ALPHA(1)-ADRENERGIC AND ALPHA(2)-ADRENERGIC CONTROL OF LARGE AND SMALL CORONARY-ARTERIES DURING EXERCISE IN CONSCIOUS DOGS UNDER BETA-BLOCKADE

The aim of this study was to determine the relative roles of alpha1-an d alpha2-adrenoceptors at the level of large epicardial and small resi stance coronary arteries when sympathetic tone is increased by exercis e in conscious dogs. The responses of left circumflex coronary artery diameter and blood flow were investigated at rest and during graded tr eadmill exercise (5, 10 and 12 km/h) in six chronically instrumented d ogs during control conditions (saline) and after administration of pro pranolol (1 mg/kg) either alone or in combination with either prazosin (50 mu/kg), or idazoxan (300 mug/kg), or the association of prazosin + idazoxan (same doses). In control conditions, graded treadmill exerc ise resulted in a progressive increase in coronary artery diameter (3.8 +/- 0.6% from 3479 +/- 80 mum) and in a decrease in coronary vascu lar resistance (- 46.0 +/- 4.5% from 8.49 +/- 1.51 mmHg/cm/s). Propran olol significantly constricted large (- 4.4 +/- 0.6% from 3486 +/- 87 mum) and limited dilation of small coronary arteries during exercise. These coronary effects of propranolol remained unchanged following add itional alpha2-adrenoceptor blockade by idazoxan but were abolished fo llowing alpha1-adrenoceptor blockade by prazosin, given either alone o r combined with idazoxan. Thus, alpha1- but not alpha2-adrenoceptors a re responsible for propranolol-induced constriction of large coronary arteries and limitation of small coronary arteries dilation during exe rcise in conscious dogs.