Stimulatory release of hepatic lipase activity from cultured rat hepatocytes by sodium orthovanadate: rapid increase in cyclic adenosine monophosphate content.

The release of hepatic triacylglyceride lipase [EC 3.1.1.3] has been examined in isolated hepatocytes in primary culture. The stimulatory release of activity from the hepatocytes into the medium by sodium orthovanadate (vanadate) was observed in a time- and dose-dependent manner. However, insulin failed to have this stimulatory action. Moreover, vanadate rapidly increased the cyclic adenosine monophosphate (cyclic AMP) content in hepatocytes in a time- and dose-dependent manner. The treatment of hepatocytes with H-89, which is a potent cyclic AMP-dependent protein kinase inhibitor, decreased the stimulatory release of hepatic lipase activity by vanadate. The vanadate-stimulated release of the enzyme activity was suppressed by uncouplers. In addition, the incorporation of [3H]leucine into protein was increased in the presence of vanadate. Under the marked inhibition of protein synthesis by cycloheximide, vanadate still showed a full effect on the release of the enzyme activity. These results suggest that the vanadate-stimulated release of hepatic lipase activity from the cultured hepatocytes is associated with a rapid increase in intracellular cyclic AMP content, probably due to an activation of cyclic AMP-dependent protein kinase which requires a metabolic energy process rather than an elevation in enzyme molecule synthesis.