Moriondo A, Pelucchi B, Rispoli G
University of Ferrara, Department of Biology, Via L. Borsari 46, I-44100, Ferrara, Italy.
Eur J Neurosci. 2001 Jul;14(1):19-26. doi: 10.1046/j.0953-816x.2001.01605.x.
Vertebrate photoreceptors respond to light with a graded hyperpolarization from a membrane potential in the dark of approximately -35 mV. The present work investigates the physiological role of the Ca2+-activated K+ current in the photovoltage generation in mechanically isolated rods from salamander retina. Membrane current or voltage in isolated rods was recorded from light- and dark-adapted rods under voltage- or current-clamp conditions, respectively. In light-adapted rods of the salamander, selective blockade of Ca2+-activated K+ channels by means of charybdotoxin depolarized the plasma membrane of current-clamped rods by approximately 30 mV, from a resting potential of approximately -35 mV. A similar depolarization was observed if external Ca2+ (1 mM) was substituted with Ba2+ or Sr2+. Under control conditions, the injection of currents of increasing amplitude (up to -100 pA, to mimic the current entering the rod outer segment) could not depolarize the membrane potential beyond a saturating value of approximately -20 mV. However, in the presence of charybdotoxin, rods depolarized up to +20 mV. In experiments with dark-adapted current-clamped rods, charybdotoxin perfusion lead to transient depolarizations up to 0 mV and steady-state depolarizations of approximately 5 mV above the dark resting potential. Finally, the recovery phase of the voltage response to a flash of light in the presence of charybdotoxin showed a transient overshoot of the membrane potential. It was concluded that Ca2+-activated K+ current is necessary for clamping the rod photovoltage to values close to the dark potential, thus allowing faithful single photon detection and correct synaptic transmission.
脊椎动物的光感受器对光的反应是从黑暗中的约 -35 mV 膜电位产生分级超极化。本研究调查了蝾螈视网膜机械分离的视杆细胞中 Ca2+ 激活的 K+ 电流在光电压产生中的生理作用。分别在电压钳或电流钳条件下，记录了暗适应和光适应视杆细胞的膜电流或电压。在蝾螈的光适应视杆细胞中，用蝎毒素选择性阻断 Ca2+ 激活的 K+ 通道，使电流钳制视杆细胞的质膜从约 -35 mV 的静息电位去极化约 30 mV。如果用 Ba2+ 或 Sr2+ 替代外部 Ca2+（1 mM），也会观察到类似的去极化。在对照条件下，注入幅度逐渐增加的电流（高达 -100 pA，以模拟进入视杆细胞外段的电流）不能使膜电位去极化超过约 -20 mV 的饱和值。然而，在存在蝎毒素的情况下，视杆细胞去极化至 +20 mV。在暗适应电流钳制视杆细胞的实验中，蝎毒素灌注导致瞬态去极化至 0 mV，并在暗静息电位以上产生约 5 mV 的稳态去极化。最后，在存在蝎毒素的情况下，对光闪光的电压响应的恢复阶段显示膜电位有瞬态过冲。得出的结论是，Ca2+ 激活的 K+ 电流对于将视杆细胞光电压钳制在接近暗电位的值是必要的，从而允许进行可靠的单光子检测和正确的突触传递。
