Voltage-dependent InsP_3- insensitive calcium channels in membranes of pancreatic endoplasmic reticulum vesicles

Abstract

STIMULUS–SECRETION coupling in exocrine glands involves Ca²⁺ release from intracellular stores^1,2. In endoplasmic reticulum vesicle preparations from rat exocrine pancreas, an inositol 1,4,5-trisphosphate(InsP₃)-sensitive, as well as an InsP₃-insensitive, Ca²⁺ pool has been characterized³. But Ca²⁺ channels in the endoplasmic reticulum of rat exocrine pancreas have not been demonstrated at the level of single-channel current. We have now used the patch-clamp technique on endoplasmic reticulum vesicles fused by means of the dehydration–rehydration method^4–6. In excised patches, single Ba²⁺- and Ca²⁺-selective channels were recorded. The channel activity was markedly voltage-dependent. Caffeine increased channel open-state probability, whereas ruthenium red and Cd²⁺ blocked single-channel currents. Ryanodine, nifedipine and heparin had no effect on channel activity. The channel activity was not dependent on the free Ca²⁺ concentration, the presence of InsP₃, or pH. We conclude that this calcium channel mediates Ca²⁺ release from an intracellular store through an InsP₃-insensitive mechanism.