Extracellular carbonic anhydrase activity facilitates lactic acid transport in rat skeletal muscle fibres

1. In skeletal muscle an extracellular sarcolemmal carbonic anhydrase (CA) has been demonstrated. We speculate that this CA accelerates the interstitial CO2/HCO3- buffer system so that H+ ions can be rapidly delivered or buffered in the interstitial fluid. Because > 80% of the lactate which crosses the sarcolemmal membrane is transported by the H+-lactate cotransporter, we examined the contributions of extracellular and intracellular CA to lactic acid transport, using ion-selective microelectrodes for measurements of intracellular pH (pHi) and fibre surface pH (pHs) in rat extensor digitorum longus (EDL) and soleus fibres. 2. Muscle fibres were exposed to 20 mM sodium lactate in the absence and presence of the CA inhibitors benzolamide (BZ), acetazolamide (AZ), chlorzolamide (CZ) and ethoxzolamide (EZ). The initial slopes (dpHs/dt, dpHi/dt) and the amplitudes (ΔpHs, ΔpHi) of pH changes were quantified. From dpHi/dt, ΔpHi, and the total buffer factor (BFtot) the lactate fluxes (mM min-1) and intracellular lactate concentrations ([lactate]i) were estimated. 3. BFtot was obtained as the sum of the non-HCO3- buffer factor (BFnon-HCO3) and the HCO3- buffer factor (BFHCO3). BFnon-HCO3 was 35±4 mM ΔpH-1 for the EDL (n = 14) and 86±16 mM ΔpH-1 for the soleus (n = 14). 4. In soleus, 10 mM cinnamate inhibited lactate influx by 44% and efflux by 30%; in EDL, it inhibited lactate influx by 37% and efflux by 20%. Cinnamate decreased [lactate]i, in soleus by 36% and in EDL by 45%. In soleus, 1 mM DIDS reduced lactate influx by 18% and efflux by 16%. In EDL, DIDS lowered the influx by 27% but had almost no effect on efflux. DIDS reduced [lactate]i by 20% in soleus and by 26% in EDL. 5. BZ (0.01 mM) and AZ (0.1 mM), which inhibit only the extracellular sarcolemmal CA, led to a significant increase in dpHs/dt and ΔpHs by about 40%-150% in soleus and EDL. BZ and AZ inhibited the influx and efflux of lactate by 25%-50% and reduced [lactate]i by about 40%. The membrane-permeable CA inhibitors CZ (0.5 mM) and EZ (0.1 mM), which inhibit the extracellular as well as the intracellular CAs, exerted no greater effects than the poorly permeable inhibitors BZ and AZ did. 6. In soleus, 10 mM cinnamate inhibited the lactate influx by 47%. Addition of 0.01 mM BZ led to a further inhibition by only 10%. BZ alone reduced the influx by 37%. 7. BZ (0.01 mM) had no influence on the Km value of the lactate transport, but led to a decrease in maximal transport rate (Vmax). In EDL, BZ reduced Vmax by 50% and in soleus by about 25%. 8. We conclude that the extracellular sarcolemmal CA plays an important role in lactic acid transport, while internal CA has no effect, a difference most likely attributable to the high internal vs. low extracellular BFnon-HCO3. The fact that the effects of cinnamate and BZ are not additive indicates that the two inhibitors act at distinct sites on the same transport pathway for lactic acid.