An investigation of the ionic mechanism of intracellular pH regulation in mouse soleus muscle fibres

Intracellular pH (pH i ) of surface fibres of the mouse soleus muscle was measured in vitro by recessed‐tip pH‐sensitive micro‐electrodes. pH i was displaced in an acid direction by removal of external (NH 4 ) 2 SO 4 after a short exposure, and the mechanism of recovery from this acidification was investigated. 2. Removal of external K caused a very slow acidification (probably due to the decreasing Na gradient) but had no effect on the rate of pH i recovery following acidification. This indicates that K + —H + exchange is not involved in the pH i regulating system. 3. Short applications of 10 −4 M ouabain had no obvious effect on pH i and did not alter the rate of pH i recovery following acidification. This suggests that there is no direct connexion between the regulation of pH i and the Na pump. 4. Reduction of external Ca from 10 to 1 m M caused a transient fall in pH i , but the rate of pH i recovery following acidification was unaffected. This suggests that Ca 2+ —H + exchange is not involved in the pH i regulating system. 5. An 11% reduction in external Na caused a significant slowing of pH i recovery following acidification. 90% or complete removal of external Na almost stopped pH i recovery. This suggests that Na + —H + exchange is involved in pH i regulation. 6. Amiloride (10 −4 M ) reversibly reduced the rate of pH i recovery to much the same extent as removal of external Na. Its effect was not additive to that of removal of external Na. 7. Internal Na ion concentration (Na + i ), measured using Na + ‐sensitive micro‐electrodes, fell on application of (NH 4 ) 2 SO 4 and increased on its removal. The increase transiently raised Na + i above the level recorded before (NH 4 ) 2 SO 4 application. This overshoot of Na + i was almost completely inhibited by amiloride. This is consistent with the involvement of Na + —H + exchange in the pH i regulating system. 8. Removal of external CO 2 or application of SITS (10 −4 M ) caused some slowing of the rate of pH i recovery following acidification by removal of (NH 4 ) 2 SO 4 . The effect of SITS was additive to that of Na‐free Ringer or amiloride. These results suggest that Cl − —HCO 3 − exchange is also involved in the pH i regulating system and that it is a separate mechanism. Under the conditions used, Cl − —HCO 3 − exchange formed about 20% of the pH i regulating system. 9. Decreasing the temperature from 37 to 28 °C not only caused an increase in pH i , but also considerably slowed the rate of pH i recovery following acidification. We have calculated a Q 10 for Na + —H + exchange of 1·4 and for Cl − —HCO 3 − exchange, 6·9. 10. We conclude that the pH i regulating system is comprised of two separate ionic exchange mechanisms. The major mechanism is Na + —H + exchange, which is probably driven by the transmembrane Na gradient. The other mechanism is Cl − —HCO 3 − exchange, which probably requires metabolic energy.