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 importanc e of this mechanism in vivo remains controversial. Methods: The left a nterior descending coronary arteries of 20 anesthetized dogs whose che sts had been opened were perfused at 80 mmHg, Myocardial segmental sho rtening was measured with ultrasonic crystals and coronary blood now w ith a Doppler now transducer. The coronary arteriovenous oxygen differ ence was determined and used to calculate myocardial oxygen consumptio n and the myocardial oxygen extraction ratio, The myocardial oxygen ex traction ratio served as an index of effectiveness of metabolic vasodi lation, Data were obtained during intracoronary infusions of CaCl2 (5, 10, and 15 mg/min) and compared with those during intracoronary infus ions of dobutamine (2.5, 5.0, and 10.0 mu g/min). Results: CaCl2 cause d dose-dependent increases in segmental shortening, accompanied by pro portional increases in myocardial oxygen consumption. Although CaCl2 a lso increased coronary blood now, these increases were less than propo rtional to those in myocardial oxygen consumption, and therefore the m yocardial oxygen extraction ratio increased. Dobutamine caused dose-de pendent increases in segmental shortening and myocardial oxygen consum ption that were similar in magnitude to those caused by CaCl2. In cont rast to CaCl2, however, the accompanying increases in coronary blood n ow 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 o xygen demand by metabolic vascular control mechanisms. Dobutamine is a n inotropic agent with no apparent direct action on coronary resistanc e vessels in vivo.