Church J
Department of Anatomy, University of British Columbia, Vancouver, Canada.
Neuroscience. 1999 Mar;89(3):731-42. doi: 10.1016/s0306-4522(98)00344-3.
The effects of changes in extra- and intracellular pH (pHo and pHi, 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 >- 60 mV, which in part reflects activation of a low-voltage-activated Ca2+ conductance, was reduced at pHo 6.9 and enhanced at pHo 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 PCO2. 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 pHi at a constant pHo (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 pHi at a constant pHo 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 pHo. These effects of changes in pHo are not dependent upon changes in pHi consequent upon the changes in pHo. Changes in pHo also affect the magnitudes of fast and slow afterhyperpolarizations mediated by Ca2+-dependent K+ conductances. In these cases, however, the effects of changes in pHo are mimicked by changes in pHi at a constant pHo, suggesting in turn that the effects of changes in pHo on fast and slow afterhyperpolarizations may be mediated both by changes in Ca2+ influx (reflecting mainly changes in pHo) and by direct effects of changes in pHi (consequent upon changes in pHo) on Ca2+-dependent K+ conductances.
在大鼠海马切片CA1锥体神经元的细胞内“电流钳”记录中，研究了细胞外和细胞内pH值（分别为pHo和pHi）变化对由Ca2+离子内流介导的电位的影响。在pH 7.3时对去极化电流注入表现出“规则放电”的神经元中，灌注pH 7.7的培养基会导致爆发式放电。相反，在pH 7.3时“爆发式放电”的神经元在暴露于pH 6.9的培养基后变为规则放电。此外，电流诱发的超极化至>-60 mV后的反弹去极化，其部分反映了低电压激活的Ca2+电导的激活，在pHo 6.9时降低，在pHo 7.7时增强。在灌注pH 7.7培养基期间观察到的爆发式放电模式和增强的反弹去极化，均不是由于在恒定PCO2下[HCO3-]o增加导致[Cl-]o降低所致。与在pH 7.3时获得的反应相比，在灌注pH 6.9和pH 7.7培养基期间，单个去极化电流诱发动作电位后的快速超极化后电位的幅度以及一串动作电位后的慢速超极化后电位的幅度分别减弱和增强。在恒定pHo下降低pHi（通过暴露于用30 mM HEPES缓冲的pH 7.3无HCO3-/CO2培养基）也会减弱快速和慢速超极化后电位。在河豚毒素和四乙铵中毒的切片中，与在pH 7.3时获得的反应相比，灌注pH 6.9和pH 7.7培养基分别降低和增加了电流诱发的Ca2+依赖性去极化电位的幅度及其相关的慢速超极化后电位。相反，在恒定pHo下降低pHi仅使Ca2+尖峰的幅度略有降低，但明显减弱了随后的慢速超极化后电位。结果表明，在大鼠CA1海马锥体神经元中，由Ca2+离子通过电压激活的Ca2+通道内流介导的Ca2+依赖性去极化电位对pHo的变化敏感。pHo变化的这些影响不依赖于pHo变化导致的pHi变化。pHo的变化也会影响由Ca2+依赖性K+电导介导的快速和慢速超极化后电位的幅度。然而，在这些情况下，pHo变化的影响在恒定pHo下被pHi的变化所模拟，这反过来表明pHo变化对快速和慢速超极化后电位的影响可能既由Ca2+内流的变化（主要反映pHo的变化）介导，也由pHi的变化（由于pHo的变化）对Ca2+依赖性K+电导的直接影响介导。
