1. The intracellular pH (pHi) of live fibres from the anterior tibialis of the frog Rana temporaria was measured at 10 degrees C (using pH-sensitive microelectrodes) in Ringer solutions containing a fixed bicarbonate concentration (20 mM) and varying PCO2 concentrations of 0.5-54%. As extracellular pH was changed from 7.99 to 6.00, mean pHi changed from 7.24 to 5.97. Similar results were obtained at 20 degrees C. 2. In parallel experiments force and rate of heat production in 4 s isometric tetani at 10 degrees C were measured, and compared to control observations (5% CO2, pHi 6.80). 3. As the fibres became more acid (to pHi 5.95), force and heat rate were progressively reduced (to 0.75 and 0.71 of the control values, respectively). 4. As the fibres became more alkaline (to pHi 7.26), force increased slightly (by a maximum of 0.03 of the control value) but heat rate did not increase. 5. When the dependence on pH of the molar enthalpy change for phosphocreatine splitting is taken into account, these results indicate that the force-time integral per cross-bridge cycle increases with pHi over this range.
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