Is calcium a coronary vasoconstrictor in vivo?

Background: Calcium produces constriction in isolated coronary vessels and in the coronary circulation of isolated hearts, but the importance of this mechanism in vivo remains controversial. Methods: The left anterior descending coronary arteries of 20 anesthetized dogs whose chests had been opened were perfused at 80 mmHg, Myocardial segmental shortening was measured with ultrasonic crystals and coronary blood now with a Doppler now transducer. The coronary arteriovenous oxygen difference was determined and used to calculate myocardial oxygen consumption and the myocardial oxygen extraction ratio, The myocardial oxygen extraction ratio served as an index of effectiveness of metabolic vasodilation, Data were obtained during intracoronary infusions of CaCl2 (5, 10, and 15 mg/min) and compared with those during intracoronary infusions of dobutamine (2.5, 5.0, and 10.0 mu g/min). Results: CaCl2 caused dose-dependent increases in segmental shortening, accompanied by proportional increases in myocardial oxygen consumption. Although CaCl2 also increased coronary blood now, these increases were less than proportional to those in myocardial oxygen consumption, and therefore the myocardial oxygen extraction ratio increased. Dobutamine caused dose-dependent increases in segmental shortening and myocardial oxygen consumption that were similar in magnitude to those caused by CaCl2. In contrast to CaCl2, however, the accompanying increases in coronary blood now were proportional to the increases in myocardial oxygen consumption, with the result that the myocardial oxygen extraction ratio remained constant. Conclusions: Calcium has a coronary vasoconstricting effect and a positive inotropic effect in vivo. This vasoconstricting effect impairs coupling of coronary blood flow to the augmented myocardial oxygen demand by metabolic vascular control mechanisms. Dobutamine is an inotropic agent with no apparent direct action on coronary resistance vessels in vivo.