Hormonal regulation of hepatic glycogenolysis in the toad, Xenopus laevis, is mediated by cyclic AMP and not Ca2+.
Hepatic glycogenolysis and glycogen phosphorylase a activity were stimulated by arginine vasotocin (AVT) in liver pieces from Xenopus laevis cultured in vitro. In each case, the EC50 was about l nM. The increased rate of glycogenolysis brought about by either AVT or adrenaline was maintained for at least 6 hr and was unchanged when Ca2+ salts were omitted from the medium or when 2.5 mM EGTA was added. Neither the Ca2+ ionophore, A23187, nor the Ca2+ channel blocker, verapamil, had any effect on the rate of glycogenolysis in the presence or the absence of either hormone. Tissue cyclic AMP levels were unchanged by addition of AVT alone but were doubled in the presence of AVT plus either of the phosphodiesterase inhibitors, isobutylmethylxanthine or RO20-1724. These findings suggest that hormones regulating hepatic glycogenolysis in X. laevis use cyclic AMP, and not Ca2+, as an intracellular messenger. We would argue that cytosolic Ca2+ may not have become involved in regulation of hepatic glycogenolysis until after the ancestors of present day amphibians separated from those of present day mammals.
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000242:Cyclic AMP / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism; D066298:In Vitro Techniques; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008112:Liver Glycogen / Q000378:metabolism*; D010762:Phosphorylase a / Q000378:metabolism; D014668:Vasotocin / Q000494:pharmacology*; D014700:Verapamil / Q000494:pharmacology; D014982:Xenopus laevis / Q000378:metabolism*
D008112:Liver Glycogen; D000001:Calcimycin; D014700:Verapamil; D000242:Cyclic AMP; D010762:Phosphorylase a; D005947:Glucose; D002118:Calcium; D014668:Vasotocin