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In a previous study [G. C. M. Beaufort-Krol, J. Takens, M. C. Molenkamp, G. B. Smid, J. J. Meuzelaar, W. G. Zijlstra, and J. R. G. Kuipers. Am. J. Physiol. 275 (Heart Circ. Physiol. 44): H1503-H1512, 1998], a lower systemic O-2 supply was found in lambs with aortopulmonary left-to-right shunts. To determine whether the lower systemic O-2 supply results in increased anaerobic metabolism, we used [1-C-13]lactate to investigate lactate kinetics in eight 7-wk-old lambs with shunts and eight control lambs, at rest and during moderate exercise [treadmill; 50% of peak O-2 consumption ((V) over dot O-2)]. The mean left-to-right shunt fraction in the shunt lambs was 55 +/- 3% of pulmonary blood flow. Arterial lactate concentrations and the rate of appearance (R-a) and disappearance (R-d) Of lactate were similar in shunt and control lambs, both at rest (lactate: 1,201 +/- 76 vs. 1,214 +/- 151 mu mol/l; R-a = R-d: 12.97 +/- 1.71 vs. 12.55 +/- 1.25 mu mol.min(-1).kg(-1)) and during a similar relative workload. We found a positive correlation between R-a and systemic blood flow, O-2 supply, and (V) over dot O-2 in both groups of lambs. In conclusion, shunt lambs have similar lactate kinetics as do control lambs, both at rest and during moderate exercise at a similar fraction of their peak (V) over dot O-2, despite a lower systemic O-2 supply.

In a previous study [G. C. M. Beaufort-Krol, J. Takens, M. C. Molenkamp, G. B. Smid, J. J. Meuzelaar, W. G. Zijlstra, and J. R. G. Kuipers. Am. J. Physiol. 275 (Heart Circ. Physiol. 44): H1503-H1512, 1998], a lower systemic O-2 supply was found in lambs with aortopulmonary left-to-right shunts. To determine whether the lower systemic O-2 supply results in increased anaerobic metabolism, we used [1-C-13]lactate to investigate lactate kinetics in eight 7-wk-old lambs with shunts and eight control lambs, at rest and during moderate exercise [treadmill; 50% of peak O-2 consumption ((V) over dot O-2)]. The mean left-to-right shunt fraction in the shunt lambs was 55 +/- 3% of pulmonary blood flow. Arterial lactate concentrations and the rate of appearance (R-a) and disappearance (R-d) Of lactate were similar in shunt and control lambs, both at rest (lactate: 1,201 +/- 76 vs. 1,214 +/- 151 mu mol/l; R-a = R-d: 12.97 +/- 1.71 vs. 12.55 +/- 1.25 mu mol.min(-1).kg(-1)) and during a similar relative workload. We found a positive correlation between R-a and systemic blood flow, O-2 supply, and (V) over dot O-2 in both groups of lambs. In conclusion, shunt lambs have similar lactate kinetics as do control lambs, both at rest and during moderate exercise at a similar fraction of their peak (V) over dot O-2, despite a lower systemic O-2 supply.

Increased myocardial fatty acid uptake during acute exercise could adversely affect myocardial O2 consumption in lambs with left-to-right shunts, which would be unfavorable in view of their decreased coronary blood flow reserve. Therefore, we studied myocardial substrate uptake (glucose, lactate, pyruvate, free fatty acids, triglycerides, beta-hydroxybutyrate, and acetoacetate) in 10 7-wk-old lambs with an aortopulmonary left-to-right shunt [61 +/- 3% (SE) of left ventricular output] and 9 control lambs during strenuous treadmill exercise. The hemodynamic reaction to exercise was similar in shunt and control lambs. The peripheral metabolic response to exercise was also similar in the two groups: glucose, free fatty acids, and, most prominently, lactate concentrations increased. Myocardial O2 consumption increased but less in shunt than in control lambs because of a smaller increase in heart rate. In both groups myocardial lactate uptake increased substantially at the cost of other substrates, providing the heart with 40% of its oxidative metabolism. Fatty acid uptake was not different between the two groups. In conclusion, our data reveal no essential differences in myocardial substrate uptake between shunt and control lambs during a substantial circulatory load.

Increased myocardial fatty acid uptake during acute exercise could adversely affect myocardial O2 consumption in lambs with left-to-right shunts, which would be unfavorable in view of their decreased coronary blood flow reserve. Therefore, we studied myocardial substrate uptake (glucose, lactate, pyruvate, free fatty acids, triglycerides, beta-hydroxybutyrate, and acetoacetate) in 10 7-wk-old lambs with an aortopulmonary left-to-right shunt [61 +/- 3% (SE) of left ventricular output] and 9 control lambs during strenuous treadmill exercise. The hemodynamic reaction to exercise was similar in shunt and control lambs. The peripheral metabolic response to exercise was also similar in the two groups: glucose, free fatty acids, and, most prominently, lactate concentrations increased. Myocardial O2 consumption increased but less in shunt than in control lambs because of a smaller increase in heart rate. In both groups myocardial lactate uptake increased substantially at the cost of other substrates, providing the heart with 40% of its oxidative metabolism. Fatty acid uptake was not different between the two groups. In conclusion, our data reveal no essential differences in myocardial substrate uptake between shunt and control lambs during a substantial circulatory load.