Sodium‐dependent control of intracellular pH in Purkinje fibres of sheep heart.

Intracellular pH (pHi) of Purkinje fibres from sheep heart was recorded with pH‐sensitive glass micro‐electrodes. The cells were acidified by one of three methods: (1) exposure to and subsequent removal of NH4Cl, (2) exposure to solutions containing 5% CO2 or (3) exposure to an acidic Tyrode solution. The pHi recovery from these acidifications was studied. The time constant of recovery from an acidification induced by NH4Cl was almost twice as long as that from one induced by CO2 or acid extracellular pH. Following an acidification induced by exposure to CO2 the time constant of pHi recovery was not changed when the cell was depolarized to ‐40 mV (by replacement of some Na+ by K+). An intracellular acidification was produced when extracellular Na+ was removed and replaced by quaternary ammonium ions or K+. Such Na+‐free solutions also inhibited pHi recovery from an acidification. A 50% inhibition of the rate of recovery was produced by lowering the Na+o to 8 mM. When used as a Na+ substitute, Li+ could permit recovery. Tris (22 mM) changed pHi in the alkaline direction. Amiloride (1 mM) or a decrease in temperature slowed the recovery from an acidification (Q10 = 2.65). There was no effect of SITS (4‐acetamido‐4'‐isothiocyanatostilbene‐2,2'‐disulphonic acid disodium salt; 100 microM) on the recovery. Na+‐sensitive glass micro‐electrodes were used to measure the intracellular Na+ activity when Na+o was lowered to levels used in our pHi recovery experiments. From these data we have calculated the apparent Na+ electrochemical gradient at different values of Na+o. If this gradient is responsible for H+ efflux from the cell then, by applying thermodynamic considerations, it can be shown that only low concentrations (1‐2mM) of extracellular Na+ are required. Solutions containing a very low Ca2+o (less than 10(‐8) M, buffered with EGTA) were used to prevent large rises of Ca2+i which may occur on removal of external Na+. Under these conditions pHi recovery is still dependent upon Na+o, and the apparent inhibition of pHi recovery by removal of Na+ is not simply due to rises in Ca2+i. The intracellular acidification which occurs on removal of Na+ does not occur when Ca2+o is very low (less than 10(‐8) M).(ABSTRACT TRUNCATED AT 400 WORDS)