Fatty acid kinetics and carbohydrate metabolism during electrical exercisein spinal cord-injured humans

Motor center activity and reflexes from contracting muscle have been shown to be important for mobilization of free fatty acids (FFA) during exercise. We studied FFA metabolism in the absence of these mechanisms: during invol untary, electrically induced leg cycling in individuals with complete spina l cord injury (SCI). Healthy subjects performing voluntary cycling served a s controls (C). Ten SCI (level of injury: C-5-T-7) and six C exercised for 30 min at comparable oxygen uptake rates (similar to1 l/min), and [1-C-14] palmitate was infused continuously to estimate FFA turnover. From femoral a rteriovenous differences, blood flow, muscle biopsies, and indirect calorim etry, leg substrate balances as well as concentrations of intramuscular sub strates were determined. Leg oxygen uptake was similar in the two groups du ring exercise. In SCI, but not in C, plasma FFA and FFA appearance rate fel l during exercise, and plasma glycerol increased less than in C (P< 0.05). Fractional uptake of FFA across the working legs decreased from rest to exe rcise in all individuals (P< 0.05) but was always lower in SCI than in C (P < 0.05). From rest to exercise, leg FFA uptake increased less in SCI than i n C subjects (14 +/- 3 to 57 +/- 20 vs. 41 +/- 13 to 170 +/- 57 <mu>mol . m in(-1) . leg(-1); P< 0.05). Muscle glycogen breakdown, leg glucose uptake, carbohydrate oxidation, and lactate release were higher (P< 0.05) in SCI th an in C during exercise. Counterregulatory hormonal changes were more prono unced in SCI vs. C, whereas insulin decreased only in C. In conclusion, FFA mobilization, delivery, and fractional uptake are lower and muscle glycoge n breakdown and glucose uptake are higher in SCI patients during electrical ly induced leg exercise compared with healthy subjects performing voluntary exercise. Apparently, blood-borne mechanisms are not sufficient to elicit a normal increase in fatty acid mobilization during exercise. Furthermore, in exercising muscle, FFA delivery enhances FFA uptake and inhibits carbohy drate metabolism, while carbohydrate metabolism inhibits FFA uptake.