Vasopressin and angiotensin are able to lower the glucagon-induced increase of cyclic AMP levels in isolated hepatocytes. Results presented are in favour of an enhanced phosphodiesterase activity to account for this cyclic AMP lowering effect. In particular, vasopressin prevents exogenous cyclic AMP from activating glycogen phosphorylase: in the presence of phosphodiesterase inhibitors, the hormone becomes unable to decrease glucagon-induced cyclic AMP levels. This anti-glucagon effect of vasopressin and angiotensin might be physiologically more important than their glycogenolytic effect; indeed, the latter is very transient in nature and, in addition, requires higher hormone concentrations [Bréant, Keppens & De Wulf (1981) Biochem. J. 200, 509-514] than those needed for the anti-glucagon effect, as reported here.

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