Tombaugh G C
Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.
J Neurophysiol. 1998 Oct;80(4):1702-12. doi: 10.1152/jn.1998.80.4.1702.
Voltage-gated calcium (Ca) channels are highly sensitive to cytosolic H+, and Ca2+ influx through these channels triggers an activity-dependent fall in intracellular pH (pHi). In principle, this acidosis could act as a negative feedback signal that restricts excessive Ca2+ influx. To examine this possibility, whole cell current-clamp recordings were taken from rat hippocampal interneurons, and dendritic Ca2+ transients were monitored fluorometrically during spike trains evoked by brief depolarizing pulses. In cells dialyzed with elevated internal pH buffering (high beta), trains of >15 action potentials (Aps) provoked a significantly larger Ca2+ transient. Voltage-clamp analysis of whole cell Ca currents revealed that differences in cytosolic pH buffering per se did not alter baseline Ca channel function, although deliberate internal acidification by 0.3 pH units blunted Ca currents by approximately 20%. APs always broadened during a spike train, yet this broadening was significantly greater in high beta cells during rapid but not slow firing rates. This effect of internal beta on spike repolarization could be blocked by cadmium. High beta also 1) enhanced the slow afterhyperpolarization (sAHP) seen after a spike train and 2) accelerated the decay of an early component of the sAHP that closely matched a sAHP conductance that could be blocked by apamin. Both of these effects on the sAHP could be detected at high but not low firing rates. These data suggest that activity-dependent pHi shifts can blunt voltage-gated Ca2+ influx and retard submembrane Ca2+ clearance, suggesting a novel feedback mechanism by which Ca2+ signals are shaped and coupled to the level of cell activity.
电压门控钙（Ca）通道对胞质H⁺高度敏感，通过这些通道的Ca²⁺内流会引发细胞内pH（pHi）的活性依赖性下降。原则上，这种酸中毒可作为一种负反馈信号，限制过多的Ca²⁺内流。为了检验这种可能性，我们对大鼠海马中间神经元进行了全细胞电流钳记录，并在短暂去极化脉冲诱发的动作电位串期间用荧光法监测树突状Ca²⁺瞬变。在用升高的细胞内pH缓冲（高β）进行透析的细胞中，超过15个动作电位（APs）的序列引发了明显更大的Ca²⁺瞬变。全细胞Ca电流的电压钳分析表明，胞质pH缓冲本身的差异并没有改变基线Ca通道功能，尽管故意将细胞内酸化0.3个pH单位会使Ca电流减弱约20%。在动作电位串期间APs总是会变宽，但在快速而非缓慢放电率时，高β细胞中的这种变宽明显更大。细胞内β对动作电位复极化的这种作用可被镉阻断。高β还1）增强了动作电位串后出现的缓慢超极化后电位（sAHP），以及2）加速了sAHP早期成分的衰减，该成分与可被蜂毒明肽阻断的sAHP电导紧密匹配。对sAHP的这两种作用在高放电率而非低放电率时均可检测到。这些数据表明，活性依赖性的pHi变化可减弱电压门控Ca²⁺内流并延缓膜下Ca²⁺清除，提示了一种新的反馈机制，通过该机制Ca²⁺信号被塑造并与细胞活性水平相耦合。
