Effect of intracellular pH on force and heat production in isometric contraction of frog muscle fibres.

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Abstract
The intracellular pH (pHi) of live fibres from the anterior tibialis of the frog Rana temporaria was measured at 10.degree.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.degree.C. In parallel experiments force and rate of heat production in 4 s isometric tetani at 10.degree.C were measured, and compared to control observations (5% CO2, pHi 6.80). 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). As the fibres became more alkaline (to pHi 7.26), force increased slightly (by a maximum of 0.03 of the control value) but heart rate did not increase. When the dependence on pH of the molar enthalpy change for phosphocreatine spitting is taken in to account, these results indicate that the force-time integral per cross-bridge cycle increases with pHi over this range.