Abstract
Blockade of cardiac adrenoceptor subtypes, coronary or myocardial, might elicit compensatory interaction from remaining unblocked subtypes. An attempt to explore this interplay was made by studying regional myocardial blood flow alterations associated with beta 2-adrenergic blockade followed by alpha 1-adrenergic blockade in anaesthetized cats with acute coronary occlusion. In order to maintain constant needs for perfusion, atrial pacing was established and the aortic blood pressure was kept constant. In myocardium remote from the ischaemic region, beta 2-adrenergic blockade produced higher endocardial blood flow whereas no flow changes were observed close to the ischaemic region. With subsequent alpha 1-adrenergic blockade, blood flow increased endocardially in non-ischaemic regions, but remained unchanged in epicardial tissue. Control experiments without coronary ligation revealed no increase in left ventricular oxygen consumption during the experiments and support the theory that the observed blood flow increase in the coronary ligation group, following drug interventions, was not caused by increased cardiac work. This study indicates that combined beta 2- and alpha 1-adrenergic blockade alters the balance between receptor subtypes. Unopposed beta 1-mediated vasodilation is the most likely candidate to explain why endocardial flow was increased.
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