Hormones regulating hepatic glycogenolysis in two chelonians use cyclic AMP, and not Ca2+, as intracellular messenger

In teleosts, lungfish, amphibians, and a reptile, Amphibolurus nuchalis, hormonal stimulation of hepatic glycogenolysis is mediated by a rise in intracellular cyclic AMP concentration. In mammals, by contrast, the inositol trisphosphate/Ca2+/diacylglycerol signal transduction pathways are also involved. The present study describes the hormonal regulation of hepatic glycogenolysis in adult long-necked turtles, Chelodina longicollis, and hatchlings of the loggerhead turtle, Caretta caretta. Adrenaline and glucagon, but not neurohypophysial peptides, stimulated glycogenolysis, glycogen phosphorylase activity, and accumulation of cAMP in cultured liver pieces from either C. longicollis or C. caretta. The actions of adrenaline were blocked by a beta-adrenergic antagonist, propranolol, but were unaffected by an alpha-adrenergic antagonist, phentolamine. The effects of adrenaline were maintained in Ca(2+)-free medium containing EGTA, and were not mimicked by the Ca2+ ionophore, A23187. The beta-adrenergic ligand, [125I]iodocyanopindolol (ICP), specifically bound to membranes prepared from C. longicollis liver, with a calculated KD of 59 pM and a Bmax of 171 fmol/mg protein. The adrenergic ligands, propranolol, isoprenaline, adrenaline, phenylephrine, phenoxybenzamine, noradrenaline, and phentolamine displaced ICP with KD's of 50 nM, 5 microM, 22 microM, 140 microM, 180 microM, 250 microM, and 1 mM, respectively. The alpha-adrenergic ligands, prazosin and yohimbine, did not bind specifically to the membranes, although prazosin did bind to membranes prepared similarly from rat liver. Thus the glycogenolytic actions of adrenaline are mediated via beta-adrenergic receptors in liver from C. longicollis and C. caretta and alpha-adrenergic receptors may play no role in the control of hepatic metabolism in these chelonians.