Mechanism of the decrease in coronary blood flow after beta-blockade in conscious dogs

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Abstract
In order to gain further information on the mechanism responsible for the reduction in coronary blood flow produced by β-blockade, the responses of the coronary vascular bed to Butidrine and its stereoisomers and phenylephrine, were studied in 10 conscious dogs with electromagnetic flowmeters chronically implanted around the ascending aorta and the circumflex branch of the left coronary artery. The results confirmed that β-blockade decreases cardiac work and thus, since the metabolic requirements of the myocardium are reduced, coronary blood flow falls in proportion. After-blockade coronary vessels can, however, respond normally to a transient occlusion with a reactive hyperaemia. This indicates that the coronary vessels do not lose their ability to dilate. The reduction in coronary blood flow and cardiac work was elicited both by d-1 Butidrine and its isomer provided with-blocking activity as well as by that lacking in this activity. Alpha-receptors seem to be involved very little in reducing coronary blood flow. Although the increase in coronary resistance elicited by β-blockade can be eliminated by an α-blockade with phentolamine, the administration of an α-stimulating drug such as noradrenaline or phenylephrine further increases coronary resistance. This suggests that α-receptor activity is unmasked only to a very small extent by β-blockade.

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