Characterization of β-Adrenoceptor Subtype in Isolated Ring Preparations of Intramural Rat Coronary Small Arteries

Abstract

Summary: We characterized the β-adrenoceptor subtype in isolated ring preparations of small intramyocardial flow-regulating (resistance) arteries (i.d. ≃ 239 μm) from rats by using β-adrenoceptor agonists and β-adrenoceptor antagonists exhibiting selectivity towards either β₁-(pafenolol) or β₂-adrenoceptors (ICI 118,551). The relative order of potency of selected agonists in producing β-adrenoceptor– mediated relaxation of prostaglandin F_2α (10⁻⁵M) contracted coronary vessels was: isoprenaline > noradrenaline = adrenaline. ICI 118,551, but not pafenolol, relaxed coronary resistance vessels precontracted with prostaglandin F_2α (10⁻⁵M) or high potassium (125 mM) solutions. The IC₅₀ values were 8.59⋅10⁻⁶M and 2.96⋅10⁻⁵M, respectively (p < 0.0005). This indicates that ICI 118,551 had membrane-stabilizing effects. Both ICI 118,551 and pafenolol antagonized in a concentration-dependent manner the isoprenaline-induced relaxation of prostaglandin F_2α (10⁻⁵M) contracted coronary resistance vessels. The slopes and regression lines of the Schild plots were not significantly different from unity and the calculated pA₂ values were 7.55 and 6.59 (p < 0.005) for pafenolol and ICI 118,551, respectively. The results are compatible with the suggestion that β-adrenoceptors mediating relaxation in rat coronary resistance vessels belong to the β₁-adrenoceptor subtype.