Intracellular pH and buffer power of type 1 and 2 fibres from skeletal muscle ofXenopus laevis

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Abstract
Intracellular pH (pHi) and buffering power of type 1 and type 2 fibres from the iliofibularis muscle of the clawed frog,Xenopus laevis, have been measured using pH-sensitive microelectrodes. In phosphate buffered Ringer's solution (extracellular pH 7.25, 20–22°C), mean pHi and its variance were similar in the two fibre types (6.86±SD 0.15±SEM 0.03,n=24, type 1, and 6.86±SD 0.12±SEM 0.03,n=15, type 2). On changing to Ringer's solution containing CO2 and HCO 3 − (extracellular pH 7.25, 20–22°C), pHi became more acid in both fibre types. Although H+ ions were not at electrochemical equilibrium across the surface membrane, active transport did not return pHi to its original value during exposure to CO2. The buffering powers calculated from the changes in pHi were not significantly different, 41.6 mmol·l−1 per pH unit (±SEM 4.0,n=17) for type 1 and 49.3 mmol·l per pH unit (±SEM 7.2,n=11) for type 2 fibres. Thus differences in the mechanical properties of these fibre types are not due simply to a difference of the intracellular pH or buffering of resting fibres. Other possible explanations are discussed for the changes in some contractile properties that occur when pHi is acidified.