Effect of carnitine loading on long-chain fatty acid oxidation,maximal exercise capacity,and nitrogen balance

Summary Carnitine has a potential effect on exercise capacity due to its role in the transport of long-chain fatty acids into the mitochondria for-oxidation, the export of acyl-coenzyme A compounds from mitochondria and the activation of branched-chain amino acid oxidation in the muscle. We studied the effect of carnitine supplementation on palmitate oxidation, maximal exercise capacity and nitrogen balance in rats. Daily carnitine supplementation (500 mg - kg^–1 body mass for 6 weeks) was given to 30 rats, 15 of which were on an otherwise carnitine-free diet (group 1) and 15 pair-fed with a conventional pellet diet (group II). A control group (group III,n = 6) was fed ad libitum the pellet diet. Palmitate oxidation was measured by collecting¹⁴CO₂ after an intraperitoneal injection of [1-¹⁴C]palmitate and exercise capacity by swimming to exhaustion. After carnitine supplementation carnitine concentrations in serum were supranormal [group I, total 150.8 (SD 48.5), free 78.9 (SD 18.4); group II, total 170.9 (SD 27.9), free 115.8 (SD 24.6) gmol·1^–1] and liver carnitine concentrations were normal in both groups [group I, total 1.6 (SD 0.3), free 1.2 (SD 0.2); group II, total 1.3 (SD 0.3), free 0.9 (SD 0.2) mol·g^–1 dry mass]. In muscle carnitine concentrations were normal in group I [total 3.8 (SD 1.2), free 3.2 (SD 1.0) mol · g^–1 dry mass] and increased in group II [total 6.6 (SD 0.5), free 4.9 (SD 0.9) mol·g^–1 dry mass]. Despite the difference in muscle carnitine concentrations there were no differences among the groups in cumulative palmitate oxidation after 3 h [group I, 39.7 (SD 11.6)%, group II, 29.6 (SD 14.0)%; group III, 36.5 (SD 10.8)% of injected activity] or swimming time to exhaustion [group I, 9.7 (SD 2.9); group II, 8.4 (SD 3.6); group III, 7.1 (SD 2.8) h]. A borderline increase in nitrogen balance was observed in group II. We concluded that increasing carnitine tissue concentrations by carnitine supplementation had no effect on palmitate oxidation and maximal exercise capacity in the rats studied.