Skeletal muscle Na+/H+ exchange in rats: pH dependency and the effect of training.

Skeletal muscle Na+/H+ exchange was studied using giant sarcolemmal vesicles obtained from rat hind limb muscle. Experiments with either the 22Na tracer technique or with the Na+ sensitive fluorescent probe SBFI were conducted to determine the activity of the Na+/H+ exchanger, which was quantified from the amiloride or amiloride derivative 5-N-ethyl-N-propylamiloride (EIPA) sensitive Na+ influx. At a constant external pH of 7.4 the exchange system was close to half-activation at an internal pH of 7.2. A further activation was observed at lower internal pH values. The activity of the muscle Na+/H+ exchanger was elevated after 6 weeks of high-intensity treadmill training. In contrast, the activity of the system was unaffected by endurance training. The enhanced initial rate of amiloride-sensitive Na+/H+ exchange appears to be involved in the elevated in vivo (dynamical) buffer capacity reported for trained rats and human subjects, indicating that adaptive changes in the exchange system are of importance for pH regulation in association with high-intensity exercise.