Pre- and postsynaptic effects of alpha-agonists in the anococcygeus muscle of the pithed rat.
In the pithed rat electrically induced contractions of the anococcygeus muscle were inhibited by clonidine (0.1--3.0 microgram/kg, i.v.), mecamylamine (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 microgram/kg, i.v.) clonidine increased the resting tension of the tissue; this effect was antagonised by phentolamine (1 mg/kg, i.v.). The anococcygeus muscle of the pithed rat was used to simultaneously assess pre- and postsynaptic alpha-adrenoceptor agonist activity. Guanfacin was more selective and tiamenidine less selective than clonidine for presynaptic alpha-adrenoceptors. Naphazoline and oxymetazoline were selective postsynaptic alpha-adrenoceptor agonists.
D000315:Adrenalectomy; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D003000:Clonidine / Q000031:analogs & derivatives / Q000494:pharmacology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects; D051381:Rats; D013116:Spinal Cord / Q000502:physiology; D013569:Synapses / Q000187:drug effects*; D014649:Vas Deferens / Q000187:drug effects
D000316:Adrenergic alpha-Agonists; D003000:Clonidine