Metabolic responses to moderate exercise in lambs with aortopulmonary shunts

In a previous study we found, after an overnight fast of 18 hours, a lower arterial glucose concentration and a depressed glycogenolysis in lambs with aortopulmonary left-to-right shunts. During exercise, glucose and free fatty acids (FFA) concentrations normally increase. The aim of this study was to investigate whether the shunt lambs could compensate for a depressed glycogenolysis by increasing gluconeogenesis and by increasing levels of blood substrates such as FFA and glycerol during exercise. Therefore, we investigated glucose kinetics, with [U-13C]glucose, in five 7-week-old shunt and 7 control lambs of a similar age, at rest and during moderate exercise (treadmill; 50% of o2 peak). The glucose production rate and the rate of disappearance of glucose were lower in shunt than in control lambs, both at rest and during exercise. We found no difference in metabolic clearance rate of glucose, glucose recycling, or gluconeogenesis between both groups of lambs. Glycogenolysis was at rest lower in shunt than in control lambs and tended to be lower during exercise. The arterial concentrations of pyruvate, lactate, FFA, and total and free glycerol increased during exercise in both groups of lambs. In conclusion, shunt lambs have lower arterial glucose concentrations than control lambs, both at rest and during moderate exercise. This was due to a lower glucose production rate, in particular a lower glycogenolysis. In addition, the reduced glycogenolysis rate was not offset by an increase in gluconeogenesis nor by an increase in other substrates that can be utilized by working muscles.