Calcitonin, a diabetogenic hormone?

The effect of calcitonin on oral glucose tolerance and on insulin, C-peptide, glucagon, and GH secretion has been investigated in man. Eight subjects with normal glucose tolerance and eight with impaired glucose tolerance (IGT) were studied. Each subject received two oral glucose tolerance tests (100 g) in random order, one under basal conditions and the other during the simultaneous in administration of salmon calcitonin (100 Medical Research Council Units). In all subjects, calcitonin exaggerated the rise in plasma glucose after oral sugar. The integrated areas under the plasma glucose curves were 4,400 +/- 840 mg/dl.min (normals) and 8,708 +/- 1,840 mg/dl.min (IGT) without calcitonin, and 8,208 +/- 1,700 mg/dl. min (normals) and 19,500 +/- 3,500 mg/dl. min (IGT) with calcitonin (P less than 0.01). Plasma insulin and C-peptide responses to glucose were significantly reduced (P less than 0.01) by calcitonin at all times after the start of the test in both normal and IGT groups. The inhibitory action of oral glucose on glucagon secretion was partially prevented by calcitonin (P less than 0.01). Moreover, calcitonin completely blunted the GH rebound occurring at the end of the test. These findings demonstrate that calcitonin impairs glucose tolerance in man by both inhibiting glucose-induced insulin secretion (primary effect) and reducing glucose-mediated glucagon suppression (accessory effect). These effects of calcitonin could be explained by a decrease in the cytosolic Ca2+ concentration in both alpha- and beta-cells.