EFFECTS OF CYCLOPIAZONIC ACID, A NOVEL CA-2+-ATPASE INHIBITOR, ON CONTRACTILE RESPONSES IN SKINNED ILEAL SMOOTH-MUSCLE

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 beta-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-mu-M 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-mu-M CPA. The dose of CPA required to inhibit the contraction induced by caffeine or IP3 by 50% was approximately 0.6-mu-M. The CPA-sensitive Ca2+-uptake completely depended upon the presence of ATP in the solution during Ca2+-uptake. 4 When 1-mu-M 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-mu-M CPA. 6 The relationship between pCa and contraction was not affected by the presence of 10-mu-M 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-mu-M 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.