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Hormonal regulation of hepatic glycogenolysis inAmphibolurus nuchalis, the western netted dragon: an in vitro study

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In mammals hepatic glycogenolysis is controlled by several hormones using cyclicAMP, Ca2+ and/or diacylglycerol as intracellular messengers. In contrast, in teleost fish, lungfish and amphibians fewer hormones promote hepatic glycogenolysis, and cyclicAMP is the sole intra-cellular messenger. This suggests that the α-adrenergic mechanism became associated with the liver after amphibians separated from the vertebrate line. Reptiles separated later, and the aim of this study is to elucidate the hormonal control of hepatic glycogenolysis in a reptile,Amphibolurus nuchalis, and especially to determine which adrenergic receptor system is operative.

InA. nuchalis liver pieces cultured in vitro, adrenaline and glucagon stimulated glycogen breakdown and glucose release, glycogen phosphorylase activity and accumulation of cyclicAMP in the tissue. Neurohypophysial peptides did not affect these parameters. These actions of adrenaline were completely blocked by the β-adrenergic antagonist, propranolol and slightly reduced by the α-adrenergic antagonist, phentolamine. Removal of Ca2+ from the medium and addition of the Ca2+ chelator, EGTA, did not block the actions of adrenaline, and the Ca2+ ionophore A23187 did not mimic these actions.

The β-adrenegic ligand [125I]-iodocyanopindolol (ICP) bound specifically to an isolated membrane preparation fromA. nuchalis liver with a calculated KD of 100 pM and a Bmax of 37.6 fmol·mg protein−1. The adrenergic ligands propranolol, isoprenaline, adrenaline, noradrenaline, phenylephrine and phentolamine displaced ICP with KD's of 20 nM, 1 μM, 4.5 μM, 32 μM, 35 μM and 500 μM, respectively. The α2-adrenergic ligand yohimbine did not bind specifically to the membrane, but at 1 nM and 100 pM, specific binding of the α1-adrenergic ligand prazosin was 45% of total with a mean of 11.3 fmoles·mg protein−1 specifically bound.

These findings indicate that the glycogenolytic actions of adrenaline are mediated primarily via β-adrenergic receptors inA. nuchalis, but that α-adrenergic receptors may also play some role in the control of hepatic metabolism.

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Janssens, P.A., Giuliano, M. Hormonal regulation of hepatic glycogenolysis inAmphibolurus nuchalis, the western netted dragon: an in vitro study. J Comp Physiol B 159, 323–331 (1989).

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Key words

  • Glycogenolysis
  • Cyclic AMP
  • Antagonist
  • Adrenergic
  • Phosphorylase