LACTATE EFFLUX FROM FATIGUED FAST-TWITCH MUSCLE-FIBERS OF XENOPUS-LAEVIS UNDER VARIOUS EXTRACELLULAR CONDITIONS

1. Isolated, fast-twitch, low-oxidative muscle fibres from the iliofib ularis muscle of Xenopus laevis were fatigued by intermittent tetanic stimulation at 20 degrees C in different Ringer solutions and the amou nt of lactate released nas determined. 2. The rate of lactate efflux m as constant during 10 min of intermittent stimulation while lactate in the fibres accumulated, and lactate efflux was not hampered by an uns tirred layer surrounding the isolated muscle fibre. 3. The rate of lac tate efflux at extracellular pH 7.2 was the same as that at pH 7.8, bu t depended on the type of buffer used; the highest efflux rate (mean /- S.E.M., 7.4 +/- 2.2 mu mol min(-1) (g dry weight)(-1), n=8) was obs erved in bicarbonate-buffered Ringer solution. This rate was about 2.5 times higher than the rate in phosphate-buffered Ringer solution (2.9 +/- 1.3 mu mol min(-1) (g dry weight)(-1), n=8), indicating that lact ate-bicarbonate exchange is the most important route for lactate extru sion in vivo. 4. The highest rate of lactate efflux corresponds to a r ate of glycolytic ATP production which is only about 30% of the oxidat ive rate of ATP production (calculated from the maximum rate of oxygen consumption determined previously). 5. In the presence of 5 mM alpha- cyano-4-hydroxycinnamate (CHC) the lowest lac tate efflux rate (1.5 +/ - 0.6 mu mol min(-1) (g dry weight)(-1), n=16) was found. This rate ma s independent of the composition of the Ringer solution. Assuming that 5 mM CHC completely inhibits lactate transporters in the sarcolemma, the rate of lactate efflux in the presence of 5 mM CHC can be explaine d by passive diffusion, but only if most lactate is extruded via the T -tubules.