Studies of skinned fibers suggest that the rate of
ATP turnover in skeletal muscle is depressed by
acidosis. To examine whether this occurs in intact muscles, the
ATP cost of isometric contractions was measured in ex vivo, arterially perfused cat biceps (predominantly fast-twitch) and soleus (slow-twitch) muscles under normocapnic (5% CO2) and hypercapnic (70% CO2) conditions.
Hypercapnia decreased extracellular pH from 7.4 to 6.7 and intracellular pH from 7.1 to 6.5 (soleus) or 6.6 (biceps) but had no significant effect on the
phosphocreatine (PCr)-to-
ATP ratio in muscles at rest. The
ATP cost of contraction was estimated from PCr changes, measured by gating the acquisition of 31P-nuclear magnetic resonance spectra to times before and after brief tetani (1 s at 100 Hz and 2 s at 25 Hz for biceps and soleus, respectively) or 10-s trains of twitches (2 and 1 Hz, respectively). Peak isometric force and the
ATP cost of tetanic contraction (PCr/force x time integral) were not significantly different under hypercapnic compared with normocapnic conditions in either muscle (mean: 7.97 and 2.44 micromol x kg(-1) x s(-1) for biceps and soleus, respectively). Twitch force and the
ATP cost per twitch decreased by nearly 50% during hypercapnic perfusion in both muscle types. The results indicate that hypercapnic
acidosis has no significant effect on the
ATPase rate per active
myosin head in intact mammalian skeletal muscle.