Presynaptic agonist effect of phentolamine in the rabbit vas deferens and rat cerebral cortex.
Clonidine inhibited the electrically-induced twitch response of the rabbit and rat isolated vas deferens preparations and also the K+-evoked release of [3H]noradrenaline from rat cortical slices. This effect of clonidine was antagonized competitively by yohimbine. Phentolamine inhibited the electrically-induced twitch response of the rabbit, but not the rat, vas deferens and in low concentrations (less than 0.1 microM) also inhibited the K+-evoked release of [3H]NA from rat cortical slices. These inhibitory effects of phentolamine were antagonized by yohimbine in a competitive manner but were not antagonized by indoramin, an alpha 1-adrenoceptor antagonist. In the rabbit vas deferens, the effects of phentolamine were shown not to be due to the stimulation of beta-, H1-, H2-, 5-HT- or muscarinic receptors. These results are consistent with the view that phentolamine can act as an agonist at presynaptic alpha 2-adrenoceptors in the rabbit vas deferens and rat cortex but not in the rat vas deferens.
D000818:Animals; D001923:Brain Chemistry / Q000187:drug effects*; D002540:Cerebral Cortex / Q000187:drug effects* / Q000378:metabolism; D003891:Desipramine / Q000494:pharmacology; D066298:In Vitro Techniques; D007217:Indoramin / Q000494:pharmacology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects; D009638:Norepinephrine / Q000378:metabolism; D010646:Phentolamine / Q000494:pharmacology*; D011188:Potassium / Q000494:pharmacology; D011817:Rabbits; D051381:Rats; D011919:Rats, Inbred Strains; D014649:Vas Deferens / Q000187:drug effects*; D015016:Yohimbine / Q000378:metabolism
D007217:Indoramin; D015016:Yohimbine; D011188:Potassium; D003891:Desipramine; D009638:Norepinephrine; D010646:Phentolamine