EFFECTS OF FLOW AND CONTRACTION ON LACTATE TRANSPORT IN THE PERFUSED RAT HINDLIMB

A perfused rat hindlimb preparation was used to assess the effects of perfusate flow and electrical stimulation to mimic exercise on the rat es of lactate influx (measured by a dual tracer technique with [H-3]ma nnitol as the extracellular marker) and net lactate production. The sa me perfused muscle system was also used for assessing the effects of a lpha-cyano-4-hydroxycinnamate (CIN, 15 mM), phloretin (0.6 mM), and py ruvate on tracer lactate influx. Unidirectional lactate influx, oxygen uptake (VO2), and net lactate flux were all significantly dependent o n perfusate flow rate (all P < 0.05). The hindlimb was in net lactate production at all flow rates studied. Electrical stimulation (60 Hz, 1 00 ms, 20 V trains at 0.6 min(-1)) at perfusate lactate concentration of 1 mM significantly increased the hindlimb riot from 8.0 +/- 1.1 to 16.0 +/- 2.2 ml.kg(-1).min(-1) and production of lactate from -69 +/- 31 to -823 +/- 77 nmol.min(-1).g(-1) (both P < 0.001) but did not affe ct tracer-measured unidirectional lactate influx (nonstimulated: 235.4 +/- 78.1; stimulated: 235.0 +/- 31.0 nmol.min(-1).g(-1)). At a perfus ate flow of 0.55 ml.g(-1).min(-1) the unidirectional influx of 1 mM la ctate was markedly inhibited (90 +/- 5%) by 15 mM CIN. CIN also signif icantly reduced VO2 from 6.2 +/- 0.16 to 4.45 +/- 0.57 ml.kg(-1).min(- 1) (P < 0.05, n = 5). Phloretin (0.6 mM, n = 3) had no significant eff ect on lactate influx. Varying perfusate pyruvate concentration from 5 to 50 mM inhibited tracer lactate influx equally effectively as simil ar concentrations of lactate; the affinity of the process seemed to be similar for both molecules. These results add further experimental ev idence to suggest the presence of a lactate transporter in skeletal mu scle membrane and provide additional information concerning the physio logical characteristics of lactate transport in and out of muscle.