The effects of the alpha-adrenoreceptor antagonists, prazosin and yohimbine, have been examined on the isometric contraction of the rabbit pulmonary artery produced by alpha 1-adrenoreceptor agonists. Cocaine, corticosterone and propranolol were present throughout and either a single or two concentration-response curve method was used. 2. Prazosin and yohimbine competitively antagonized the responses to agonists but did not reveal differences between agonists. 3. Using the single concentration-response curve method, prazosin had pA2 values of 9.06 against clonidine and 8.76 against methoxamine. 4. Using the two concentration-response curve method the absolute pA2 values of prazosin (8.65 against phenylephrine, 8.78 against clonidine) and yohimbine (5.73 against phenylephrine, 5.72 against clonidine), as well as the relative potencies of the two antagonists (prazosin approximately 1000 times more potent than yohimbine), suggest that both agonists act on alpha 1-adrenoreceptors. 5. There is no evidence from this study to support the suggestion that the rabbit pulmonary artery contains two subtypes of alpha 1-adrenoreceptor.
