The rabbit anococcygeus muscle and its response to field stimulation and to some drugs.

1. The response of the rabbit anococcygeus muscle to field stimulation of its intramural nerves and to some drugs has been examined and compared with results previously obtained in the rat and the cat. 2. The rabbit muscle possesses an adrenergic innervation as demonstrated histologically by the Falck and Hillarp fluorescence technique. This innervation is sparser than in the rat or cat. 3. In vitro the muscle usually shows little tone but if suitably stretched will develop a maintained contraction. The response to field stimulation depends on the level of tone. When this is low purely motor responses are obtained. In the presence of tone the response depends on its level and the frequency of stimulation; low frequencies are purely inhibitory, with increasing frequency the response becomes biphasic and high frequencies produce a purely motor response. The higher the tone the more prominent the inhibitory components. Guanethidine 10(-5)M abolishes the motor component. 4. The muscle is caused to contract by noradrenaline, acting through alpha receptors, by hista,ome actomg through H1 receptors and by 5-hydroxy-tryptamine. Part of the effect of the latter appears to be due to the release of noradrenaline. 5. The muscle is caused to relax by acetylcholine acting through muscarinic receptors, by isoprenaline through beta receptors, by histamine in the presence of mepyramine through H2 receptors and by ATP and bradykinin. 6. The significance of these findings for the motor and inhibitory innervation of the muscle is discussed.