Glucose‐induced first phase insulin release in the absence of extracellular Ca 2+ in rat islets

Islets of Langerhans respond to a rapid and constant increase of the glucose concentration with biphasic release of insulin, a pattern that is observed in the portal vein in humans [l ] and in isolated pancreatic preparations [2-41. The presence of extracellular Ca” has been thought essential for both phases of insulin release [2,5,6]. However, recent studies have suggested that the first phase is dependent upon the utilization of cellular calcium, whereas the second phase depends both on cellular calcium and increased uptake of Ca2+ from the extracellular fluid [3,7], Those experiments were performed using either inhibitors of Ca2* uptake [3] or islets with increased calcium stores [7]. In islets with normal calcium stores, a preserved first phase insulin release in the absence of extracellular Ca2+ has not been demonstrated. This study was performed to examine the differential sensitivity of the two phases to Cap deprivation. It was found that first phase insulin release could occur in the absence of extracellular Ca2+, when Ca2* was removed from the medium before the glucose-induced rise of insulin release.