CLASS-A CALCIUM-CHANNEL VARIANTS IN PANCREATIC-ISLETS AND THEIR ROLE IN INSULIN-SECRETION

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