Effect of Calcium on the Secretion of Somatostatin and Insulin from Pancreatic Islets*

Calcium ionophore A23187 (20 micrometer) evoked the secretion of somatostatin (SRIF) as well as insulin from isolated rat pancreatic islets in a medium containing a relatively low concentration of calcium (0.9 mM) and a low concentration of glucose (5.5 mM). A high level of extracellular calcium (7.5 mM) also had a stimulatory effect on SRIF and insulin release. On the other hand, in the presence of high glucose (16.7 mM), A23187 had different effects on D and B cells; insulin release was markedly suppressed by A23187, but SRIF secretion was significantly enhanced. A high concentration of glucose (16.7 mM) did not stimulate SRIF secretion at low extracellular calcium concentration (0.25 mM), at which level insulin release is significantly enhanced. These findings indicate that calcium may play an important role in the regulation of the secretion of SRIF as well as insulin and suggest that the B and D cells differ in their sensitivity to the calcium ion.