A study of the sympathomimetic action of guanethidine on the isolated anococcygeus muscle of the rat.

Guanethidine, acting on the rat isolated anococcygeus, causes adrenergic neurone blockade (slowly terminated by washing), noradrenaline potentiation and, with higher concentrations, spasm (both rapidly terminated by washing). 2 The spasm is an indirect sympathomimetic action, for it is sensitive to phentolamine and reserpine and shows tachyphylaxis. 3 The concentration of cocaine equieffective with the spasmogenic concentration of guanethidine as an inhibitor of noradrenaline uptake caused much less spasm. Moreover, it did not enhance noradrenaline efflux from anococcygeus loaded with (-)-[3H]-noradrenaline, as guanethidine did. 4 The spasm induced by guanethidine in excess of cocaine is due to guanethidine-evoked noradrenaline release.