1729-P: Hypertonicity Mediates Glucose-Induced First-Phase Insulin Secretion

Biphasic insulin secretion is an intrinsic characteristic of the pancreatic islet and has clinical relevance due to the loss of first phase in patients with Type 2 diabetes. As it has long been shown both in vitro and in vivo that first phase insulin secretion only occurs in response to rapid, but not gradual, changes in glucose, we tested the hypothesis that hypertonicity is driving first phase insulin secretion. The hypothesis was tested with perifusion analysis of rat. mouse and human islets by counterbalancing increases in D-glucose with decreases in L-glucose (a membrane impermeable form of glucose) so that insulin secretion rate (ISR) and other parameters associated with its regulation (NAD(P)H, calcium, cAMP production rate (cAMP PR)) could be measured in response to D-glucose in the absence of a change in tonicity. In response to a rapid rise in D-glucose, biphasic increases were observed for all 4 measured parameters. Remarkably, the appearance of the first phase component was wholly dependent on changes in tonicity: no first phase in NAD(P)H, cytosolic calcium, cAMP PR, or ISR was observed when increased D-glucose concentration was counterbalanced by decreases in either L-glucose (or another membrane-impermeable compound mannitol). Notably, rapid increases in L-glucose without any change in D-glucose resulted in first phase responses in all measured parameters that were kinetically similar to those observed in response to D-glucose. The effects of changes in osmolarity on calcium and ISR were blocked by a specific inhibitor of L-type calcium channels. Taken together, it appears that first phase insulin secretion is a result of a short-lived hypertonicity occurring during rapidly increasing glucose that enhances glucose metabolism, activation of L-type calcium channels and elevation of cAMP, that briefly augments the same mechanisms mediating second phase insulin secretion.