Regulation of inositol 1,4,5-trisphosphate receptors in rat basophilic leukemia cells. I. Multiple conformational states of the receptor in a microsomal preparation

A detailed characterization of the inositol 1,4,5-trisphosphate (IP₃) receptor in rat basophilic leukemia (RBL) cells, a neoplastic mast cell line, has been possible through the growth of solid RBL cell tumors which provide a rich source of IP₃ receptor. Equilibrium binding studies show a 1.6 ± 0.1 pmol/mg of protein maximal binding capacity for [³H]IP₃ at optimal Ca²⁺ (10 μM). The specificity of the RBL cell IP₃ receptor towards phosphoinositides, ATP and heparin parallels those previously described with excitable and nonexcitable tissues. [³H]IP₃ binding is slightly enhanced from < 1 nM to 10 μM Ca²⁺ and inhibited by > 10 μM Ca²⁺. Kinetic and equilibrium studies provide evidence for at least two classes or conformational states of binding sites with pico- and nanomolar affinities. At nM concentrations of IP₃, neither binding to the IP₃ receptor nor IP₃-induced Ca²⁺ efflux from permeabilized cells demonstrates cooperativity. In contrast, at pM concentrations, IP₃ binding kinetics deviate from simple mass action suggesting a complex interaction among binding sites for IP₃ on the receptor-channel oligomer. The mechanism that regulate [³H]IP₃ binding in RBL cells are unique when compared to what has been reported in other cells.