Abstract
A current theory of regulation of coronary blood flow is that of adenosine-induced coronary vasodilation (1). In particular, the hypothesis that a local increase in the extracellular concentration of adenosine is responsible for the reactive hyperemia of temporary coronary occlusion has gained support from the recent work of several investigators (2, 3). Although there is evidence that adenosine is formed through the degradation of the adenine nucleotides in response to myocardial hypoxia (3), the mechanisms by which coronary vasodilation is produced is unknown. A recently reported study in isolated guinea pig and rat hearts suggested that adenosine causes coronary vasodilation by inhibition of alpha adrenergic activity in the coronary resistance vessels and that the effect can be competitively blocked by alpha adrenergic blockade (4). Since myocardial reactive hyperemia has been shown to be unaffected by alpha adrenergic blockade (5), confirmation of inhibition of adenosine induced coronary vasodilation by alpha adrenergic blocking agents would negate its proposed role in the reactive hyperemia response. The present study was designed to investigate the effect of alpha adrenergic block on the coronary vasodilatory response to infused adenosine and to its nucleotide precursor, ATP.
Methods. Mongrel dogs weighing between 18 and 22 kg were anesthetized with morphine (1 mg/kg sc) and pentobarbital (20 mg/kg sc) and positive pressure breathing was established through an occlusive intratracheal tube. After the left chest was opened and the pericardium incised, heparin (10 mg/kg) was given and the left jugular vein and right common carotid artery were cannulated. Where possible the common left coronary artery was cannulated through its aortic ostium via the left subclavian artery; when this was not feasible the left circumflex branch was directly cannulated just below the origin of the left anterior atrial artery.
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