Calcitonin Increases Calcium Content and Phosphorylase a Activity in the Hepatic Particulate Glycogen of Rats

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The effect of calcitonin (CT) on hepatic glycogenolysis was investigated after a single subcutaneous administration of the hormone to intact rats. Amdinistration of CT (porcine CT; 80 MRC mU/100g 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/100g BW)-treated rats was significantly enhanced by the addition of calcium ion (10μM). Furthermore, a single intraperitoneal administration of calcium chloride (2.0 mg Ca/100g BW) to intact rats produced α 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 α activity mediated by cellular calcium.
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