INCREASED MYOCARDIAL LACTATE OXIDATION IN LAMBS WITH AORTOPULMONARY SHUNTS AT REST AND DURING EXERCISE

Free fatty acids are the major fuels for the myocardium, but during a higher load carbohydrates are preferred. Previously, we demonstrated t hat myocardial net lactate uptake was higher in lambs with aortopulmon ary shunts than in control lambs. To determine whether this was caused by an increased lactate uptake and oxidation or by a decreased lactat e release, we studied myocardial lactate and glucose metabolism with C -13-labeled substrates in 36 lambs in a fasting, conscious state. The lambs were assigned to two groups: a resting group consisting of 8 shu nt and 9 control lambs, and an exercise group (50% of peak O-2 consump tion) consisting of 9 shunt and 10 control lambs. Myocardial lactate o xidation was higher in shunt than in control lambs (mean +/- SE, rest: 10.33 +/- 2.61 vs. 0.17 +/- 0.82, exercise: 38.05 +/- 8.87 vs. 16.89 +/- 4.78 mu mol.min(-1).100 g(-1); P < 0.05). There was no difference in myocardial lactate release between shunt and control lambs. Oxidati on of exogenous glucose, which was approximately zero at rest, increas ed during exercise in shunt and control lambs. The contribution of glu cose and lactate to myocardial oxidative metabolism increased during e xercise compared with at rest in both shunt and control lambs. We conc lude that myocardial lactate oxidation is higher in shunt than in cont rol lambs, both at rest and during exercise, and that the contribution of carbohydrates in myocardial oxidative metabolism in shunt lambs is higher than in control lambs. Thus it appears that this higher contri bution of carbohydrates occurs not only in the case of pressure-overlo aded hearts but also in myocardial hypertrophy due to volume overloadi ng.