Responses of goat ruminal musculature to bradykinin and serotonin in vitro and in vivo.

Concentration-response curves obtained for contraction of goat rumen strips indicated that both bradykinin (BK) and serotonin (5-hydroxytryptamine creatinin sulphate; 5-HT) are potent agents. Sodium meclofenamate antagonized the smooth muscle-stimulating action of BK; the 5-HT blocking agents xylamidine and methysergide diminished the contractile effects of 5-HT. The results indicated that the antagonisms are noncompetitive in nature. In vivo, both BK and 5-HT caused a dose-dependent inhibition of the rumen contraction sequences. The BK was the more potent of the two compounds. This inhibition was probably due to an increase in ruminal smooth muscle tone, resulting in reflex inhibition of the normal cyclical contraction sequences. In conscious goats, BK induced tachycardia, whereas 5-HT produced only minor changes in heart rate. Treatment with sodium meclofenamate or phenylbutazone prevented the BK-induced inhibition of rumen motility. Treatment with the 5-HT blocking agents only partly reduced the 5-HT-induced effects.