LACTATE UPTAKE BY SKELETAL-MUSCLE SARCOLEMMAL VESICLES DECREASES AFTER 4 WK OF HINDLIMB UNWEIGHTING IN RATS

We investigated the effects of 4 wk of hypodynamia on the rate of lact ate transport in skeletal muscle sarcolemmal vesicles from control and hindlimb-suspended rats. Characterization of the sarcolemmal preparat ions was achieved with a marker enzyme (K+-p-nitrophenylphosphatase) a nd measurement of 1 mM [U-C-14]lactate transport activity under zero-t rans conditions with or without a pH gradient or the transport inhibit or alpha-hydroxycinnamate. Preparations from the two groups were not s ignificantly different concerning yield and purification. Based on the se results, we used this model to analyze the lactate transport activi ty after hypodynamia by tail suspension. Hindlimb suspension caused a shift from slow to fast myosin heavy chain isoforms in soleus muscles with a 40% decrease in the citrate synthase activity (from 35.3 +/- 3. 7 to 21.4 +/- 2.1 mu mol . g(-1). min(-1); P < 0.05). Lactate (1 mM) u ptake in vesicles from the two groups was a function of time, and the rate after hindlimb suspension was significantly decreased in the susp ended compared with the control group (2.25 +/- 0.44 and 3.50 +/- 0.26 nmol . min(-1). mg protein(-1), respectively; P < 0.05). These differ ences were not observed for a higher lactate concentration (50 mM). Th ese results suggest that the level of physical activity plays a role i n the regulation of sarcolemmal lactate transport activity implicated in the exchanges of lactate between producing and utilizing cells, org ans, and tissues, which are major ways of carbohydrate energy distribu tion in humans and others species.