Inositol 1,4,5-trisphosphate receptor isoforms show similar Ca2+ release kinetics

The inositol 1,4,5-trisphosphate receptor (InsP(3)R) is an intracellular Ca2+ release channel which upon activation initiates many cellular functions. Multiple InsP(3)R subtypes are expressed in most cell types but the physiological significance of this heterogeneity is poorly understood. This study has directly compared the functional properties of the three different InsP(3)R isoforms by analyzing their InsP(3)-induced Ca2+ release (IICR) properties in cell lines which predominantly express each isoform subtype. The InsP(3)-dependence of the amount or extent of IICR was InsP(3)R isoform-specific, with the type III isoform having the lowest affinity with respect to Ca2+ release. The transient kinetics of IICR, measured using stopped-flow spectrofluorimetry, however, were similar for all three InsP(3)R isoforms. At maximal InsP(3) concentrations (20 muM) the rate constants where between 0.8 and 1.0 s(-1) for the fast phase and 0.25-0.45 s(-1) for the slow phase. The concentration of InsP(3) required to induce half-maximal rates of Ca2+ release (EC50) were also similar for the three isoforms (0.2-0.4 muM for the fast phase and 0.75-0.95 muM for the slow phase). These results indicate the InsP(3)R channel does not significantly differ functionally in terms of Ca2+ release rates between isoforms. The temporal and spatial features of intracellular Ca2+ Signals are thus probably achieved through InsP(3)R isoform-specific regulation or localization rather than their intrinsic Ca2+ efflux properties. (C) 2001 Harcourt Publishers Ltd.