Pharmacological study of the anococcygeus muscle of the dog

1 The response of the dog anococcygeus muscle to field stimulation and to some drugs has been studied. The results are compared with those reported previously in the rat, cat and rabbit. 2 Field stimulation produced frequency-dependent contractions which were inhibited by guanethidine and phentolamine. When the tonus of the muscle was increased with guanethidine, field stimulation always produced frequency-dependent relaxation. The relaxation was not prevented by propranolol. 3 The muscle was contracted by noradrenaline, tyramine, acetylcholine, histamine (H1), 5-hydroxy-tryptamine, prostaglandin E2 and vasopressin. Phentolamine, atropine, promethazine (but not cimetidine) and methysergide inhibited the effect of the respective agonists. 4 After increasing the tonus of the muscle, it was relaxed by low concentrations of isoprenaline. The relaxation was antagonized by propranolol. 5 The response to adenosine triphosphate (ATP) was variable. In some preparations, it relaxed the muscle, in others it contracted the muscle prior to relaxation, in others still it only contracted the muscle. Indomethacin did not prevent ATP-induced contraction. 6 It is concluded that the anococcygeus of the dog, like that of rat, cat and rabbit, has an adrenergic motor innervation and an inhibitory innervation, the transmitter of which is not identified.