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

Abstract

Intracellular pH (pH_i) 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 pH_i 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), pH_i became more acid in both fibre types. Although H⁺ ions were not at electrochemical equilibrium across the surface membrane, active transport did not return pH_i to its original value during exposure to CO₂. The buffering powers calculated from the changes in pH_i 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 pH_i is acidified.