The present study was performed to investigate the effect of
acidosis on the efflux of
ATP from skeletal muscle. Infusion of
lactic acid to the perfused hindlimb muscles of anaesthetised rats produced dose-dependent decreases in pH and increases in the interstitial
ATP of extensor digitorum longus (EDL) muscle: 10 mM
lactic acid reduced the venous pH from 7.22 ± 0.04 to 6.97 ± 0.02 and increased interstitial
ATP from 38 ± 8 to 67 ± 11 nM. The increase in interstitial
ATP was well-correlated with the decrease in pH (r(2) = 0.93; P < 0.05). Blockade of cellular uptake of
lactic acid using α-cyano-
hydroxycinnamic acid abolished the
lactic acid-induced
ATP release, whilst infusion of
sodium lactate failed to depress pH or increase interstitial
ATP, suggesting that intracellular pH depression, rather than
lactate, stimulated the
ATP efflux. Incubation of cultured skeletal myoblasts with 10 mM
lactic acid significantly increased the accumulation of
ATP in the bathing medium from 0.46 ± 0.06 to 0.76 ± 0.08 μM, confirming the skeletal muscle cells as the source of the released
ATP.
Acidosis-induced
ATP efflux from the perfused muscle was abolished by
CFTR(inh)-172, a specific inhibitor of the
cystic fibrosis transmembrane conductance regulator (CFTR), or
glibenclamide, an inhibitor of both K(
ATP) channels and CFTR, but it was not affected by
atractyloside, an inhibitor of the mitochondrial
ATP transporter. Silencing of the CFTR gene using an
siRNA abolished the
acidosis-induced increase in
ATP release from cultured myoblasts. CFTR expression on skeletal muscle cells was confirmed using immunostaining in the intact muscle and Western blotting in the cultured cells. These data suggest that depression of the intracellular pH of skeletal muscle cells stimulates
ATP efflux, and that CFTR plays an important role in the release mechanism.