The influence of muscle respiration and glycolysis on surface and intracellular pH in fibres of the rat soleus.

Extracellular pH (pHo) and intracellular pH (pHi) of superficial fibres of the rat soleus muscle were measured in vitro using pH-sensitive glass micro-electrodes. The origin of the pH gradient existing between the bulk phase of extracellular solution and the surface of muscle fibres was investigated. The pHo decreased almost linearly over a distance of 285 microns from bulk solution to fibre surface. The magnitude of the bulk-surface pH gradient is greater in the mid region of the muscle than close to the tendon. Decreasing the superfusate velocity increased the magnitude of the pH gradient. Reducing the buffer capacity of the superfusing solution had the same effect. Inhibiting the aerobic metabolism or stimulating it acidified the fibre surface. Inhibiting glycolysis alone, or both aerobic metabolism and glycolysis, alkalinized the fibre surface. Inhibiting the membrane ionic exchange process involved in pHi regulation had no effect on surface pH. Changing the rate of aerobic or anaerobic metabolism quickly modified pHi in most cases. In conclusion the bulk-surface pH gradient seems to result mainly from diffusion of CO2 and lactic acid across an unstirred layer of fluid covering the surface of muscle fibres.