ACTION-POTENTIAL BROADENING AND FREQUENCY-DEPENDENT FACILITATION OF CALCIUM SIGNALS IN PITUITARY NERVE-TERMINALS
ACTION-POTENTIAL BROADENING AND FREQUENCY-DEPENDENT FACILITATION OF CALCIUM SIGNALS IN PITUITARY NERVE-TERMINALS
复制标题
DOI：
10.1073/pnas.88.2.380
复制
发表时间：
1991-01-01
影响因子：
11.1
通讯作者：
AUGUSTINE, GJ
中科院分区：
文献类型：
--
作者：
JACKSON, MB;KONNERTH, A;AUGUSTINE, GJ
神经垂体神经末梢的激素释放是动作电位频率的敏感函数。我们通过结合膜片钳和荧光法测量大鼠垂体后叶薄片单个神经分泌终末的钙离子浓度，研究了这种频率依赖的便利化的细胞机制。在这些终末，高频刺激均能增强动作电位引起的细胞内钙离子浓度([Ca~(2+)]i)和动作电位时程的改变，其频率依赖关系与激素释放类似。此外，短暂的电压钳位脉冲以非常相似的频率依赖关系失活K+电流。这些结果支持频率依赖性易化模型，在该模型中，K+电流的失活扩大了动作电位，导致[Ca~(2+)]i信号增强。进一步的实验测试了动作电位加宽和促进[Ca2+]i变化之间的因果关系。首先，延长去极化时间，无论是用K+通道阻滞剂四乙基铵加宽动作电位，还是延长去极化电压钳步骤，都会增加[Ca~(2+)]i的变化。其次，通过用恒定的持续时间脉冲电压钳制终端，消除持续时间的频率依赖变化，大大减少了[Ca~(2+)]i变化的频率依赖增强。这些结果表明，动作电位展宽有助于频率依赖性地促进[Ca~(2+)]i的变化。然而，持续时间刺激的小残留频率依赖性的[Ca~(2+)]i变化表明，与动作电位加宽不同的第二个过程也有助于易化。这两种频率依赖的机制也可能有助于突触终末的活动依赖可塑性。
Hormone release from nerve terminals in the neurohypophysis is a sensitive function of action potential frequency. We have investigated the cellular mechanisms responsible for this frequency-dependent facilitation by combining patch clamp and fluorimetric Ca2+ measurements in single neurosecretory terminals in thin slices of the rat posterior pituitary. In these terminals both action potential-induced changes in the intracellular Ca2+ concentration ([Ca2+]i) and action potential duration were enhanced by high-frequency stimuli, all with a frequency dependence similar to that of hormone release. Furthermore, brief voltage clamp pulses inactivated a K+ current with a very similar frequency dependence. These results support a model for frequency-dependent facilitation in which the inactivation of a K+ current broadens action potentials, leading to an enhancement of [Ca2+]i signals. Further experiments tested for a casual relationship between action potential broadening and facilitation of [Ca2+]i changes. First, increasing the duration of depolarization, either by broadening action potentials with the K+-channel blocker tetraethylammonium or by applying longer depolarizing voltage clamp steps, increased [Ca2+]i changes. Second, eliminating frequency-dependent changes in duration, by voltage clamping the terminal with constant duration pulses, substantially reduced the frequency-dependent enhancement of [Ca2+]i changes. These results indicate that action potential broadening contributes to frequency-dependent facilitation of [Ca2+]i changes. However, the small residual frequency dependence of [Ca2+]i changes seen with constant duration stimulation suggests that a second process, distinct from action potential broadening, also contributes to facilitation. These two frequency-dependent mechanisms may also contribute to activity-dependent plasticity in synaptic terminals.