Existence of two types of postjunctional α-adrenoceptors in the isolated canine internal carotid artery

Abstract

The pharmacological characteristics of postjunctional α-adrenoceptors in isolated canine internal carotid arteries were investigated by the use of selective agonists and antagonists for α₁ and α₂-adrenoceptors. Norepinephrine, phenylephrine, and xylazine caused concentration-dependent contractions in the helical strips. The contraction induced by 10⁻⁴ M xylazine was significantly smaller than that produced by 10⁻⁴ M norepinephrine or 10⁻⁴ M phenylephrine. The contraction induced by 10⁻⁴ M phenylephrine was almost the same value as that induced by 10⁻⁴ M norepinephrine. Phentolamine (10⁻⁸ and 10⁻⁷ M) caused a parallel shift to the right of the concentration–response curve to norepinephrine. The contractile responses to low concentrations of norepinephrine were significantly suppressed by pretreatment with an α₂-antagonist such as yohimbine (10⁻⁹ and 10⁻⁸ M) or DG 5128(10⁻⁷ and 10⁻⁶ M). On the other hand, the responses to higher concentrations of norepinephrine were mainly reduced by low concentrations of an α₁-antagonist, prazosin (3 × 10⁻¹⁰ and 3 × 10⁻⁹ M). These results suggest that both α₁- and α₂-adrenoceptors are located on the plasma membrane of smooth muscle cells in canine internal carotid arteries and that the norepinephrine-induced contractions at low and high concentrations are mainly mediated by activation of α₂- and α₁-adrenoceptors, respectively.