Novel effect of 2-aminoethoxydiphenylborate through inhibition of calcium sensitization induced by Rho kinase activation in human detrusor smooth muscle

Since the introduction of 2-aminoethoxydiphenylborate (2-APB) as a membrane permeable modulator of inositol (1,4,5)-trisphosphate receptors, subsequent studies have revealed additional actions of this chemical on multiple Ca ²⁺-permeable ionic channels in the plasma membrane. However, no reports have yet examined 2-APB as a modulator targeting contractile machinery in smooth muscle, independent of Ca²⁺ mobilization, namely Ca ²⁺ sensitization. Here, we assessed whether or not 2-APB affects intracellular signaling pathways of Ca²⁺ sensitization for contraction using α-toxin permeabilized human detrusor smooth muscle. Although contractions were induced by application of Ca²⁺-containing bath solutions, 2-APB had little effect on contractions induced by 1 μM Ca²⁺ alone but significantly reversed the carbachol-induced augmentation of Ca²⁺-induced contraction in the presence of guanosine triphosphate (carbachol-induced Ca²⁺ sensitization). The rho kinase inhibitor Y-27632 and protein kinase C inhibitor GF-109203X also reversed the carbachol-mediated Ca²⁺ sensitization. Additional application of 2-APB caused a small but significant further attenuation of the contraction in the presence of GF-109203X but not in the presence of Y-27632. Like carbachol, the rho kinase activator; sphingosylphosphorylcholine, protein kinase C activator; phorbol 12,13 dibutyrate, and myosin light chain phosphatase inhibitor; calyculin-A all induced Ca²⁺ sensitization. However, the inhibitory activity of 2-APB was limited with sphingosylphosphorylcholine- induced Ca²⁺ sensitization. This study revealed a novel inhibitory effect of 2-APB on smooth muscle contractility through inhibition of the rho kinase pathway.