Intracellular pH in depolarized cardiac Purkinje strands

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Abstract
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 NH +4 and CO2/HCO −3 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 NH +4 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 NH +4 , pHi recovery also was slightly slower than in the normally polarized cell. In the depolarized fiber, removal and readdition of CO2/HCO −3 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/HCO −3 containing and in CO2/HCO −3 free solution tended to become identical. These experiments support the hypothesis that in the normally polarized Purkinje fiber passive shuttle movement of NH +4 /NH3 and CO2/HCO −3 occurs and could perhaps at least be partly responsible for the lower steady state pHi as compared to that reached in NH +4 -free and CO2/HCO −3 -free solutions respectively.