A Novel Hyperpolarization-Activated Current Identified in Pancreatic β Cells Modulates Insulin Secretion
Abstract Number: 1639-P
Authors: WASIM EL-KHOLY, TIAN XUE, JINGYU DIAO, HEE C. CHO, CATHERINE B. CHAN, PATRICK E. MACDONALD, ROBERT G. TSUSHIMA, RONALD A. LI, MICHAEL B. WHEELER
Institutions: Toronto, ON, Canada; Baltimore, MD; Charlottetown, PE, Canada
Results: Hyperpolarization-activated cyclic-nucleotide-modulated (HCN1-4) channels mediate the pacemaker current (Ih or If) in electrically rhythmic cardiac and neuronal cells. In pancreatic β cells, it has been demonstrated that membrane potential oscillations play an important role in glucose-stimulated insulin secretion, although the underlying mechanisms are poorly defined. Here we demonstrate the presence of a hyperpolarization-activated time-dependent cationic current, β-Ih, in pancreatic β cells. Using RT-PCR, transcripts for HCN1-4 were detected in human, rat and mouse islets as well as in MIN6 and INS-1 cells, while HCN4 was shown to be present in rat β cells by immunohistochemistry. β-Ih measured in MIN6 and rat β cells displayed biophysical and pharmacological properties similar to those of the cardiac and neuronal Ih counterparts. β-Ih was a sustained current of 45 pA/pF at -140mV, with was half-maximally activated at -99mV. β-Ih had an activation tau of approximately 250msec at -140mV. Inhibition of β-Ih with 50 µM ZD7288, an established pharmacological inhibitor of HCN channels, caused a 57% and 52% reduction in MIN6 and rat β cells, respectively. Cesium, another known inhibitor of HCN channels, reduced β-Ih in MIN6 (2mM) and rat β cells (5mM) by 82% and 78%, respectively. Expression of an HCN family dominant negative channel (HCN1-GYG349-352AAA) caused a 92% reduction in sustained MIN6 β-Ih. Membrane potential recordings in MIN6 cells demonstrated that inhibition of β-Ih with ZD7288 or HCN1-GYG349-352AAA caused a significant reduction in glucose-stimulated action potential activity. To assess a role of β-Ih on β cell function, rat islets and MIN6 cells were treated with 50µM ZD7288, which inhibited glucose-stimulated insulin secretion by 65% and 59%, respectively. These findings provide novel insights into the electrogenic modulation of insulin secretion, and may lead to new therapies for type 2 diabetes.