Effects of cyclopiazonic acid, a novel Ca2+‐ATPase inhibitor, on contractile responses in skinned ileal smooth muscle

Abstract

1 Effects of cyclopiazonic acid (CPA), a specific inhibitor of the Ca²⁺-ATPase in sarcoplasmic reticulum (SR) of skeletal and cardiac muscles, on contractile responses induced by Ca²⁺-release from intracellular storage sites were examined in the longitudinal smooth muscle strip of the guinea-pig ileum skinned with β-escin. 2 Ca²⁺-loading of storage sites (Ca²⁺-uptake) was performed in pCa 6.3 solution. The amount of Ca²⁺ taken up was monitored by use of the amplitude of contraction following application of 25 mm caffeine or 25 μm inositol 1,4,5-trisphosphate (IP₃). 3 Contractile responses to caffeine or IP₃ were reduced or abolished when the preceding Ca²⁺-uptake was performed in the presence of 0.1–10 μm CPA. The dose of CPA required to inhibit the contraction induced by caffeine or IP₃ by 50% was approximately 0.6 μm. The CPA-sensitive Ca²⁺-uptake completely depended upon the presence of ATP in the solution during Ca²⁺-uptake. 4 When 1 μm CPA was added after Ca²⁺-uptake, the subsequent caffeine- or IP₃-induced contraction was not significantly affected by the presence of CPA. 5 Acetylcholine-induced contraction was also almost abolished when the preceding Ca²⁺-uptake was performed in the presence of 10 μm CPA. 6 The relationship between pCa and contraction was not affected by the presence of 10 μm CPA in skinned fibres where Ca²⁺ storage sites had been destroyed by treatment with A23187. The enhancement of contraction in pCa 6.0 solution by calmodulin was not affected by 10 μm CPA. 7 These results suggest that CPA selectively inhibits ATP-dependent Ca²⁺-uptake into intracellular storage sites in skinned ileal smooth muscle strips. CPA appears to be a potent, reversible, and very specific inhibitor of the Ca²⁺-pump in the storage sites of smooth muscle, and is an extremely valuable pharmacological tool.