Phasic changes in intracellular pH during action potentials of sheep Purkinje fibres
Pressler, M.L.
Pflugers Archiv European Journal of Physiology 411(1): 69-75
1988
ISSN/ISBN: 0031-6768 PMID: 3353214 Document Number: 309183
Regulation of intracellular pH (pHi) and the relationship between H+ and Ca2+ may vary during activity. Ion-selective microelectrodes were used to record pHi during action potentials of sheep Purkinje fibers prolonged by low temperature (21 C) and elevated CO2 content. Intracellular pH also was measured during changes in extracellular calcium concentration, [Ca2+]o. Cytosolic alkalinization (peak pHi change, 0.03-0.05) was observed during the long action-potential plateau and transient acidification (0.01-0.02 units) upon repolarization. Potassium-induced depolarization to plateau potentials (i.e. to -15 .+-. 2 mV) simulated the peak magnitude of the alkalinization. However, compensation for the alkalinization occurred at a faster rate during the action potential (8.9 .+-. 4.3 nM/min) than during K+ depolarization (1.2 .+-. 0.5 nM/min). In comparison, the cytoplasm acidified in resting fibers (0.06-0.07 log units) during changes of [Ca2+]o thought to increase intracellular calcium concentration. Alterations of pHi were translated into changes of proton concentration ([H+]i). Ten- to twenty-fold elevation of [Ca2+]o evoked a comparable change in [H+]i (mean increase, 5.7 nM) but oppositely directed from that during the plateau (mean decrease, 8.8 nM). The findings in resting fibers seem consistent with displacement of bound protons by Ca2+. In contrast, the initial change in pHi, during the plateau is proposed to be consequent to Ca2+-release from sarcoplasmic reticulum and or phosphocreatine hydrolysis coupled to ATP regeneration.