This study was carried out to investigate the action of adenosine triphosphate (ATP) on the motility of immature pig uterine smooth muscle. ATP appeared contractile responses in a dose-dependent manner, showing the maximal contraction at the concentration of 10<sup>-3</sup>M in the uterine smooth muscle strip. The contractile responses by ATP(10<sup>-4</sup>M) were not affected by atropine (10<sup>-6</sup>M), phentolamine (10<sup>-6</sup>M), propranolol (10<sup>-6</sup>M), pyrilamine (10<sup>-6</sup>M), cimetidine (10<sup>-6</sup>M), and theophyulline (5 X 10<sup>-5</sup>M), but were inhibited uncompetitively by quinidine. The effects of these drugs on the contractile responses by prostaglandin F<sub>2α</sub>(PGF<sub>2α</sub>) were also comparable to those observed with ATP. When muscle strips were pretreated with indomethacin (5 X 10<sup>-5</sup>M) for 20 min., the contractile responses by ATP(10<sup>-4</sup>M) were completely inhibited. But the contractile responses by PGF<sub>2α</sub> were not affected by indomethacin. These results suggest that ATP elicited the contraction through noncholinergic- and nonadrenergic-receptor mediated by prostaglandin F<sub>2α</sub> in pig uterine smooth muscle.
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