Inhibitory effect of neurohypophysial peptides on cyclic AMP accumulation by isolated hepatocytes of the trout.

Cyclic AMP levels were measured in freshly isolated hepatocytes of the rainbow trout. Compared with basal values, the average levels were increased up to 60 times in a dose-dependent manner either by mammalian glucagon (concentration range 1 nmol-1 mumol/l; dose giving half maximum response (EC50) 0.18 mumol/l) or by forskolin (concentration range 0.1-100 mumol/l; EC50 about 10 mumol/l). These stimulatory effects were partially inhibited by fish or mammalian neurohypophysial hormones used at relatively high concentrations (1-5 mumol/l). It is suggested that these results are evidence for the presence of V1-type receptors in fish hepatocytes. Together with previous results obtained with gills on the hormonal inhibition of adenylate cyclase activity, they suggest that teleost fish may possess only V1-type receptors (or two V1-related types), while the V2 receptors have evolved (or have become functional) in higher vertebrates.