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

The effect of calcitonin (CT) on hepatic glycogenolysis was investigated after a single subcutaneous administration of the hormone to intact rats. Administration of CT (porcine CT; 80 MRC mU/100 g BW) produced a significant decrease in glycogen content of the liver, and corresponding increases in phosphorylase a activity and calcium content of the particulate glycogen fraction in the liver. These alterations were observed with the dose of CT at physiological level. The removal of calcium by 1 mM EGTA treatment of the hepatic particulate glycogen caused a clear reduction in the increase in phosphorylase a activity produced by CT administration. Meanwhile, 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 calcium ion (10 microM). Furthermore, a single intraperitoneal administration of calcium chloride (2.0 mg Ca/100 g BW) to intact rats produced a remarkable increase in phosphorylase a activity and calcium content of the hepatic glycogen particulate fraction. These results suggest that the promotion of hepatic glycogenolysis by CT administration may result from the increase in phosphorylase a activity mediated by cellular calcium.