Calcium antagonists and hormone release. IV. The role of calcium in glucose-stimulated early phase insulin release in vivo.

Extensive in vitro studies have demonstrated that an increase in the concentration of Ca2+ in the cytosol of the beta-cell of islets of Langherhans is essential for the glucose-stimulated insulin release. However, there are controversies as to whether both phases of insulin release are equally dependent upon glucose-stimulated uptake of extracellular calcium. Previous studies performed in vivo, have demonstrated an inhibitory effect of verapamil, an organic antagonist of calcium transport into cells, on the release of insulin induced by an oral glucose load. The present study was designed to investigate whether calcium antagonists are capable of inhibiting the rapid release of insulin that follows the iv infusion of glucose. Verapamil, infused into normal subjects for different periods of time before the iv administration of glucose was ineffective in inhibiting the rapid release of insulin, even when it was infused for 1 h before the glucose stimulus was applied. The present results obtained in vivo confirm some previous in vitro data showing that the first phase insulin release is not inhibited by calcium antagonist, agents known to block the uptake of calcium from extracellular sources.