Wang T L, Sterling P, Vardi N
Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
J Neurosci. 1999 Jun 1;19(11):4221-8. doi: 10.1523/JNEUROSCI.19-11-04221.1999.
Calcium enters the outer segment of a vertebrate photoreceptor through a cGMP-gated channel and is extruded via a Na/Ca, K exchanger. We have identified another element in mammalian cones that might help to control cytoplasmic calcium. Reverse transcription-PCR performed on isolated photoreceptors identified mRNA for the SII- splice variant of the type I receptor for inositol 1,4,5-triphosphate (IP3), and Western blots showed that the protein also is expressed in outer segments. Immunocytochemistry showed type I IP3 receptor to be abundant in red-sensitive and green-sensitive cones of the trichromatic monkey retina, but it was negative or weakly expressed in blue-sensitive cones and rods. Similarly, the green-sensitive cones expressed the receptor in dichromatic retina (cat, rabbit, and rat), but the blue-sensitive cones did not. Immunostain was localized to disk and plasma membranes on the cytoplasmic face. To restore sensitivity after a light flash, cytoplasmic cGMP must rise to its basal level, and this requires cytoplasmic calcium to fall. Cessation of calcium release via the IP3 receptor might accelerate this fall and thus explain why the cone recovers much faster than the rod. Furthermore, because its own activity of the IP3 receptor depends partly on cytoplasmic calcium, the receptor might control the set point of cytoplasmic calcium and thus affect cone sensitivity.
钙离子通过一种环鸟苷酸门控通道进入脊椎动物光感受器的外段,并通过钠钙钾交换体排出。我们在哺乳动物视锥细胞中发现了另一种可能有助于控制细胞质钙离子的成分。对分离出的光感受器进行逆转录聚合酶链反应,鉴定出肌醇1,4,5-三磷酸(IP3)I型受体的SII剪接变体的信使核糖核酸,蛋白质免疫印迹显示该蛋白质也在外段表达。免疫细胞化学显示I型IP3受体在三色猴视网膜的红敏和绿敏视锥细胞中大量存在,但在蓝敏视锥细胞和视杆细胞中呈阴性或弱表达。同样,在双色视网膜(猫、兔和大鼠)中,绿敏视锥细胞表达该受体,而蓝敏视锥细胞不表达。免疫染色定位于细胞质面的盘膜和质膜。为了在闪光后恢复敏感性,细胞质环鸟苷酸必须升至其基础水平,这需要细胞质钙离子下降。通过IP3受体停止钙离子释放可能会加速这种下降,从而解释了视锥细胞比视杆细胞恢复得快得多的原因。此外,由于IP3受体自身的活性部分取决于细胞质钙离子,该受体可能控制细胞质钙离子的设定点,从而影响视锥细胞的敏感性。