Single-Channel Properties in Endoplasmic Reticulum Membrane of Recombinant Type 3 Inositol Trisphosphate Receptor

The inositol 1,4,5-trisphosphate receptor (InsP(3)R) is an intracellular Ca(2+)-release channel localized in endoplasmic reticulum (ER) with a central role in complex Ca(2+) signaling in most cell types. A family of InsP(3)Rs encoded by several genes has been identified with different primary sequences, subcellular locations, variable ratios of expression, and heteromultimer formation. This diversity suggests that cells require distinct InsP(3)Rs, but the functional correlates of this diversity are largely unknown. Lacking are single-channel recordings of the recombinant type 3 receptor (InsP(3)R-3), a widely expressed isoform also implicated in plasma membrane Ca(2+) influx and apoptosis. Here, we describe functional expression and single-channel recording of recombinant rat InsP(3)R-3 in its native membrane environment. The approach we describe suggests a novel strategy for expression and recording of recombinant ER-localized ion channels in the ER membrane. Ion permeation and channel gating properties of the rat InsP(3)R-3 are strikingly similar to those of Xenopus type 1 InsP(3)R in the same membrane. Using two different two-electrode voltage clamp protocols to examine calcium store-operated calcium influx, no difference in the magnitude of calcium influx was observed in oocytes injected with rat InsP(3)R-3 cRNA compared with control oocytes. Our results suggest that if cellular expression of multiple InsP(3)R isoforms is a mechanism to modify the temporal and spatial features of [Ca(2+)](i) signals, then it must be achieved by isoform-specific regulation or localization of various types of InsP(3)Rs that have relatively similar Ca(2+) permeation properties.