Kir2.1 Channels Compensate for the Loss of KATP Channels in SUR1 Null Islets

Abstract

Pancreatic islets of SUR1-/- mice exhibit electrical oscillations even though they lack KATP channels. However, while we confirmed that electrical oscillations persisted in freshly isolated SUR1-/- islets, after overnight culture in media containing 11.1 mM glucose, SUR1-/- islets exhibited continuous spiking in place of oscillations, as has been observed in human congenital hyperinsulinemia (CHI). Microarray analysis suggested upregulation of kcnj2, the gene encoding the inward rectifier Kir2.1, in SUR1-/- islets. To test whether Kir2.1 was upregulated in SUR1-/- islets islet lysates were immunoblotted for Kir2.1 protein. Kir2.1 was expressed in mouse and human islets and increased in lysates from freshly isolated SUR1 islets. Using patch clamping, we found that an inwardly rectifying K current was present in freshly isolated SUR1-/- islets that was absent in wild type islets. This current was sensitive to the Kir2.1 inhibitors ML133 and barium. Notably, rectifying current, enhanced Kir2.1 protein levels, and the expected Kir2.1 drug sensitivity were all lost along with oscillations after overnight culture in media with 11.1 mM glucose. Similar results were found using fura-2 loaded islets to examine Ca2+ oscillations. To test whether the suppression of Kir2.1 activity in cultured islets was a response to culture in high glucose, islets were cultured in 5 mM instead of 11.1 mM glucose. Lowering glucose preserved Kir2.1 protein and maintained the islet oscillations. These results show that the loss of KATP channels produced by deleting SUR1 is at least partially compensated for by the upregulation of Kir2.1 channels in mouse beta cells. We propose pharmacological activation of compensatory Kir2.1 channels as a novel way to inhibit the unrestrained insulin secretion characteristic of beta cells from human CHI patients.