Inhibition of the type 1 inositol 1,4,5-trisphosphate receptor by 2-aminoethoxydiphenylborate

2-Aminoethoxydiphenylborate (2-APB) inhibits the extent of inositol 1,4,5-trisphosphate (InsP(3))-induced Ca2+ release from cerebellar microsomes with a potency that is dependent upon the InsP(3) concentration used. At high InsP(3) concentrations (10 muM), the concentration of 2-APB required to cause half-maximal InsP(3)-induced Ca2+ release (IC50) was greater than 1 mM, while at 0.25 muM InsP(3) this reduced to 220 muM. The fact that the inhibition of the extent of InsP(3)-induced Ca2+ release (IICR) by 2-APB was not restored to control levels by high concentrations of InsP(3), in addition to the fact 2-APB did not substantially inhibit [H-3]InsP(3) binding to its receptor, indicates that the inhibition is not competitive in nature. Since the cooperativity of IICR as a function of InsP(3) was reduced in the presence of 2-APB (Hill coefficient changing from 1.9 in the absence of 2-APB to 1.4 in the presence of 1 mM 2-APB), this suggests that it is acting as an allosteric inhibitor. 2-APB also reduces the rate constants for IICR. In cerebellar microsomes this release process is biphasic in nature, with a fast and slow phase. 2-APB appears particularly to affect the fast-phase component. Although 2-APB does not inhibit the ryanodine receptor, it does inhibit the Ca2+ ATPase activity as well store-operated Ca2+ entry channels, which may limit its use as a specific membrane permeant InsP(3) receptor inhibitor. (C) 2002 Elsevier Science Inc. All rights reserved.