PRESYNAPTIC AND POSTSYNAPTIC EFFECTS OF ALPHA-AGONISTS IN THE ANOCOCCYGEUS MUSCLE OF THE PITHED RAT

In the pithed rat electrically induced contractonis of the anococcygeus muscle were inhibited by clonidien (0.1-0.3 μg/kg, i.v.), mechamylamine (1 mg/kg, i.v.) and guanethidine (1 mg/kg, i.v.). Only the inhibition produced by clonidine was antagonised by yohimbine (0.3 mg/kg, i.v.). At higher concentrations (10 μg/kg, i.v.) clonidine increased the resting tension of the tissue; this effect was antagonised by phentollamine (1 mg/kg, i.v.). The anococcygeus muslce of the pithed rat was used to simultaneously asses pre- and postsynaptic α-adrenoceptor agonist activity. Guanfacin was more selective and tiamenidien less selective than clonidine for presynaptic α-adrenoceptors. Naphazoline and oxymetazoline were selective postsynaptic α-adrenoceptor agonists. © 1979.