THE EFFECTS OF CLONIDINE, GUANFACINE AND PHENYLEPHRINE ON THE EXCITATORY AND INHIBITORY RESPONSES OF THE RAT ANOCOCCYGEUS MUSCLE

The effects of clonidine, guanfacine and phenylephrine on twitch respo nses and the basal tone of the rat anococcygeus muscle were investigat ed. Clonidine (10(-9)-3 x 10(-8) M) and guanfacine (10(-9)-10(-7) M) i nhibited the twitch responses with the same potency, whereas phenyleph rine (10(-9)-10(-7) M) was found ineffective. The inhibitory effect of clonidine and guanfacine was antagonized by yohimbine. Higher concent rations of clonidine and guanfacine increased the muscle tone and elic ited inhibitory responses during field stimulation. Phenylephrine at c oncentrations greater than 10(-7) M also increased the muscle tone but induced biphasic responses. Clonidine (10(-7)-3 x 10(-5) M), guanfaci ne(3 x 10(-7)-3 x 10(-5) M) and phenylephrine (3 x 10(-7)-10(-5) M) ca used concentration-dependent increases in the basal tone. The order of potency of these agonists in increasing the basal tone was clonidine > guanfacine > phenylephrine. Both yohimbine (10(-8)-10(-5) M) and pra zosin (10(-9)-10(-7) M) antagonized these tonic contractions. Prazosin was found to be 39-, 122- and 83-fold more potent than yohimbine in a ntagonizing clonidine, guanfacine and phenylephrine-induced tonic cont ractions, respectively. Clonidine and guanfacine inhibited twitch resp onses through stimulation of presynaptic alpha-2 adrenoceptors. Postsy naptic alpha-1 adrenoceptors seem responsible for the contractile effe cts of clonidine, guanfacine and phenylephrine in the rat anococcygeus muscle.