l(+)-Lactate transport perfused rat skeletal muscle: kinetic characteristics and sensitivity to pH and transport inhibitors

We have examined lactate uptake (as the rate of net muscle lactate accumulation) and unidirectional inward transport (measured by a paired-tracer dilution method) in muscle of the perfused skinned rat hindlimb. Inhibition of tracer influx (fractional uptake at 1 mM l(+)-lactate, 43.3 ± 3.1% but only 32.9 ± 1.8% at 50 mM lactate) suggested some competition between tracer and native forms of the carboxylate for transport. d(-)-lactate (50 mM) did not inhibit uptake of tracer l(+)-lactate. Pyruvate (25 mM), but none of five other monocarboxylates, inhibited uptake of tracer lactate, by 22% (P < 0.01). Altering perfusate pH from 7.4 to 6.8 caused a 36% increase (P < 0.001) in the unidirectional l(+)-lactate transport at 1 mM l(+)-lactate, whereas increasing pH to 7.7 reduced transport by 18% (P < 0.01). Tracer lactate influx was inhibited by 500 μM 4-acetamido-4′-isothiocyanostilbene (SITS) (19%), 5 mM α-cyano-4-hydroxycinnamic acid (CIN) (20-30%), 1 mM amiloride (27%) and by a thiol group reagent p-chloromercuribenzenesulphonic acid (pCMBS) (26%). Overall the results indicate that at least two processes are involved in the transfer of lactate: one, saturable, with a V_max of 0.84 μmol · min^-1 · g^-1 and an apparent K_m of 21 mM was sensitive to SITS, CIN, and a thiol group reagent; the other was non-saturable and insensitive to SITS and CIN with an apparent rate constant of 0.1 min^-1.