Hormones regulating hepatic glycogenolysis in two chelonians use cyclic AMP, and not Ca 2+, as intracellular messenger

Abstract

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/Ca 2+/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 turtles, 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 β-adrenergic antagonist, propranolol, but were unaffected by an α-adrenergic antagonist, phentolamine. The effects of adrenaline were maintained in Ca 2+-free medium containing EGTA, and were not mimicked by the Ca 2+ ionophore, A23187. The β-adrenergic ligand, [ 125I]iodocyanopindolol (ICP), specifically bound to membranes prepared from C. longicollis liver, with a calculated K D of 59 p M and a B max of 171 fmol/mg protein. The adrenergic ligands, propranolol, isoprenaline, adrenaline, phenylephrine, phenoxybenzamine, noradrenaline, and phentolamine displaced ICP with K D's of 50 n M, 5 μ M, 22 μ M, 140 μ M, 180 μ M, 250 μ M, and 1 m M, respectively. The α-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 β-adrenergic receptors in liver from C. longicollis and C. caretta and α-adrenergic receptors may play no role in the control of hepatic metabolism in these chelonians.