CYCLOPIAZONIC ACID, AN INHIBITOR OF CA(2+)-ATPASE IN SARCOPLASMIC-RETICULUM, INCREASES EXCITABILITY IN ILEAL SMOOTH-MUSCLE

1 Effects of cyclopiazonic acid (CPA), a specific inhibitor of Ca2+-ATPase in endo- and sarcoplasmic reticulum (ER/SR), on contractile responses, cytosolic Ca2+ concentration and spontaneous electrical activity were examined in ileal longitudinal smooth muscle strips. 2 After intracellular stored Ca2+ intact ileal strips was depleted by application of 25 mM caffeine in Ca2+ -free solution, Ca2+-loading was performed in the absence or presence of 10 muM CPA in a standard solution containing 2.2 mM Ca2+. Subsequent application of caffeine in Ca2+-free solution induced a phasic contraction which was significantly smaller in the strip pretreated with CPA than that in the control. 3 Spontaneous and 20 mM K+-induced contractions in the presence of 1 muM atropine were markedly enhanced by 1-30 muM CPA, whereas that induced by 80 mM K+ was not. The magnitude of repetitive transient elevation of cytosolic Ca2+ concentration ([Ca2+]i) and concomitant phasic contractions were markedly enhanced by CPA. The effects were abolished by 10 muM verapamil and restored by 10 muM Bay K 8644. 4 Application of 10 muM CPA depolarized the cell by about 5 mV, decreased the action potential (AP) afterhyperpolarization and markedly increased the frequency of spontaneous AP. These effects were mimicked by 100 nM charybdotoxin. 5 The rate of decay of [Ca2+]i and tension after the bathing solution was changed from one containing 140 mM K+ and 2.2 mM Ca2+ to one containing 5.9 mM K+ and 0 mM Ca2+ was significantly slowed when 10 muM CPA was added to the latter solution. 6 These results indicate that CPA enhances ileal smooth muscle excitability and increases Ca2+-influx through voltage-dependent Ca2+ channels. The effect may be consistent with the hypothesis that CPA-induced decrease in stored Ca due to Ca-pump inhibition reduces the Ca2+-dependent K+ current and indirectly enhances Ca2+-influx through membrane activity resulting from the increased excitability. Direct evidence for the regulation of Ca2+ channel activity by intracellular Ca storage sites was not obtained in the present study.