Interactions of antagonists with subtypes of inositol 1,4,5-trisphosphate ( IP3) receptor.

Background and Purpose Inositol 1,4,5-trisphosphate receptors ( IP3Rs) are intracellular Ca2+ channels. Interactions of the commonly used antagonists of IP3Rs with IP3R subtypes are poorly understood. Experimental Approach IP3-evoked Ca2+ release from permeabilized DT40 cells stably expressing single subtypes of mammalian I P3R was measured using a luminal Ca2+ indicator. The effects of commonly used antagonists on IP3-evoked Ca2+ release and 3 H-IP3 binding were characterized. Key Results Functional analyses showed that heparin was a competitive antagonist of all IP3R subtypes with different affinities for each ( IP3R3 > IP3R1 ≥ IP3R2). This sequence did not match the affinities for heparin binding to the isolated N-terminal from each IP3R subtype. 2-aminoethoxydiphenyl borate (2- APB) and high concentrations of caffeine selectively inhibited IP3R1 without affecting IP3 binding. Neither Xestospongin C nor Xestospongin D effectively inhibited IP3-evoked Ca2+ release via any IP3R subtype. Conclusions and Implications Heparin competes with IP3, but its access to the IP3-binding core is substantially hindered by additional IP3R residues. These interactions may contribute to its modest selectivity for IP3R3. Practicable concentrations of caffeine and 2- APB inhibit only IP3R1. Xestospongins do not appear to be effective antagonists of IP3Rs. [ABSTRACT FROM AUTHOR]