The actions of some vasoactive polypeptides and their antagonists on the anococcygeus muscle

1 The action of three polypeptides, bradykinin, substance P and eledoisin known to inhibit vascular smooth muscle has been examined on the anococcygeus muscle of the rat, cat and rabbit.2 In the atonic rat muscle, bradykinin and substance P had little or no effect on tone but eledoisin produced a sustained dose-related contraction which could be abolished by phentolamine (1 muM) and is, therefore, probably an indirect sympathomimetic effect. On the motor response to field stimulation of adrenergic nerves, bradykinin had no effect whereas both substance P and eledoisin reduced this response. The mechanism of action was further analysed with eledoisin by examining its effect on the response to noradrenaline. Eledoisin did not alter the dose-response curve to noradrenaline and its inhibitory action is likely, therefore, to be presynaptic.3 In the rat anococcygeus muscle in which the tone was raised by guanethidine or carbachol, bradykinin and substance P reduced this tone whereas eledoisin continued to exert a motor action. Compared with substance P the inhibitory effect of bradykinin appeared at lower concentrations (threshold 0.01 mug/ml), developed more rapidly and the size of the response was greater.4 The effect of bradykinin on the tonically contracted cat and rabbit anococcygeus muscles was examined in addition to that of the rat. In all three species bradykinin caused inhibition and the magnitude of the response was equal to the maximum effect of inhibitory nerve stimulation. None of the peptides affected the inhibitory response to nerve stimulation itself.5 The effects of three substances, hesperitin, khellin and apiin, reported in other tissues to antagonize the action of bradykinin were examined both on the inhibitory response to bradykinin and to field stimulation. None of them was able to inhibit either response, although they reduced tone when given by themselves. During these experiments it was found that ethanol antagonized the inhibitory response to field stimulation.6 The possibility that bradykinin or some related peptide might play a part in the inhibitory response to nerve stimulation in the anococcygeus is discussed.