Subtypes of β-adrenergic receptors in bovine coronary arteries

Abstract

Whether large coronary artery dilation induced by β-adrenergic stimulation is mediated by β1- or β2-adrenergic receptors remains controversial. This problem is particularly difficult to address in vivo due to the concomitant increase in coronary blood flow with β-adrenergic stimulation, which by itself can dilate large coronary arteries. To reconcile this problem, 5 calves were instrumented with intraaortic and intracoronary (i.c.) catheters, ultrasonic diameter transducers, Doppler flow transducers, and hydraulic occluders on the left circumflex coronary artery. Two to six weeks following surgery, β-adrenergic agonists were administered i.c. to avoid complicating systemic effects. Isoproterenol (0.0025 μg/kg, a β1 + β2-adrenergic agonist) increased coronary diameter (7.1 ± 0.8% from 5.80 ± 0.58 mm) (p < 0.01). Similar increases (p < 0.01) in coronary diameter occurred with prenalterol (0.4 μg/kg, β1-adrenergic agonist) (9.5 ± 1.4%) and pirbuterol (0.25 μg/kg, β2-adrenergic agonist) (8.1 ± 1.2%). When coronary blood flow was prevented from rising with the hydraulic constrictor, increases in coronary diameter to all three β-adrenergic agonists were not attenuated. Large coronary artery dilation with prenalterol and pirbuterol was abolished with β1- and β2-adrenergic receptor blockade, respectively, while neither β1- or β2-adrenergic blockade alone abolished the large coronary artery dilation with isoproterenol. To identify the predominant subtype of β-adrenergic receptor, competitive inhibition curves utilizing 125I-cyanopindolol (125I-CYP) as the radiolabel versus isoproterenol, epinephrine, and norepinephrine were generated in membrane preparations from calf heart (predominant β1), calf lung (predominant β2) and calf coronary artery. The coronary artery membrane preparations demonstrated an intermediate pattern. Competition curves with selective β1- and β2-adrenergic receptor agonists and antagonists again demonstrated a pattern for coronary artery intermediate to that of heart lung, further confirming the presence of both β-adrenergic receptor subtypes in large coronary arteries, with a ratio of β1:β2 of 1.5-2.0:1.0. Thus, large coronary arteries of the calf contain both β1- and β2-adrenergic receptors identified utilizing ligand binding techniques, and stimulation of both receptor subtypes in the intact conscious animal results in large coronary artery dilation, independent of blood-flow-mediated vasodilation.