MUSCLE BUFFER CAPACITY ESTIMATED FROM PH CHANGES DURING REST-TO-WORK TRANSITIONS

Gated phosphorus nuclear magnetic resonance (31P-NMR) spectra were acquired after 5 or 9 s of 5-Hz stimulation in rat and cat skeletal muscles, respectively. Net phosphocreatine (PCr) hydrolysis was associated with an intracellular alkalinization of 0.08 ± 0.01 and 0.05 ± 0.003 pH units in isolated perfused cat biceps and soleus, respectively, and 0.12 ± 0.02 in the superficial predominantly fast-twitch white portion of gastrocnemius of anesthetized rats. The net change in [H+] expected from PCr hydrolysis was calculated, and apparent buffer capacity (β) in intact muscles was calculated from β = Δ[H+]/ΔpH. The β of the same muscle types was also estimated from titration of muscle homogenates between pH 6.0 and 8.0. The contribution of P(i) to total β of the homogenates was subtracted to ascertain the non-P(i) β for each muscle. The non-P(i) β values were added to the actual amount of P(i) present in the stimulated muscles to calculate a predicted β at pH 7. The apparent β calculated from PCr and pH changes in intact muscles and the predicted β from homogenate titrations were in good agreement (38 ± 9 vs. 38 slykes in cat biceps, 21 ± 7 vs. 30 in cat soleus, and 30 ± 6 vs. 27 in rat gastrocnemius). The results indicate that changes in pH during the first few seconds of contraction can be entirely accounted for by proton consumption via net PCr hydrolysis.