Article date: October 1993
By: Yoshiaki Uyama, Yuji Imaizumi, Minoru Watanabe, in Volume 110, Issue 2, pages 565-572
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.
After intracellular stored Ca2+ in 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 μm 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.
Spontaneous and 20 mm K+‐induced contractions in the presence of 1 μm atropine were markedly enhanced by 1–30 μm 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 μm verapamil and restored by 10 μm Bay K 8644.
Application of 10 μm 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.
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 μm CPA was added to the latter solution.
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.
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.
After intracellular stored Ca2+ in 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 μm 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.
Spontaneous and 20 mm K+‐induced contractions in the presence of 1 μm atropine were markedly enhanced by 1–30 μm 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 μm verapamil and restored by 10 μm Bay K 8644.
Application of 10 μm 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.
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 μm CPA was added to the latter solution.
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.
DOI: 10.1111/j.1476-5381.1993.tb13848.x
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