Alpha 1- and Alpha 2-vascular Adrenoceptors in the Dog

Prazosin inhibited contractions of isolated electrically stimulated dog pulmonary artery preincubated with (-)-7-3H-norepinephrine, but had no effect on 3H-overflow, whereas yohimbine enhanced both responses. In pithed dogs the cumulative doses of yohimbine required for 50% inhibition of the pressor effects of clonidine, phenylephrine and norepinephrine were 23, 188 and 35 micrograms/kg i.v., and those for prazosin were 9, 1.5 and 5 micrograms/kg i.v., respectively. The agonists could be ranked in pairs depending on the extent to which the antagonists discriminated members of the pair: clonidine-phenylephrine > phenylephrine-norepinephrine > clonidine-norepinephrine. The results suggest that pressor effects in dogs are mediated by both alpha 1- and alpha 2-vascular (i.e. postsynaptic) adrenoceptors.