Characteristics of inositol 1,4,5-trisphosphate binding to rat cerebellar and bovine adrenal cortical membranes: evidence for the heterogeneity of binding sites.

The equilibrium and kinetic binding characteristics of D-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) have been studied in membrane preparations of rat cerebellum and bovine adrenal cortex. Saturation analysis of isotopic dilution binding data demonstrated apparent KD values for Ins(1,4,5)P3 binding of 23 +/- 5 nM and 3.0 +/- 1.3 nM for cerebellar and adrenal cortical preparations, respectively, with approximately 20-fold greater receptor density present in the cerebellar preparation (Bmax: 10.2 +/- 2.5 pmol/mg protein). Kinetic analysis confirmed the equilibrium binding-derived KD value for cerebellum (KD: 39.9 nM), but revealed a second, very high affinity site (KD: 0.06 nM) to be present in adrenal cortex. The affinity differences between the investigated preparations was also observed with respect to the IC50 values obtained for inhibition of specific [3H]Ins(1,4,5)P3 binding by a number of inositol polyphosphate analogues including D-inositol 2,4,5-trisphosphate, DL-inositol 1,4,5-trisphosphorothioate and L-Ins(1,4,5)P3. In contrast, the Ins(1,4,5)P3-receptor antagonist heparin displayed greater potency for the cerebellar (IC50: 16.5 +/- 6.2 micrograms . ml-1) compared to the adrenal cortical preparation (IC50: 51.0 +/- 6.1 micrograms . ml-1). The apparent differences between the Ins(1,4,5)P3 receptors characterized in the two tissue preparations are discussed.