A STUDY OF LACTATE METABOLISM WITHOUT TRACER DURING PASSIVE AND ACTIVE POSTEXERCISE RECOVERY IN HUMANS

Tracers have been used extensively to study lactate metabolism in huma ns during rest and exercise. Nevertheless, quantification of in vivo l actate kinetics as measured by lactate tracers remains controversial a nd new data are necessary to clarify the issue. The present study has developed a simple kinetic model which does not require labelled molec ules and which yields proportional and quantitative information on lac tate metabolism in humans during postexercise recovery performed at di fferent levels of intensity. Five subjects took part in six experiment s each of which began with the same strenuous exercise (StrEx; 1 min, 385 W, 110 rpm). The StrEx of each session was followed by a different intensity of recovery: passive recovery (PR) and active recoveries (A R) with power outputs of 60, 90, 120, 150 and 180 W, respectively. Blo od lactate concentration was measured prior to and immediately after S trEX and regularly during the Ist h of recovery. Oxygen uptake (VO2) w as measured every 30 s during the whole session. The results showed th at the disappearance rate constant (k(e)) increases abruptly from PR [ 0.080 (SEM 0.004) min(-1)] to moderate AR [60 W: 0.189 (SEM 0.039) min (-1)] and decreases slowly during more intense AR [180 W: 0.125 (SEM 0 .027) min(-1)]. The lactate apparent clearance (Cl . F-1) was calculat ed from the area under the lactate concentration-time curve. The Cl . F-1 increased 1.81 (SEM 0.17) fold from PR to moderate AR (60 W) and o nly 1.31 (SEM 0.14) from PR to the most intense AR (180 W). Using the model, the apparent lactate production (F '' K-o) was also calculated. The F '' K-o increased regularly following a slightly curvilinear fun ction of VO2 and was 2.61 (SEM 0.53) fold greater during the most inte nse AR (180 W) than during PR. Because of the lack of data concerning the size of apparent lactate distribution volume (V-d), the apparent t urnover rate (R(bl)) has been presented here related to V-d. The R(bl) . V-d(-1) increased also following a slightly curvilinear function of VO2. The R(bl). V-d(-1) was 85.90 (SEM 14.42) mu mol . min(-1). l(-1) during PR and reached 314.09 (SEM 153.95) mu mol . min(-1). l(-1) duri ng the most intense AR (180 W). In conclusion the model presented here does not require labelled molecules and firstly makes it possible to follow the proportional change of apparent lactate clearance and appar ent lactate production during active postexercise recovery in comparis on with passive recovery conditions and secondly to estimate the blood lactate turnover.