Reiff D F, Guenther E
Department of Pathophysiology of Vision and Neuro-Ophthalmology, University Eye Hospital, Tübingen, Germany.
Neuroscience. 1999;92(3):1103-17. doi: 10.1016/s0306-4522(99)00044-5.
Ca2(+)-independent voltage-activated potassium currents were investigated during the differentiation of rat retinal ganglion cells. Whole-cell patch-clamp recordings of Ca2(+)-independent voltage-activated potassium currents and their individual current components, i.e. a sustained, tetraethylammonium-sensitive current, a transient, 4-aminopyridine-sensitive current, and a slowly decaying current that was blocked by Ba2+, revealed distinct ontogenetic modifications in current densities and in activation and inactivation parameters. All three current types were expressed simultaneously at embryonic day 17/18 and were present in all retinal ganglion cells thereafter without showing any significant changes until the end of the first postnatal week. Ca2(+)-independent voltage-activated potassium current densities then increased strongly from postnatal day 8 onwards. Tetraethylammonium-sensitive current density increased about eightfold from 74 pA/pF in embryonic stages to 586 pA/pF in adult cells, whereas the transient potassium currents blocked by 4-aminopyridine increased only about 2.5-fold from 174 pA/pF to 442 pA/pF. The Ba2(+)-sensitive current increased simultaneously from 35 pA/pF to 332 pA/pF. The much higher increase in the sustained current components during retinal ganglion cell differentiation accounted for the changes in decay kinetics of Ca2(+)-independent voltage-activated potassium current observed in later postnatal stages. Alterations in current densities were paralleled by pronounced changes in current kinetics. From postnatal day 8 onwards, activation of Ca2(+)-independent voltage-activated potassium current was right-shifted for about 10 mV owing to a shift in tetraethylammonium-sensitive current-activation, whereas activation of other K+ components remained unaltered. Tetraethylammonium-sensitive current steady-state inactivation was incomplete at all developmental stages. About 50% of the tetraethylammonium-sensitive current elicited by a depolarization to +36 mV did not inactivate after prepulse potentials positive to -10 mV. In contrast, transient potassium current blocked by 4-aminopyridine almost fully inactivated during embryonic stages, whereas in adult retinal ganglion cells about 40% of this current component did not inactivate after prepulse potentials positive to -20 mV. Parallel investigation of the resting membrane potential during retinal ganglion cells differentiation showed an exponential increase from -3 mV at embryonic day 15/16 when no voltage-activated ion currents were expressed to a final value of -58 mV at postnatal day 8. These results show that fundamental potassium current modifications occur relatively late in retinal ganglion cell development and only after the resting potential is at its final value.
在大鼠视网膜神经节细胞分化过程中，对不依赖Ca2(+)的电压激活钾电流进行了研究。对不依赖Ca2(+)的电压激活钾电流及其各个电流成分，即持续的、对四乙铵敏感的电流、瞬时的、对4-氨基吡啶敏感的电流以及被Ba2+阻断的缓慢衰减电流进行全细胞膜片钳记录，结果显示电流密度以及激活和失活参数存在明显的个体发育变化。所有这三种电流类型在胚胎第17/18天同时表达，此后在所有视网膜神经节细胞中均存在，直到出生后第一周结束都没有显示出任何显著变化。从出生后第8天起，不依赖Ca2(+)的电压激活钾电流密度开始大幅增加。对四乙铵敏感的电流密度从胚胎期的74 pA/pF增加到成年细胞中的586 pA/pF，增加了约8倍，而被4-氨基吡啶阻断的瞬时钾电流仅从174 pA/pF增加到442 pA/pF，增加了约2.5倍。对Ba2(+)敏感的电流同时从35 pA/pF增加到332 pA/pF。在视网膜神经节细胞分化过程中，持续电流成分的大幅增加解释了在出生后后期观察到的不依赖Ca2(+)的电压激活钾电流衰减动力学的变化。电流密度的改变伴随着电流动力学的显著变化。从出生后第8天起，由于对四乙铵敏感的电流激活发生了偏移，不依赖Ca2(+)的电压激活钾电流的激活向右偏移了约10 mV，而其他K+成分的激活保持不变。对四乙铵敏感的电流稳态失活在所有发育阶段都是不完全的。在去极化至+36 mV时，由预脉冲电位正向至-10 mV引发的对四乙铵敏感的电流中，约50%不会失活。相比之下，被4-氨基吡啶阻断的瞬时钾电流在胚胎期几乎完全失活，而在成年视网膜神经节细胞中，该电流成分在预脉冲电位正向至-20 mV后约40%不会失活。在视网膜神经节细胞分化过程中对静息膜电位进行的平行研究表明，静息膜电位从胚胎第15/16天不表达电压激活离子电流时的-3 mV呈指数增加，到出生后第8天达到最终值-58 mV。这些结果表明，基本的钾电流修饰在视网膜神经节细胞发育过程中相对较晚发生，且仅在静息电位达到其最终值之后。
