Intracellular pH in depolarized cardiac Purkinje strands

In isolated sheep cardiac Purkinje strands the effect of membrane depolarization on intracellular pH (pHi) and on pHi changes produced by addition and withdrawal of NH4+ and Co2/HCO3- was investigated. pHi was continuously measured with double-barreled glass microelectrodes. Repetitive stimulation at high rate resulted in a moderate intracellular acidification (approximately 0.03 pH unit after a 3 Hz train of 2 min), whereafter pHi returned toward its pre-stimulus level. Prolonged depolarization, evoked either by current injection or by superfusion with high K+ solutions, was accompanied by a small acid shift. In the depolarized cell, addition of NH4+ to the superfusate caused intracellular alkalinization followed by re-acidification which was slower than at normal membrane potential. Following intracellular acidification caused by withdrawal of NH4+, pHi recovery also was slightly slower than in the normally polarized cell. In the depolarized fiber, removal and readdition of CO2/HCO3- produced the expected intracellular alkalinization and acidification respectively. Recovery from CO2-induced acidosis was slowed somewhat in high K+ (low Na+) superfused fibers, not in current depolarized fibers. In the depolarized cell, steady state pHi in CO2/HCO3- containing and in CO2/HCO3- free solution tended to become identical. These experiments support the hypothesis that in the normally polarized Purkinje fiber passive shuttle movement of NH4+/NH3 and CO2/HCO3- occurs and could perhaps at least be partly responsible for the lower steady state pHi as compared to that reached in NH4+-free and CO2/HCO3- -free solutions respectively.