Biphasic Insulin Release in Rat Islets of Langerhans and the Role of Intracellular Ca++Stores*

Abstract

The role of extracellular Ca⁺⁺ in the development of biphasic insulin release has been studied previously by using Verapamil to block the glucose-induced stimulation of Ca⁺⁺ uptake. It was found that glucose-stimulated uptake of Ca⁺⁺ from the extracellular fluid was important for the second phase of insulin release but had no role in the first phase. In the current study, the role of intracellular Ca⁺⁺ stores in biphasic insulin release has been studied. Isolated rat islets were maintained in tissue culture for 46 h with normal (1 HIM) or high (5 RIM) phosphate concentrations. The intracellular calcium content of islets maintained in high phosphate was 50 times greater than the content of control islets. Insulin release was studied by perifusion, and the Ca⁺⁺ concentration in the medium was decreased to 0.1 mM during stimulation with high glucose to minimize the effect of extracellular Ca⁺⁺. The islets maintained in normal phosphate responded with a transient subnormal first phase insulin release. In contrast, 6.7 mM glucose elicited a 3.5-fold greater first phase release from the Ca⁺⁺-loaded islets. This release was quantitatively similar to the first phase response of islets perifused with 1 mM Ca⁺⁺ throughout. Ca⁺⁺ loading also increased the rate of second phase insulin release, but this was less marked than that observed for the first phase. It is concluded that the handling of intracellular Ca⁺⁺ is primarily responsible for the first phase of insulin release, whereas both intracellular Ca⁺⁺ and increased uptake of Ca⁺⁺ from the extracellular compartment are involved in the genesis and development of the second phase.