Role of alpha 1A- and alpha 1B-adrenoceptors in phenylephrine-induced positive inotropic response in isolated rat left atrium.

We determined the distribution of alpha 1A- and alpha 1B-adrenoceptor subtypes and their functional roles in phenylephrine (PE)-induced positive inotropic responses in rat atrium. Radioligand binding assays in membrane preparations of rat atria showed that 62% of [125I]BE 2254 binding sites were irreversibly inactivated by pretreatment with 20 microM chloroethylclonidine (CEC). Inhibition curves for WB 4101 and 5-methyl-urapidil (5-MU) were better fit by a two-site model, comprising 29-35% high-affinity sites and 65-71% low-affinity sites, suggesting that rat atria contains both alpha 1A and alpha 1B subtypes in a ratio of approximately 1:2. In isolated perfused atria, pretreatment with CEC inhibited the maximum PE-induced positive inotropic response by 55%, and pA2 values for WB 4101 and 5-MU in inhibiting this response were 8.26 +/- 0.4 and 7.85 +/- 0.07, respectively, between the KD values for alpha 1A and alpha 1B subtypes. PE shifted the concentration-contractile response curve for Ca2+ to the left and upward. Pretreatment with CEC completely abolished whereas 1 nM WB 4101 did not alter, the effect of PE on Ca2+ sensitivity. These results demonstrate that both alpha 1A- and alpha 1B- adrenoceptor subtypes are involved in the PE-induced positive inotropic response. Only activation of the alpha 1B subtype potentiates the positive inotropic effect induced by increasing extracellular Ca2+, however, which suggests that the mechanisms involved in the action of the two subtypes may differ at least in part.