Effects of carbon dioxide on tetanic contraction of frog skeletal muscles studied by phosphorus nuclear magnetic resonance.

1. Bullfrog skeletal muscles were stimulated tetanically for 2 s at a sarcomere length of 2.4 microns in Ringer solution saturated with a gas mixture of O2 and CO2 of various proportions, at 4 degrees C. Using 31P nuclear magnetic resonance (31P-NMR), concentrations of inorganic phosphate (P(i)) and phosphocreatine (PCr) and intracellular pH (pHi) in whole muscle were measured in order to correlate the changes in these metabolites with the mechanical responses. 2. Resting muscles were first equilibrated to 5% CO2, then the CO2 concentration was increased up to 30%. This resulted in a rapid increase in [H+] (pH 6.72 in 30%, and 7.21 in 5% CO2) with little change in P(i) or PCr concentrations. When these muscles were given a single tetanic stimulation force fell in proportion to the decrease in pHi. 3. When a series of tetanic stimulations, with a long recovery period between tetani, was applied to the muscles in high CO2, force declined until a steady level was attained. A considerable increase in [P(i)] accompanied this whereas the decrease in pHi was relatively small. The force decline was much greater than could be predicted by the decrease in pHi alone. The concentration of ATP did not change significantly. 4. By subtracting the direct effect of [H+] on force a relationship between force and total P(i) concentration in the steady state of repeated contraction-recovery cycles was obtained. Force was suppressed linearly by increasing P(i) up to 30 mmol (1 fibre water)-1, while this relationship suggested an interaction between the effects of [P(i)] and [H+]. 5. These results have also shown that the inhibitory effects of CO2 on muscle contraction are dual. While elevation of [H+] directly suppresses contraction, this also accelerates P(i) accumulation in actively contracting muscles, which further suppresses contraction.