REGULATION OF CELLULAR PH IN SKELETAL-MUSCLE FIBER TYPES, STUDIED WITH SARCOLEMMAL GIANT VESICLES OBTAINED FROM RAT MUSCLES

Sarcolemmal giant vesicles obtained from rat hindlimb muscles were used as a model for the study of pH regulation in skeletal muscle. The transport systems involved in the recovery from 40 mM lactate and pH(i) 6.5 were quantified from both flux measurements of the co-transported ions and counter-ions, and from measurements of the rate of the internal pH change. The diffusion of lactic acid plus the carrier-mediated co-transport of lactate and H+ had the highest capacity to transport protons (240 nmol H+/mg protein per min). These systems are therefore responsible for a large part of the H+ efflux in periods with a high lactate production. The capacity of the HCO3--dependent systems was 47 nmol/mg per min, and the capacity of the Na+/H+ exchange system was 33 nmol/mg per min in vesicles from mixed muscles. The capacity to remove H+ by the lactate/H+ co-transport system and by the bicarbonate-dependent systems was significantly higher in vesicles from predominantly red fibers than in vesicles from white fibers, whereas the distribution of the Na+/H+ exchange system was independent of fiber type. These observations demonstrate that the pH regulation during muscle activity in red muscles is more effective than in white muscles.