Calcitonin gene-related peptide: occurrence in pancreatic islets in the mouse and the rat and inhibition of insulin secretion in the mouse.

The intrapancreatic cellular distribution and effects on basal and stimulated insulin secretion of the 37-amino-acid polypeptide, calcitonin gene-related peptide (CGRP), were investigated in the mouse. The cellular localization of CGRP was also studied in the rat pancreas. In both species, CGRP was demonstrated in pancreatic islet cells and nerve fibers. Immunocytochemical double staining experiments revealed the CGRP-immunoreactive cells in the mouse to be identical with a majority population of the insulin cells. In the rat, on the other hand, CGRP-immunoreactive cells were identical with somatostatin cells. CGRP-immunoreactive nerve fibers were observed, in both species, running in the exocrine parenchyma, particularly around blood vessels, and they were occasionally seen also within the islets. In in vivo experiments, CGRP was found to inhibit both basal and stimulated insulin secretion in the mouse. Thus, 6 min after the iv injection of CGRP (0.85 nmol/kg), plasma insulin levels were 13 +/- 2 (SE) microU/ml compared to 30 +/- 4 microU/ml in controls (P less than 0.01). At this dose level, CGRP inhibited the insulin secretory response to carbachol, leaving that to glucose unaffected. However, at a higher dose level (4.25 nmol/kg), CGRP inhibited glucose-induced insulin secretion as well. We conclude that CGRP occurs in islet cells and in intrapancreatic nerve fibers of both the mouse and the rat, and inhibits both basal and stimulated insulin secretion in vivo in the mouse.