Class A calcium channel variants in pancreatic islets and their role in insulin secretion

Abstract

The initiation of insulin release from rat islet β cells relies, in large part, on calcium influx through dihydropyridine-sensitive (α(1D)) voltage-gated calcium channels. Components of calcium-dependent insulin secretion and whole cell calcium current, however, are resistant to L-type channel blockade, as well as to ω-conotoxin GVIA, a potent inhibitor of α(1B) channels, suggesting the expression of additional exocytotic calcium channels in the islet. We used a reverse transcription-polymerase chain reaction-based strategy to ascertain at the molecular level whether the α(1A) calcium channel isoform was also present. Results revealed two new variants of the rat brain α(1A) channel in the islet with divergence in a putative extracellular domain and in the carboxyl terminus. Using antibodies and cRNA probes specific for α(1A) channels, we found that the majority of cells in rat pancreatic islets were labeled, indicating expression of the α(1A) channels in β cells, the predominant islet cell type. Electrophysiologic recording from isolated islet cells demonstrated that the dihydropyridine-resistant current was sensitive to the α(1A) channel blocker, ω-agatoxin IVA. This toxin also inhibited the dihydropyridine- resistant component of glucose-stimulated insulin secretion, suggesting functional overlap among calcium channel classes. These findings confirm the presence of multiple high voltage-activated calcium channels in the rat islet and implicate a physiologic role for α(1A) channels in excitation-secretion coupling in β cells.