ACIDOSIS HAS NO EFFECT ON THE ATP CAST OF CONTRACTION IN CAT FAST-TWITCH AND SLOW-TWITCH SKELETAL-MUSCLES

Studies of shinned fibers suggest that the rate of ATP turnover in ske letal 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 (predomi nantly fast-twitc h) and soleus (slow-twitch) muscles under normocapnic (5% CO2) and hyp ercapnic (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 estimate d from PCr changes, measured by gating th acquisition of P-31-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) w ere not significantly different under hypercapnic compared with normoc apnic conditions in either muscle (mean: 7.97 and 2.44 mu mol . kg(-1) . s(-1) for biceps and soleus, respectively). Twitch force and the AT P cost per twitch decreased by nearly 50% during hypercapnic perfusion in both muscle types. Tile results indicate that hypercapnic acidosis has no significant effect on the ATPase rate per active myosin head i n intact mammalian skeletal muscle.