Miller D L, Korenbrot J I
Department of Physiology, University of California Medical School, San Francisco 94143.
J Gen Physiol. 1987 Sep;90(3):397-425. doi: 10.1085/jgp.90.3.397.
We measured simultaneously in single toad rods the membrane photocurrent and the Ca concentration in a small volume surrounding the outer segment. Illumination causes a rise in the extracellular Ca concentration. Photocurrents and Ca concentration changes occur over the same range of light intensities. Analysis of the time course of the Ca concentration changes suggests that these concentration changes arise from the difference in the transport rates of light-activated Ca influx and efflux across the outer segment plasma membrane. The Ca influx occurs through the light-sensitive channels of the outer segment membrane and the efflux through Na/Ca exchangers. In 0.1 mM external Ca, approximately 1-2% of the dark current is carried by Ca ions. The Ca efflux in the dark is identical to the influx, approximately 2 X 10(6) ions/s. Upon illumination, the Ca influx decreases with a time course and light sensitivity identical to those of the photocurrent. The Ca efflux, on the other hand, has very different kinetics from those of the photocurrent. Upon illumination, the Ca efflux decreases with a time course and light sensitivity determined by the change in membrane voltage and in the free cytoplasmic Ca concentration near the plasma membrane. In response to bright stimuli, which saturate the photocurrent for prolonged periods of time, the Ca efflux decays with an exponential time course from its value in darkness. The average time constant of this decay is 2.5 s. From the kinetics of the light-activated Ca fluxes, it is possible to predict that illumination causes a decrease in the cytoplasmic Ca concentration. We present a model of the regulation of the cytoplasmic Ca concentration by the dynamic balance of the Ca influx and efflux from the rod outer segment. The model accounts for our experimental observations and allows us to predict the time course and extent of the light-dependent decrease in the free cytoplasmic concentration.
我们在单个蟾蜍视杆细胞中同时测量了膜光电流以及外段周围小体积区域内的钙离子浓度。光照会导致细胞外钙离子浓度升高。光电流和钙离子浓度变化发生在相同的光强范围内。对钙离子浓度变化时间进程的分析表明，这些浓度变化源于光激活的钙离子内流和外流穿过外段质膜的运输速率差异。钙离子内流通过外段膜的光敏感通道发生，外流则通过钠钙交换体。在外部钙离子浓度为0.1 mM时，暗电流中约1 - 2%由钙离子携带。黑暗中的钙离子外流与内流相同，约为2×10⁶个离子/秒。光照时，钙离子内流随时间进程和光敏感度下降，与光电流的情况相同。另一方面，钙离子外流的动力学与光电流的动力学有很大不同。光照时，钙离子外流随时间进程下降，其光敏感度由膜电压变化以及质膜附近游离细胞质钙离子浓度变化决定。对于长时间使光电流饱和的强光刺激，钙离子外流从其黑暗中的值开始以指数时间进程衰减。这种衰减的平均时间常数为2.5秒。从光激活的钙离子通量动力学可以预测，光照会导致细胞质钙离子浓度降低。我们提出了一个通过视杆细胞外段钙离子内流和外流的动态平衡来调节细胞质钙离子浓度的模型。该模型解释了我们的实验观察结果，并使我们能够预测游离细胞质浓度光依赖性降低的时间进程和程度。
