Xu-Friedman M A, Regehr W G
Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA.
Biophys J. 1999 Apr;76(4):2029-42. doi: 10.1016/S0006-3495(99)77360-1.
Strontium can replace calcium in triggering neurotransmitter release, although peak release is reduced and the duration of release is prolonged. Strontium has therefore become useful in probing release, but its mechanism of action is not well understood. Here we study the action of strontium at the granule cell to Purkinje cell synapse in mouse cerebellar slices. Presynaptic residual strontium levels were monitored with fluorescent indicators, which all responded to strontium (fura-2, calcium orange, fura-2FF, magnesium green, and mag-fura-5). When calcium was replaced by equimolar concentrations of strontium in the external bath, strontium and calcium both entered presynaptic terminals. Contaminating calcium was eliminated by including EGTA in the extracellular bath, or by loading parallel fibers with EGTA, enabling the actions of strontium to be studied in isolation. After a single stimulus, strontium reached higher peak free levels than did calcium (approximately 1.7 times greater), and decayed more slowly (half-decay time 189 ms for strontium and 32 ms for calcium). These differences in calcium and strontium dynamics are likely a consequence of greater strontium permeability through calcium channels, lower affinity of the endogenous buffer for strontium, and less efficient extrusion of strontium. Measurements of presynaptic divalent levels help to explain properties of release evoked by strontium. Parallel fiber synaptic currents triggered by strontium are smaller in amplitude and longer in duration than those triggered by calcium. In both calcium and strontium, release consists of two components, one more steeply dependent on divalent levels than the other. Strontium drives both components less effectively than does calcium, suggesting that the affinities of the sensors involved in both phases of release are lower for strontium than for calcium. Thus, the larger and slower strontium transients account for the prominent slow component of release triggered by strontium.
锶在触发神经递质释放方面可以取代钙，尽管释放峰值降低且释放持续时间延长。因此，锶在研究递质释放方面变得很有用，但对其作用机制尚不清楚。在这里，我们研究了锶在小鼠小脑切片中颗粒细胞与浦肯野细胞突触处的作用。用荧光指示剂监测突触前残余锶水平，这些指示剂均对锶有反应（fura - 2、钙橙、fura - 2FF、镁绿和mag - fura - 5）。当细胞外浴液中的钙被等摩尔浓度的锶取代时，锶和钙都会进入突触前终末。通过在细胞外浴液中加入乙二醇双乙醚二胺四乙酸（EGTA），或用EGTA加载平行纤维来消除污染的钙，从而能够单独研究锶的作用。单次刺激后，锶达到的游离峰值水平高于钙（约高1.7倍），且衰减更慢（锶的半衰期为189毫秒，钙为32毫秒）。钙和锶动力学的这些差异可能是由于锶通过钙通道的通透性更高、内源性缓冲剂对锶的亲和力更低以及锶的外排效率更低所致。突触前二价离子水平的测量有助于解释由锶诱发的释放特性。由锶触发的平行纤维突触电流在幅度上比由钙触发的小，持续时间更长。在钙和锶的情况下，释放都由两个成分组成，其中一个比另一个更强烈地依赖二价离子水平。锶驱动这两个成分的效果都不如钙，这表明在释放的两个阶段中涉及的传感器对锶的亲和力低于对钙的亲和力。因此，更大且更慢的锶瞬变解释了由锶触发的释放中突出的慢成分。
