Abstract
The effect of intracoronary administration of the selective α 2-adrenoceptor agonist B-HT 920 on coronary resistance was tested in 14 open-chest dogs before and after exhaustion of coronary dilator reserve. Under both conditions B-HT 920 increased coronary resistance to the same extent: 27 ± 4% before and 32 ± 3% after. Administration of B-HT 920 did not affect cardiovascular haemodynamics significantly, although after exhaustion of coronary dilator reserve left ventricular dP dt max decreased slightly by 12 ± 5%. While efferent cardiac sympathetic discharge remained constant during control conditions it increased by 27 ± 7% when B-HT 920 was administered into a stenotic coronary artery. We conclude that, in contrast to observations made on α 1-adrenoceptors, activation of coronary α 2-adrenoceptors may have deleterious effects after exhaustion of coronary dilator reserve because these receptors mediate a comparable constriction in well- and under-perfused coronary vessels. Thus they are unlikely to favour a redistribution of coronary blood flow into under-perfused myocardial regions. In addition, this unfavourable effect of B-HT 920 on coronary perfusion after coronary dilator reserve has been exhausted may be further exacerbated by an increase in efferent sympathetic discharge.
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