| Abstract: | The effects of glucagon and dexamethasone on the activities of the enzymes involved in cyclic adenosine 3':5'-monophosphate (cyclic AMP) metabolism in primary monolayer cell cultures of adult rat hepatocytes were examined. Short-term experiments indicated that the magnitude of the cultured cells' response to glucagon, as measured by production of cyclic AMP, was essentially the same as that for freshly isolated hepatocytes. However, the time course of this response was markedly different. Although the activity of adenylate cyclase is maintained throughout the culture period at a level similar to that of the freshly isolated hepatocytes, the activity of both low and high Km forms of phosphodiesterase decreases rapidly with length of time in vitro. This is reflected by an increase in cyclic AMP produced in response to glucagon and theophylline by cells of different ages. Dexamethasone caused an increased loss of phosphodiesterase activity, as well as increased cyclic AMP accumulation in the presence or absence of theophylline. Various agents failed to restore the lost phosphodiesterase activity. These results may indicate that phosphodiesterase activity is more sensitive to the inevitable inadequacies of the in vitro environment of cultured hepatocytes than adenylate cyclase. It was also found that a modification of the method of Seglen (1) for the preparation of isolated hepatocytes yielded cells that had less phosphodiesterase activity than those prepared by the method of Berry and Friend (2). |
