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 Ca(2+)-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+ in intact ileal strips was depleted by application of 25 mM caffeine in Ca(2+)-free solution, Ca(2+)-loading was performed in the absence or presence of 10 microns CPA in a standard solution containing 2.2 mM Ca2+. Subsequent application of caffeine in Ca(2+)-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 microM atropine were markedly enhanced by 1-30 microM 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 microM verapamil and restored by 10 microM Bay K 8644. 4. Application of 10 microM 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 microM 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.