The intracellular pH (pHi) of surface fibres of the mouse soleus muscle has been measured in vitro using recessed-tip pH-sensitive microelectrodes. 2. In 5% CO2 and pH 7-40, the mean pHi was 7-07 +/- 0-007 (S.E. of mean) at 37 degrees C and 7-23 +/- 0-01 at 28 degrees C. The difference between these tow values is the same as the change in neutral pH between 37 and 28 degrees C. 3. Alteration of the CO2 level at constant external pH caused a biphasic change in pHi with a rapid displacement followed by a slower partial recovery. Because the recovery was incomplete, different stable pHi values were recorded at different CO2 levels, the higher the CO2 the lower the pHi. The differences in pHi were highly significant both at 37 and 28 degrees C. 4. Alteration of the CO2 level at constant external pH also changed the membrane potential (Em), an increase in CO2 leading to an increased Em. The dependence of Em on the CO2 level was much smaller in the fast-twitch muscle, extensor digitorum longus, than in soleus. 5. Changing external pH, either by alteration of the bicarbonate or CO2 level of the Ringer solution, caused pHi to change by a mean 38-7% of the external pH change. The change in pHi was accomplished about 10 times more rapidly, and in the same direction, by altering CO2 than by altering the bicarbonate. 6. Application of external NH3 and NH+4 caused a rapid intracellular alkalinization followed by a slower acidification. On removal of external NH3 and NH+4, there was a large and rapid acdification, followed by a fairly rapid recovery in pHi. 7. The size of the pHi changes occurring on alteration of the CO2 level at both constant external pH and constant external bicarbonate, and on removal of external NH3 and NH+4, suggests a non-CO2 buffering power of 45m-equiv H+ ions/pH unit per litre and a constant-CO2 buffering power of 58 m-equiv H+ ions/pH unit per litre. The buffering power was apparently unaffected by a change in temperature between 37 and 28 degrees C. 8. It was concluded that H+ ions are not passively distributed across the muscle cell membrane, and that the pHi is closely controlled by the active transport of H+, OH- or HCO-3 ions.
