2APB, 2-aminoethoxydiphenyl borate, a membrane-penetrable modulator of Ins(1,4,5)P3-induced Ca2+ release

The effects of a novel membrane-penetrable modulator, 2APB (2-aminoethoxy diphenyl borate), on Ins(1,4,5)P3-induced Ca2+ release were examined. 2APB inhibited Ins(1,4,5)P3-induced Ca2+ release from rat cerebellar microsomal preparations without affecting [3H]Ins(1,4,5)P3 binding to its receptor. The IC50 value (concentration producing 50% inhibition) of 2APB for inhibition of Ins(1,4,5)P3 (100 nM) induced Ca2+ release was 42 microM. Further increase in the concentration of 2APB (more than 90 microM) caused a gradual release of Ca2+ from cerebellar microsomal preparations. Addition of 2APB to the extracellular environment inhibited the cytosolic Ca2+ ([Ca2+]c) rise in intact cells such as human platelets and neutrophils stimulated by thromboxane-mimetic STA2 or thrombin, and leukotriene B4 (LTB4) or formyl-methionine-leucine-phenylalanine (FMLP), respectively. 2APB inhibited the contraction of thoracic aorta isolated from rabbits induced by angiotensin II (AII), STA2, and norepinephrine in a non-competitive manner, but showed no effect on the contraction of potassium-depolarized muscle. 2APB had no effect on the Ca2+ release from the ryanodine-sensitive Ca2+ store prepared from rat leg skeletal muscle and heart. Although the specificity of 2APB with respect to the intracellular signaling system was not fully established, 2APB is the first candidate for a membrane-penetrable modulator of Ins(1,4,5)P3 receptor, and it should be a useful tool to investigate the physiological role of the Ins(1,4,5)P3 receptor in various cells.