Characterization of Erg K+ channels in alpha- and beta-cells of mouse and human islets.

Voltage-gated eag-related gene (Erg) K(+) channels regulate the electrical activity of many cell types. Data regarding Erg channel expression and function in electrically excitable glucagon and insulin producing cells of the pancreas is limited. In the present study Erg1 mRNA and protein were shown to be highly expressed in human and mouse islets and in alpha-TC6 and Min6 cells alpha- and beta-cell lines, respectively. Whole cell patch clamp recordings demonstrated the functional expression of Erg1 in alpha- and beta-cells, with rBeKm1, an Erg1 antagonist, blocking inward tail currents elicited by a double pulse protocol. Additionally, a small interference RNA approach targeting the kcnh2 gene (Erg1) induced a significant decrease of Erg1 inward tail current in Min6 cells. To investigate further the role of Erg channels in mouse and human islets, ratiometric Fura-2 AM Ca(2+)-imaging experiments were performed on isolated alpha- and beta-cells. Blocking Erg channels with rBeKm1 induced a transient cytoplasmic Ca(2+) increase in both alpha- and beta-cells. This resulted in an increased glucose-dependent insulin secretion, but conversely impaired glucagon secretion under low glucose conditions. Together, these data present Erg1 channels as new mediators of alpha- and beta-cell repolarization. However, antagonism of Erg1 has divergent effects in these cells; to augment glucose-dependent insulin secretion and inhibit low glucose stimulated glucagon secretion.

Growth hormone receptor regulates ß cell hyperplasia and glucose-stimulated insulin secretion in obese mice.

Insulin, growth hormone (GH), and insulin-like growth factor-1 (IGF-1) play key roles in the regulation of ß cell growth and function. Although ß cells express the GH receptor, the direct effects of GH on ß cells remain largely unknown. Here we have employed a rat insulin II promoter-driven (RIP-driven) Cre recombinase to disrupt the GH receptor in ß cells (ßGHRKO). ßGHRKO mice fed a standard chow diet exhibited impaired glucose-stimulated insulin secretion but had no changes in ß cell mass. When challenged with a high-fat diet, ßGHRKO mice showed evidence of a ß cell secretory defect, with further deterioration of glucose homeostasis indicated by their altered glucose tolerance and blunted glucose-stimulated insulin secretion. Interestingly, ßGHRKO mice were impaired in ß cell hyperplasia in response to a high-fat diet, with decreased ß cell proliferation and overall reduced ß cell mass. Therefore, GH receptor plays critical roles in glucose-stimulated insulin secretion and ß cell compensation in response to a high-fat diet.