Effects of pH changes on calcium-mediated potentials in rat hippocampal neurons in vitro

The effects of changes in extra- and intracellular pH (pH(o) and pH(i), respectively) on potentials mediated by the influx of Ca2+ ions were investigated in intracellular "current-clamp" recordings from CA1 pyramidal neurons in rat hippocampal slices. In neurons which exhibited a "regular-spiking" discharge in response to depolarizing current injection at pH 7.3, perfusion with pH 7.7 medium led to the development of burst firing. Conversely, neurons which were "burst-firing" at pH 7.3 became regular spiking upon exposure to pH 6.9 medium. In addition, the rebound depolarization following a current-evoked hyperpolarization to >-60mV, which in part reflects activation of a low-voltage-activated Ca2+ conductance, was reduced at pH, 6.9 and enhanced at pH, 7.7. Neither the burst firing pattern of discharge nor the augmented rebound depolarization observed during perfusion with pH 7.7 medium was due to the reduction in [Cl-](o) consequent upon the increase in [HCO3-](o) at a constant P-CO2. The magnitudes of the fast afterhyperpolarization which follows a single depolarizing current-evoked action potential and the slow afterhyperpolarization which follows a train of action potentials were attenuated and enhanced, respectively, during perfusion with pH 6.9 and pH 7.7 media, compared to responses obtained at pH 7.3. Reducing pH(i) at a constant pH(o) (by exposure to pH 7.3 HCO3-/CO2-free medium buffered with 30 mM HEPES) also attenuated fast and slow afterhyperpolarizations. In tetrodotoxin- and tetraethylammonium-poisoned slices, perfusion with pH 6.9 and pH 7.7 media reduced and increased, respectively, the magnitude of current-evoked Ca2+-dependent depolarizing potentials and their associated slow afterhyperpolarizations, compared with responses obtained at pH 7.3. In contrast, reducing pH(i) at a constant pH(o) elicited only a small reduction in the magnitude of Ca2+ spikes but markedly attenuated the subsequent slow afterhyperpolarization. The results suggest that, in rat CA1 hippocampal pyramidal neurons, Ca2+-dependent depolarizing potentials mediated by the influx of Ca2+ ions through voltage-activated Ca2+ channels are sensitive to changes in pH(o). These effects of changes in pH(o) are not dependent upon changes in pH(i) consequent upon the changes in pH(o). Changes in pH(o) also affect the magnitudes of fast and slow afterhyperpolarizations mediated by Ca2+-dependent K+ conductances. In these cases, however, the effects of changes in pH(o) are mimicked by changes in pH(i) at a constant pH(o), suggesting in turn that the effects of changes in pH(o) on fast and slow afterhyperpolarizations may be mediated both by changes in Ca2+ influx (reflecting mainly changes in pH(o)) and by direct effects of changes in pH(i) (consequent upon changes in pH(o)) on Ca2+-dependent K+ conductances. (C) 1999 IBRO. Published by Elsevier Science Ltd.