Alpha 1-adrenoceptor subtype involved in the positive inotropic response to phenylephrine in rat atria

Phenylephrine produced positive inotropic responses in isolated rat right and left atria. The responses were competitively inhibited by alpha 1-adrenoceptor antagonists (prazosin, WB4101 and HV723) with relatively low affinities (pA2 values close to 8.0). Chloroethylclonidine had no significant effect on the responses to phenylephrine. These results suggest that the positive inotropic response to phenylephrine in rat atria is mediated through alpha 1-adrenoceptors which cannot be defined by the alpha 1A, alpha 1B subclassification.