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

Abstract

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 mM sodium lactate in the absence and presence of the CA inhibitors benzolamide (BZ), acetazolamide (AZ), chlorzolamide (CZ) and ethoxzolamide (EZ 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 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-HCO 3 . 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.

Surface carbonic anhydrase activity on astrocytes and neurons facilitates lactate transport

“…A similar observation could be anticipated for any cell type with surface CA and a significant permeability to lactic acid, as schematized in Figure 2B. In skeletal muscle, 20 mM lactate produced an acid shift that was reduced by roughly half in the presence of 10 M benzolamide (Wetzel et al, 2001). By contrast, the LIA in astrocytes was virtually abolished by the same concentration of drug.…”

“…In a recent study of skeletal muscle by Wetzel et al (2001), inhibition of the surface CA by benzolamide reduced inward lactate-H ϩ cotransport, demonstrating that buffering of surface H ϩ by this enzyme was critical for inward operation of the monocarboxylate transporter. Since lactate-H ϩ carriers are also found on astrocytes and neurons (Broer et al, 1997), a similar dependence on extracellular buffering may be anticipated.…”

“…In muscle, the ability of benzolamide to diminish lactate influx was attributed to the requirement of a rapid surface buffer, able to replenish the local H ϩ cotransported with lactate (Wetzel et al, 2001). Accordingly, benzolamide would be expected to have little or no effect on lactate influx in nominally CO 2 /HCO 3 Ϫ -free, HEPES-buffered media.…”

“…A consequence of this requirement is a need for efficient buffering close to the surface membrane, in order that loss of local H ϩ can be rapidly replenished (Wetzel et al, 2001). In skeletal muscle, surface buffering is catalyzed by an extracellular CA (Geers et al, 1985) that has been identified as CA IV .…”

Nonenzymatic Proton Handling by Carbonic Anhydrase II during H+-Lactate Cotransport via Monocarboxylate Transporter 1

“…In muscle cells, coexpression of CAII and acid/base-transporting proteins like NBC, NHE1, and also the MCTs have been found to be pivotal in acid/base homeostasis (20). In addition, it has been shown that extracellular carbonic anhydrase activity facilitates lactic acid transport in rat skeletal muscle fibers (21). In the brain CAII is highly expressed in astrocytes where it plays a supportive role in pH regulation (22) and supports the lactate shuttle from astrocytes to neurons (23).…”