Morgans C W, El Far O, Berntson A, Wässle H, Taylor W R
Department of Neuroanatomy, Max-Planck-Institute f-60528 Frankfurt, Germany.
J Neurosci. 1998 Apr 1;18(7):2467-74. doi: 10.1523/JNEUROSCI.18-07-02467.1998.
Ribbon synapses of vertebrate photoreceptors constantly release glutamate in darkness. Transmitter release is maintained by a steady influx of calcium through voltage-dependent calcium channels, implying the presence of a mechanism that is able to extrude calcium at an equal rate. The two predominant mechanisms of intracellular calcium extrusion are the plasma membrane calcium ATPase (PMCA) and the Na+/Ca2+-exchanger. Immunohistochemical staining of retina sections revealed strong immunoreactivity for the PMCA in rod and cone terminals, whereas staining for the Na+/Ca2+-exchanger was very weak. The PMCA was localized to the plasma membrane along the sides of the photoreceptor terminals and was excluded from the base of the terminals where the active zones are located. The amplitude of a calcium-activated chloride current was used to monitor the intracellular calcium concentration. An upper limit for the time required to remove intracellular free calcium is obtained from two time constants measured for the calcium-activated chloride current tail currents: one of 50 msec and a second of 190 msec. Calcium extrusion was inhibited in the absence of intracellular ATP or in the presence of the PMCA inhibitor orthovanadate, but was unaffected by replacement of external Na+ with Li+. The data indicate that the PMCA, rather than the Na+/Ca2+-exchanger, is the predominant mechanism for calcium extrusion from photoreceptor synaptic terminals.
脊椎动物光感受器的带状突触在黑暗中持续释放谷氨酸。递质释放通过电压依赖性钙通道稳定的钙内流来维持，这意味着存在一种能够以相同速率排出钙的机制。细胞内钙排出的两种主要机制是质膜钙ATP酶（PMCA）和钠/钙交换体。视网膜切片的免疫组织化学染色显示，杆状和锥状终末对PMCA有强烈的免疫反应性，而对钠/钙交换体的染色非常弱。PMCA定位于光感受器终末侧面的质膜上，而在终末基部（即活性区所在部位）则没有。利用钙激活氯电流的幅度来监测细胞内钙浓度。从测量的钙激活氯电流尾电流的两个时间常数获得去除细胞内游离钙所需时间的上限：一个为50毫秒，另一个为190毫秒。在没有细胞内ATP或存在PMCA抑制剂原钒酸盐的情况下，钙排出受到抑制，但用Li+替代外部Na+对其没有影响。数据表明，PMCA而非钠/钙交换体是光感受器突触终末钙排出的主要机制。
