Islet Secretory Defect in Insulin Receptor Substrate 1 Null Mice Is Linked With Reduced Calcium Signaling and Expression of Sarco(endo)plasmic Reticulum Ca²⁺-ATPase (SERCA)-2b and -3

Abstract

Mice with deletion of insulin receptor substrate (IRS)-1 (IRS-1 knockout [KO] mice) show mild insulin resistance and defective glucose-stimulated insulin secretion and reduced insulin synthesis. To further define the role of IRS-1 in islet function, we examined the insulin secretory defect in the knockouts using freshly isolated islets and primary β-cells. IRS-1 KO β-cells exhibited a significantly shorter increase in intracellular free Ca²⁺ concentration ([Ca²⁺]_i) than controls when briefly stimulated with glucose or glyceraldehyde and when l-arginine was used to potentiate the stimulatory effect of glucose. These changes were paralleled by a lower number of exocytotic events in the KO β-cells in response to the same secretagogues, indicating reduced insulin secretion. Furthermore, the normal oscillations in intracellular Ca²⁺ and O₂ consumption after glucose stimulation were dampened in freshly isolated KO islets. Semiquantitative RT-PCR showed a dramatically reduced islet expression of sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA)-2b and -3 in the mutants. These data provide evidence that IRS-1 modulation of insulin secretion is associated with Ca²⁺ signaling and expression of SERCA-2b and -3 genes in pancreatic islets and provides a direct link between insulin resistance and defective insulin secretion.