Inositol 1,4,5-trisphosphate-gated channels in cerebellum: Presence of multiple conductance states

The mechanism by which inositol 1,4,5-triphosphate (InsP₃) induces calcium (Ca) release from the reticulum of canine cerebellum was examined. Reticular membrane vesicles used in these experiments accumulated Ca in the presence of ATP and then released ~30% of the accumulated Ca upon addition of micromolar concentrations of InsP₃. When these membrane vesicles were incorporated into planar lipid bilayers, InsP₃-gated Ca channels were observed. Up to four current amplitudes were observed at a given voltage, yielding conductances of 20, 40, 60, and 80 pS with 50 mM Ca as the current carrier. Thus, the cerebellar InsP₃-gated Ca channel exhibits four conductance levels that are multiples of a unit conductance step. Moreover, examination of the single-channel records showed both openings directly to each of the current levels and rapid transitions between current levels. These four conductance steps may reflect the interaction among the four InsP₃ receptors thought to comprise the InsP₃-gated Ca channel in these tissues. Examination of the InsP₃ dependence of channel openings and Ca release from vesicles, however, yielded Hill coefficients of 1-1.3. Thus, we hypothesize that it takes only one molecule of InsP₃ to open the channel. The observation that the conductance of the InsP₃-gated Ca channel assumes four levels that are multiples of a unit conductance suggests that the number of interacting InsP₃ receptors in one complex can vary from one to four and supports the hypothesis that the channel is a tetramer.