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

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Abstract
Intracellular pH (pHi) of Purkinje fibers from sheep heart was recorded with pH-sensitive glass micro-electrodes. The cells were acidified by 1 of 3 methods: exposure to and subsequent removal of NH4Cl, exposure to solutions containing 5% CO2 or 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 [extracellular Na+ concentration] to 8 mM. When used as a Na+ substitute, Li+ could permit recovery. Tris [2-amino-2-hydroxymethylpropane-1,3-diol] (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''-disulfonic acid disodium salt; 100 .mu.M) 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 pHi recovery experiments. From these data the apparent Na+ electrochemical gradient at different values of [Na+]o were calculated. If this gradient is responsible for H+ efflux from the cell then, by applying thermodynamic considerations, it can be shown that only low concentrations (1-2 mM) of extracellular Na+ are required. Solutions containing a very low [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 (<10-8M). A very large intracellular acidification occurs on removal of Na+ in the presence of strophanthidin (10-5 M). Amiloride (1 mM) inhibits the contracture produced under these conditions and slightly decreases the magnitude of the acidification. The acidification and contracture do not occur when [Ca2+]o is very low. The Na+ dependence of the pHi regulatory system in Purkinje fibers and the close relationship of intracellular levels of Na+ Ca2+ and H+ are discussed. Evidently, sarcolemmal Na+-H+ exchange is responsible for pH1 recovery from an acidosis, but other systems aiding pHi recovery, e.g., an uptake of protons by mitochondria, cannot be excluded.