Responsiveness of the coronary circulation to brief vs sustained alpha-adrenergic stimulation

The effects of brief and sustained pharmacologic alpha-adrenergic stimulation on the coronary arterial circulation were compared in awake pigs. Phenylephrine was administered into the left anterior descending coronary artery (LAD) either as a bolus (eight pigs) or as a 15-minute infusion (eight pigs), with myocardial blood flow measured by the radioactive microsphere technique. Flow in the distribution in the LAD was compared with flow in myocardium perfused by the left circumflex coronary artery (LCF) as the ratio LAD/LCF. This technique corrects for systemic factors capable of modifying oxygen demand, and hence myocardial blood flow, in both zones. After a phenylephrine bolus (50-100 microgram), LAD/LCF fell significantly, whereas no change was observed after the sustained infusion (5-10 and 50-100 microgram/min). Four additional pigs were pretreated with i.v. adenosine to raise myocardial blood flow in excess of demand before sustained stimulation. In this setting LAD/LCF fell significantly during the sustained phenylephrine infusion. Brief alpha-adrenergic stimulation could overcome normal flow regulatory mechanisms and resulted in constriction of coronary resistance vessels. Such changes did not occur after sustained stimulation and suggest an ability of the coronary circulation to offset chronic vasoconstrictive effects. When the myocardium is overperfused, sustained alpha-adrenergic stimulation does not jeopardize myocardial oxygenation and its vasoconstriction potential is unmasked.