The classification of β‐adrenoceptors in isolated ring preparations of canine coronary arteries

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Abstract
1 Relaxant responses to the β-adrenoceptor agonists isoprenaline, fenoterol, noradrenaline or procaterol were obtained on isolated ring preparations of canine coronary arteries contracted with KCl (20 mm) or 5-hydroxytryptamine (3 μm). 2 On left circumflex arterial preparations, Schild plots for the selective antagonists atenolol (β1-selective) or ICI 118,551 (β2-selective), when using noradrenaline or fenoterol as agonist, were superimposed. This suggested that only one subtype of β-adrenoceptor was involved in the responses. 3 The pA2 values on left circumflex artery preparations were: atenolol, noradrenaline as agonist 6.98, fenoterol as agonist 6.71; ICI 118,551, noradrenaline as agonist 6.66, fenoterol as agonist 7.04. These data indicated that the β-adrenoceptor subtype was β1. 4 The relative potencies of isoprenaline: noradrenaline: fenoterol were left circumflex 100: 10.0: 2.3, left ventricular branch 100: 9.7: 2.0, septal branch 100: 10.9: 2.5. These data confirmed that β1-adrenoceptors were involved in the responses of all three arterial preparations. 5 On preparations of left circumflex artery, left ventricular branch and septal branch, responses were obtained to high concentrations (1 to 100 μm) but not to low concentrations (0.001 to 0.1 μm) of procaterol. This observation confirmed the absence of β2-adrenoceptors in these arteries. 6 Responses of left circumflex artery to isoprenaline were potentiated by the extraneuronal uptake inhibitor drugs, corticosterone and metanephrine. 7 It is concluded (a) that responses of canine left circumflex artery, left ventricular branch and septal branch are mediated by a homogeneous population of β1-adrenoceptors, and (b) that modulation of responses to isoprenaline by extraneuronal uptake is not confined to responses mediated by β2-adrenoceptors.

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