Characterization of beta-adrenoceptor subtype in isolated ring preparations of intramural rat coronary small arteries

We characterized the beta-adrenoceptor subtype in isolated ring preparations of small intramyocardial flow-regulating (resistance) arteries (i.d. congruent to 239 micrometers) from rats by using beta 2 adrenoceptor agonists and beta-adrenoceptor antagonists exhibiting selectivity towards either beta 1- (pafenolol) or beta-adrenoceptors (ICI 118,551). The relative order of potency of selected agonists in producing beta-adrenoceptor-mediated relaxation of prostaglandin F2 alpha (10(-5) M) contracted coronary vessels was: isoprenaline greater than noradrenaline = adrenaline. ICI 118,551, but not pafenolol, relaxed coronary resistance vessels precontracted with prostaglandin F2 alpha (10(-5) M) or high potassium (125 mM) solutions. The IC50 values were 8.59 x 10(-6) M and 2.96 x 10(-5) M, respectively (p less than 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 F2 alpha (10(-5) M) 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 less than 0.005) for pafenolol and ICI 118,551, respectively. The results are compatible with the suggestion that beta-adrenoceptors mediating relaxation in rat coronary resistance vessels belong to the beta 1-adrenoceptor subtype.