Cyclopiazonic acid, an inhibitor of the sarcoplasmic reticulum Ca(2+)-pump, reduces Ca(2+)-dependent K+ currents in guinea-pig smooth muscle cells.
1. Effects of cyclopiazonic acid (CPA), a specific inhibitor of the Ca(2+)-ATPase in sarcoplasmic 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 microM) 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 approximately 3 microM 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 Ca(2+)-loading of the cell in a solution containing 2.2 mM Ca2+ was performed in the presence of 3 microM CPA. 4. When the Ca(2+)-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 microM CPA. When outward currents were blocked by replacement of K+ by Cs+ in the pipette solution, the remaining ICa was not affected by 10 microM CPA. 5. CPA (10 microM) did not affect the conductance of single maxi Ca(2+)-dependent K+ channels or the Cd(2+)-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 Ca24 release which is triggered by Ca24 entering through voltage-dependent Ca24 channels and therefore less activation of these K channels.7. CPA may be extremely valuable pharmacological tool for investigating intracellular Ca24 mobilization and ionic currents regulated by intracellular Ca24.
Suzuki
M
M
Department of Chemical Pharmacology, Faculty of Pharmaceutical Sciences, Nagoya City University, Japan.
Muraki
K
K
Imaizumi
Y
Y
Watanabe
M
M
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000187:drug effects*; D004553:Electric Conductivity; D004558:Electric Stimulation; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology*; D009130:Muscle, Smooth / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000187:drug effects*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000201:enzymology; D001743:Urinary Bladder / Q000187:drug effects / Q000378:metabolism
D007211:Indoles; D015221:Potassium Channels; D000252:Calcium-Transporting ATPases; D011188:Potassium; D002118:Calcium; C000543:cyclopiazonic acid