Cyclopiazonic acid, an inhibitor of the sarcoplasmic reticulum Ca2+‐pump, reduces Ca2+‐dependent K+ currents in guinea‐pig smooth muscle cells

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Abstract
1 Effects of cyclopiazonic acid (CPA), a specific inhibitor of the Ca2+-ATPase in sarcoplamic reticulum (SR), on membrane ionic currents were examined in single smooth muscle cells freshly isolated from ileal longitudinal strips and urinary bladder of the guinea-pig. 2 Under whole-cell clamp, CPA (1–10 μm) reduced peak outward current elicited by depolarization in a concentration-dependent manner. The concentration of CPA required for 50% decrease in the peak outward current was ∼ 3 μm in ileal cells under these conditions. The current reduced by CPA recovered by more than 70% after washout. 3 The transient outward current elicited by application of 5 mm caffeine at a holding potential of — 50 mV in Ca2+ free solution was almost abolished, when the preceding Ca2+-loading of the cell in a solution containing 2.2 mm Ca2+ was performed in the presence of 3 μm CPA. 4 When the Ca2+-dependent K+ current (IK-Ca) and Ca2+ current (ICa) were inhibited by addition of Ca2+, the remaining delayed rectifier type K+ current was not affected by 10 μm CPA. When outward currents were blocked by replacement of K+ by Cs+ in the pipette solution, the remaining ICa was not affected by 10 μm CPA. 5 CPA (10 μm) did not affect the conductance of single maxi Ca2+-dependent K+ channels or the Cd2+-dependence of their open probability in both inside- and outside-out configurations. 6 These results indicate that IK-Ca is selectively and strongly suppressed by CPA. Its effects may be attributed to a decrease in Ca2+-uptake into SR, resulting in a decrease in Ca2+-induced Ca2+ release which is triggered by Ca2+ entering through voltage-dependent Ca2+ channels and therefore less activation of these K channels. 7 CPA may be extremely valuable pharmacological tool for investigating intracellular Ca2+ mobilization and ionic currents regulated by intracellular Ca2+.

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