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Original Article

^a From the Department of Physiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104
^b From the Institute for Human Gene Therapy, University of Pennsylvania, Philadelphia, Pennsylvania 19104
^c Department of Pathology, Thomas Jefferson University School of Medicine, Philadelphia, Pennsylvania 19107
Department of Physiology, University of Pennsylvania, B400 Richards Building, Philadelphia, PA 19104.215-573-6808

foskett{at}mail.med.upenn.edu

The inositol 1,4,5-trisphosphate receptor (InsP₃R) is an intracellular Ca²⁺-release channel localized in endoplasmic reticulum (ER) with a central role in complex Ca²⁺ signaling in most cell types. A family of InsP₃Rs 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 InsP₃Rs, but the functional correlates of this diversity are largely unknown. Lacking are single-channel recordings of the recombinant type 3 receptor (InsP₃R-3), a widely expressed isoform also implicated in plasma membrane Ca²⁺ influx and apoptosis. Here, we describe functional expression and single-channel recording of recombinant rat InsP₃R-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 InsP₃R-3 are strikingly similar to those of Xenopus type 1 InsP₃R 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 InsP₃R-3 cRNA compared with control oocytes. Our results suggest that if cellular expression of multiple InsP₃R isoforms is a mechanism to modify the temporal and spatial features of [Ca²⁺]_i signals, then it must be achieved by isoform-specific regulation or localization of various types of InsP₃Rs that have relatively similar Ca²⁺ permeation properties.

Key Words: calcium • calcium release channel • electrophysiology • expression • Xenopus

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