Interplays between intracellular pH (pHi) and calcium ([Ca2+]i) variations remain unclear, though both proton and calcium homeostasis changes accompany physiological events such as Xenopus laevis oocyte maturation. In this report, we used NH4Cl and changes of extracellular pH (pHe) to acidify the cytosol in a physiological range. In oocytes voltage-clamped at −80 mV, NH4Cl triggered an inward current, the main component of which is a Ca2+-dependent chloride current. Calcium imaging confirmed that NH4Cl provoked a [Ca2+]i increase. The mobilized sources of calcium were discriminated using the triple-step protocol as a means to follow both the calcium-activated chloride currents (ICl-Ca) and the hyperpolarization- and acid-activated nonselective cation current (IIn). These currents were stimulated during external addition of NH4Cl. This upregulation was abolished by BAPTA-AM, caffeine and heparin. By both buffering pHi changes with MOPS and by inhibiting calcium influx with lanthanum, intracellular acidification, initiated by NH4Cl and extracellular acidic medium, was shown to trigger a [Ca2+]i increase through both calcium release and calcium influx. The calcium pathways triggered by pHe changes are similar to those activated by NH4Cl, thus suggesting that there is a robust signaling mechanism allowing the cell to adjust to variable environmental conditions.
The Journal of Membrane Biology – Springer Journals
Published: Aug 18, 2010
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