Comparison of Contractile Mechanisms by Carbachol and ATP in Detrusor Strips of Rabbit Urinary Bladder.

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Contractile mechanisms by carbachol and ATP were compared in the detrusor strips of rabbit bladder. To exclude modulation of the agonists-induced responses by intramurally synthesized prostaglandins, all the experiments were done in the presence of the potent cyclooxygenase inhibitor flurbiprofen (1 μM). The concentration-response curves for carbachol and ATP were shifted to the right by 6-10-fold by verapamil (10 μM), which abolished the K-induced contraction of the atropinized detrusor completely. A similar curve for carbachol was obtained in the absence of extracellular Ca [(Ca)<SUB>O</SUB>], but the contraction by ATP below 1 mM was more reduced by (Ca)<SUB>O</SUB>-depletion than by verapamil. Under Ca-free conditions, repeated applications of ATP resulted in no response, but those of carbachol induced reproducible contractions. These results suggest that carbachol and ATP induces Ca-influx through L-type Ca channels and releases Ca from the Ca stores. However, while carbachol might increase the sensitivity of contractile machinery to Ca on the one hand, ATP would open additional, verapamil-insensitive Ca channels.
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