Calcitonin increases calcium content and phosphorylase a activity in the hepatic particulate glycogen of rats.

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Abstract
The effect of calcitonin (CT) on hepatic glycogenolysis was investigated after a single s.c. administration of the hormone to intact rats. Administration of CT (porcine CT; 80 MRC [mouse radius calcifying] mU/100 g BW [body wt]) produced a significant decrease in glycogen content of the liver and corresponding increases in phosphorylase a activity and Ca content of the particulate glycogen fraction in the liver. These alterations were observed with the dose of CT at physiological level. The removal of Ca by 1 mM EGTA [ethyleneglycol bis(.beta.-aminoethyl ether)N,N,N'',N''-tetraacetic acid] treatment of the hepatic particulate glycogen caused a clear reduction in the increase in phosphorylase a activity produced by CT administration. The enzyme activity in 1 mM EGTA-treated particulate glycogen of the liver in both control and CT (80 MRC mU/100 g BW)-treated rats was significantly, enhanced by the addition of Ca ion (10 .mu.M). A single i.p. administration of CaCl2 (2.0 mg Ca/100 g BW) to intact rats produced a remarkable increase in phosphorylase a activity and Ca content of the hepatic glycogen particulate fraction. Apparently the promotion of hepatic glycogenolysis by CT administration may result from the increase in phosphorylase a activity mediated by cellular Ca.