EFFECT OF MUSCLE MASS ON LACTATE FORMATION DURING EXERCISE IN HUMANS

To elucidate the mechanisms of lactate formation during submaximal exe rcise, eight men were studied during one- (1-LE) and two-leg (2-LE) ex ercise (approximately ii-min cycling) using the catheterization techni que and muscle biopsies (quadriceps femoris muscle). The absolute exer cise intensity and thus the energy demand for the exercising limb was the same [mean 114 (SEM 7) Wi during both 1-LE and 2-LE. At the end of exercise partial pressure of O-2 and O-2 saturation in femoral venous blood were lower and arterial adrenaline and noradrenaline were highe r during 2-LE than during 1-LE. Mean arterial blood lactate concentrat ion increased to 10.8 (SEM 0.8) (2-LE) and 5.2 (SEM 0.4) mmol.l(-1) (1 -LE) after 10 min of exercise. The intramuscular metabolic response to exercise was attenuated during 1-LE [mean, lactate=49 (SEM 9); glucos e 6-P=3.3 (SEM 0.3); nicotinamide adenine dinucleotide, reduced=0.17 ( SEM 0.02); adenosine 5'-diphosphate 2.7 (SEM 0.1) mmol kg dry mass(-1) ] compared to 2-LE [76 (SEM 6); 6.1 (SEM 0.7); 0.21 (SEM 0.02); 3.0 (S EM 0.1) mmol.kg dry mass(-1), respectively]. To elucidate whether the lower plasma adrenaline concentration could contribute to the attenuat ed metabolic response, additional experiments were performed on four o f the eight subjects with infusion of adrenaline during 1-LE (1-LEE). Average plasma adrenaline concentration was increased during 1-LEE and reached 2-4 times higher levels than during 2-LE. Post-exercise muscl e lactate and glucose 6-P contents were higher during 1-LEE than durin g 1-LE and were similar to those during 2-LE. Also, leg lactate releas e was elevated during 1-LEE versus 1-LE. It was concluded that during submaximal dynamic exercise the intramuscular metabolic response not o nly depended on the muscle power output, but also on the total muscle mass engaged. Plasma adrenaline concentrations and muscle oxygenation were found to be dependent upon the working muscle mass and both may h ave affected the metabolic response during exercise.