title: Reversal of adrenaline-induced contraction in the rabbit pulmonary artery by clonidine.
abstract: Characterization of clonidine which reversed the adrenaline-induced contractions was assessed using the rabbit pulmonary arterial segment. Clonidine in high concentrations (10(-5)-10(-4) mol/l) contracted this vascular tissue. Nevertheless, an adrenaline (3 X 10(-6)mol/l)-induced contraction, which can be markedly depressed by phentolamine, was reversed to relaxation by pre-addition of 3 X 10(-5)mol/l clonidine. This reversal was abolished by 10(-5) mol/l propranolol, a beta-adrenoceptor antagonist. The cumulative concentration-contraction curve for adrenaline was shifted to the right by 10(-5) mol/l clonidine without further alteration at 10(-4) mol/l, suggesting a partial agonistic feature of clonidine for alpha-adrenoceptors. Contractions evoked by 5 X 10(-5) mol/l clonidine alone were effectively reduced by prazosin (IC50 = 1.7 X 10(-7) mol/l) or yohimbine, (IC50 = 3.3 X 10(-5) mol/l) and those by phenylephrine (10(-5) mol/l) were suppressed as well (prazosin, IC50 = 3.4 X 10(-8) mol/l; yohimbine, IC50 = 5.0 X 10(-6) mol/l). However, the depolarization-induced contractions with 30 mmol/l KCl were virtually unaltered by both antagonists. These results suggest that the reversal action of clonidine on the adrenaline-induced contraction is primarily mediated by a partial agonistic property for postsynaptic alpha 1-adrenoceptors.
mesh_terms: D000818:Animals; D003000:Clonidine / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004837:Epinephrine / Q000037:antagonists & inhibitors*; D066298:In Vitro Techniques; D008297:Male; D010656:Phenylephrine / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011651:Pulmonary Artery / Q000187:drug effects; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects; D014661:Vasoconstriction / Q000187:drug effects*; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D015016:Yohimbine; D011189:Potassium Chloride; D003000:Clonidine; D011224:Prazosin; D004837:Epinephrine