest‐4‐en‐3‐one 12α‐hydroxylase and chenodeoxycholine acid 6β‐hydroxylase. In cells from rats in which bile salt synthesis has been stimulated only the effect at the chenodeoxycholic acid 6β‐hydroxylase level is apparent. Bt 2 cGMP and Bt 2 cCMP had no effect on the synthesis of any of the bile salts measure, showing that the effects are specific for Bt 2 cAMP. The ratio of the amounts of the three bile salts found inside the cells to those found in the medium was decreased by about 90 % when Bt 2 cAMP was present in the hepatocyte incubations. This effect was mimicked by Bt 2 cGMP and to a lesser exent by Bt 2 cCMP. These results indicate that Bt 2 cAMP has an effect on the uptake and/or secretion of bile salts in the hepatocytes.