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Extracellular carbonic anhydrase activity facilitates lactic acid transport in rat skeletal muscle fibres
Author(s) -
Wetzel Petra,
Hasse Anke,
Papadopoulos Simon,
Voipio Juha,
Kaila Kai,
Gros Gerolf
Publication year - 2001
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2001.0743h.x
Subject(s) - chemistry , extracellular , carbonic anhydrase , lactic acid , dids , acetazolamide , intracellular , intracellular ph , ion transporter , biochemistry , biophysics , efflux , skeletal muscle , anatomy , medicine , enzyme , biology , membrane , bacteria , genetics
1 In skeletal muscle an extracellular sarcolemmal carbonic anhydrase (CA) has been demonstrated. We speculate that this CA accelerates the interstitial CO 2 /HCO 3 − 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 (pH i ) and fibre surface pH (pH s ) in rat extensor digitorum longus (EDL) and soleus fibres. 2 Muscle fibres were exposed to 20 m m sodium lactate in the absence and presence of the CA inhibitors benzolamide (BZ), acetazolamide (AZ), chlorzolamide (CZ) and ethoxzolamide (EZ). The initial slopes (dpH s /d t , dpH i /d t ) and the amplitudes (ΔpH s , ΔpH i ) of pH changes were quantified. From dpH i /d t , ΔpH i and the total buffer factor (BF tot ) the lactate fluxes (m m min −1 ) and intracellular lactate concentrations ([lactate] i ) were estimated. 3 BF tot was obtained as the sum of the non‐HCO 3 − buffer factor (BF non‐HCO3 ) and the HCO 3 − buffer factor (BF HCO3 ). BF non‐HCO3 was 35 ± 4 m m ΔpH −1 for the EDL (n = 14) and 86 ± 16 m m ΔpH −1 for the soleus (n = 14). 4 In soleus, 10 m m 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 m m 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 m m ) and AZ (0.1 m m ), which inhibit only the extracellular sarcolemmal CA, led to a significant increase in dpH s /d t and ΔpH s 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 m m ) and EZ (0.1 m m ), 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 m m cinnamate inhibited the lactate influx by 47 %. Addition of 0.01 m m BZ led to a further inhibition by only 10 %. BZ alone reduced the influx by 37 %. 7 BZ (0.01 m m ) had no influence on the K m value of the lactate transport, but led to a decrease in maximal transport rate ( V max ). In EDL, BZ reduced V max 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 BF non‐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.