THE INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR OF CEREBELLUM - MN2+ PERMEABILITY AND REGULATION BY CYTOSOLIC MN2+

The inositol 1,4,5-trisphosphate receptor (InsP(3)R), an intracellular calcium release channel, is found in virtually all cells and is abund ant in the cerebellum. We used Mn2+ as a tool to study two aspects of the cerebellar InsP(3)R First, to investigate the structure of the ion pore, Mn2+ permeation through the channel was determined. We found th at Mn2+ can pass through the InsP(3)R; the selectivity sequence for di valent cations is Ba2+ > Sr2+ > Ca2+ > Mg2+ > Mn2+. Second, to begin c haracterization of the cytosolic regulatory sites responsible for the Ca2+-dependent modulation of InsP(3)R function, the ability of Mn2+ to replace Ca2+ was investigated. We show that Mn2+, as Ca2+, modulates InsP(3)R activity with a bell-shaped dependence where the affinity of the activation site of the InsP(3)R is similar for both ions, but high er concentrations of Mn2+ were necessary to inhibit the channel. These results suggest that the two regulatory sites are structurally distin ct Our findings are also important for the understanding of cellular r esponses when Mn2+ is used to quench the intracellular fluorescence of Ca2+ indicator dyes.