MYOCARDIAL LACTATE METABOLISM DURING EXERCISE

The heart consumes lactate under resting conditions in normal healthy people. A limited number of studies have measured lactate exchange across the heart during exercise by using simultaneous arterial and coronary sinus catheterization. In general, exercise results in an increase in the rate of lactate uptake, which is due both to the increases in myocardial blood flow and lactate extraction from rest to exercise. Lactate extraction by the myocardium during submaximal exercise (40-60% VO2max) is largely dependent upon the concentration of lactate in arterial blood. Studies using a continuous infusion of C-14-lactate tracer have demonstrated that essentially all of the lactate taken up during exercise is immediately oxidized to CO2 in the myocardium. In addition, lactate tracer studies indicate that healthy myocardium simultaneously consumes and produces lactate under conditions of net lactate consumption. Moderate intensity exercise (40% VO2max) does not result in an increase in the rate of myocardial lactate production above resting values. Thus, the heart takes up lactate in proportion to the rate of lactate delivery to the myocardium both at rest and during exercise. Exercise that elicits an increase in the arterial lactate concentration above resting values results in an increase in the relative contribution of lactate oxidation to myocardial oxidative metabolism.