LACTATE AND H+ UPTAKE IN INACTIVE MUSCLES DURING INTENSE EXERCISE IN MAN

1. The present study examined how uptake of lactate and H+ in resting muscle is affected by blood flow, arterial lactate concentration and m uscle metabolism. 2. Six male subjects performed intermittent arm exer cise in two separate 32 min periods (Part I and Part II) and in one su bsequent 20 min period in which one leg knee-extensor exercise was als o performed (Part III). The exercise was performed at various intensit ies in order to obtain different steady-state arterial blood lactate c oncentrations. In the inactive leg, femoral venous blood flow (drainin g about 7.7 kg of muscles) was measured and femoral arterial and venou s blood was collected frequently. Biopsies were taken from m, vastus l ateralis of the inactive leg at rest and 10 and 30 min into both Part I and Part II as well as 10 min into recovery from Part II. 3. The art erial plasma lactate concentrations were 7, 9 and 16 mmol l(-1) after 10 min of Parts I, II and III, respectively, and the corresponding art erial-venous difference (a-v(diff)) for lactate in the resting leg was 1.3, 1.4 and 2.0 mmol l(-1). The muscle lactate concentration was 2.8 mmol (kg wet wt)(-1) after 10 min of Part I and remained constant thr oughout the experiment. During Parts I and II, a-v(diff) lactate decre ased although the arterial lactate concentration and plasma-muscle lac tate gradient were unaltered throughout each period. Thus, membrane tr ansport of lactate decreased during each period. 4. Blood flow in the inactive leg was about 2-fold higher during arm exercise compared to t he rest periods, resulting in a 2-fold higher lactate uptake. Thus, la ctate uptake by inactive muscles was closely related to blood flow. 5. Throughout the experiment a-v(diff) for actual base excess and for la ctate were of similar magnitude. Thus, in inactive muscles lactate upt ake appears to be coupled to the transport of H+.