Abstract

To determine whether the increase in cardiac output during mild to moderate exercise is related to an increase in the tissue redox potential, we compared the responses of cardiac output, total body oxygen consumption, and arterial blood lactate-to-pyruvate ratio (a measure of NADH/NAD) to treadmill exercise between dogs treated with normal saline and those treated with a hydrogen acceptor, new methylene blue. Normal saline was infused into the left atrium in the first group of dogs at a rate of 0.38 ml/min throughout the treadmill exercise (2.5 mph and 5.0 mph on a 6 incline, each for 20 min). In the second group, methylene blue was administered as a loading dose (4 mg/kg) before exercise, followed by a continuous infusion (0.15 mg/kg/min) throughout exercise. A similar infusion of methylene blue was given to a third group of dogs without exercise; it reduced the arterial lactate-to-pyruvate ratio from 6.70+-0.35 to 4.12+0.27 but had no or little effects on cardiac output, heart rate, arterial pressure, and left ventricular dP/dt and (dP/dt)/P. Treadmill exercise doubled cardiac output and increased total body O2 consumption three- to fourfold in the first two groups but increased arterial blood lactate-topyruvate ratio only in group 1 (6.0+-0.54 to 9.97+-0.91). The relationship between cardiac output and total body O2 consumption was unaffected by the simultaneous administration of methylene blue during exercise. Groups 1 and 2 also did not differ in their heart rate, left ventricular dP/dt and (dP/dt)/P, and plasma catecholamine responses to exercise. Thus we conclude that changes in arterial lactate-to-pyruvate ratios of the magnitude observed in this study are not causally related to the circulatory stimulation occurring during mild to moderate exercise. Verf.-Referat