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 rates of lactate influx (measured by a dual tracer technique with [³H]mannitol as the extracellular marker) and net lactate production. The same perfused muscle system was also used for assessing the effects of α-cyano-4-hydroxycinnamate (CIN, 15 mM), phloretin (0.6 mM), and pyruvate on tracer lactate influx. Unidirectional lactate influx, oxygen uptake (V̇O₂), and net lactate flux were all significantly dependent on perfusate flow rate (all P < 0.05). The hindlimb was in net lactate production at all flow rates studied. Electrical stimulation (60 Hz, 100 ms, 20 V trains at 0.6 min^-1) at perfusate lactate concentration of 1 mM significantly increased the hindlimb V̇O₂ 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 affect tracer-measured unidirectional lactate influx (nonstimulated: 235.4 ± 78.1; stimulated: 235.0 ± 31.0 nmol · min^-1 · g^-1). At a perfusate flow of 0.55 ml · g^-1 · min^-1 the unidirectional influx of 1 mM lactate was markedly inhibited (90 ± 5%) by 15 mM CIN. CIN also significantly reduced V̇O₂ 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 effect on lactate influx. Varying perfusate pyruvate concentration from 5 to 50 mM inhibited tracer lactate influx equally effectively as similar concentrations of lactate; the affinity of the process seemed to be similar for both molecules. These results add further experimental evidence to suggest the presence of a lactate transporter in skeletal muscle membrane and provide additional information concerning the physiological characteristics of lactate transport in and out of muscle.