Taurolithocholate-induced Ca²⁺ release is inhibited by phorbol esters in isolated hepatocytes

The monohydroxy bile acid taurolithocholate (TLC) causes a rapid and transient increase in free cytosolic Ca2+ concentration ([Ca2+]i) in suspensions of rat hepatocytes similar to that elicited by the InsP3-dependent hormone vasopressin. The effect of the bile acid is due to a mobilization of Ca2+, independent of InsP3, from the endoplasmic reticulum (ER). Short-term preincubation of cells with the phorbol ester 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA), which activates protein kinase C (PKC), blocked the increase in [Ca2+]i induced by TLC, but did not alter that mediated by vasopressin. We obtained the following results, indicating that the effect of PMA is mediated by the activation of PKC. (1) Phorbol esters were effective over a concentration range where they activate PKC (IC50 = 0.5 nM); (2) phorbol esters that do not activate PKC did not inhibit the effects of TLC; (3) the permeant analogue oleoylacetylglycerol mimicked the inhibitory effect of PMA; (4) lastly, the inhibition of the TLC-induced Ca2+ mobilization by phorbol esters was partially prevented by preincubating the cells with the PKC inhibitors H7 and AMG-C16. Preincubating hepatocytes with PMA had no effect on the cell uptake of labelled TLC, indicating that the phorbol ester does not interfere with the transport system responsible for the accumulation of bile acids. In saponin-treated liver cells, PMA added before or after permeabilization failed to abolish TLC-induced Ca2+ release from the ER. The possibility is discussed that PMA, via PKC activation, may alter the intracellular binding or the transfer of bile acids in the liver.