Single-channel properties in endoplasmic reticulum membrane of recombinanttype 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 r ole in complex Ca2+ signaling in most cell types. A family of InsP(3)Rs enc oded by several genes has been identified with different primary sequences, subcellular locations, variable ratios of expression, and heteromultimer f ormation. This diversity suggests that cells require distinct InsP(3)Rs, bu t the functional correlates of this diversity are largely unknown. Lacking are single-channel recordings of the re combinant type 3 receptor (InsP(3)R -3), a widely expressed isoform also implicated in plasma membrane Ca2+ inf lux and apoptosis. Here, we describe functional expression and single-chann el recording of recombinant rat InsP(3)R-3 in its native membrane environme nt. The approach we describe suggests a novel strategy for expression and r ecording of recombinant ER-localized ion channels in the ER membrane. ion p ermeation and channel gating properties of the rat InsP(3)R-3 are strikingl y similar to those of Xenopus type 1 InsP(3)R in the same membrane. Using t wo 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 con trol oocytes. Our results suggest that if cellular expression of multiple I nsP(3)R isoforms is a mechanism to modify the temporal and spatial features of [Ca2+](i) signals, then it must be achieved by isoform-specific regulat ion or localization of various types of InsP(3)Rs that have relatively simi lar Ca2+ permeation properties.