Picones A, Korenbrot J I
Department of Physiology, School of Medicine, University of California at San Francisco 94143, USA.
Biophys J. 1995 Jul;69(1):120-7. doi: 10.1016/S0006-3495(95)79881-2.
We studied the ionic permeability of cGMP-dependent currents in membrane patches detached from the outer segment of retinal cone and rod photoreceptors. Reversal potentials measured in membranes exposed to symmetric Na+ but with varying cytoplasmic Ca2+ concentrations reveal that the permeability ratio, PCa/PNa, is higher in the cGMP-gated channels of cones (7.6 +/- 0.8) than in those of rods (3.1 +/- 1.0). Ca2+ blocks both channels in a voltage-dependent manner. At any Ca2+ concentration, the channel block is maximal near the ionic reversal potential. The maximal block is essentially identical in channels of cones and rods with respect to its extent and voltage and Ca2+ dependence. The Ca2+ block is relieved by voltage, but the features of this relief differ markedly between rods and cones. Whereas the Boltzmann distribution function describes the relief of block by hyperpolarizing voltages, any given voltage is more effective in relieving the Ca2+ block in cones than in rods. Similarly, depolarizing voltages more effectively relieve Ca2+ block in cones than in rods. Our results suggest that channels contain two binding sites for Ca2+, one of which is similar in the two receptor types. The second site either interacts more strongly with Ca2+ than the first one or it is located differently in the membrane, so as to be less sensitive to membrane voltage. The channels in rods and cones differ in the features of this second site. The difference in Ca2+ permeability between the channels is likely to result in light-dependent changes in cytoplasmic Ca2+ concentration that are larger and faster in cones than in rods. The functional differences between channels, therefore, may be critically important in explaining the differences in the phototransduction signal of the two photoreceptor types.
我们研究了从视网膜视锥和视杆光感受器外段分离的膜片中依赖环磷酸鸟苷（cGMP）的电流的离子通透性。在暴露于对称钠离子（Na⁺）但细胞质钙离子（Ca²⁺）浓度不同的膜片中测量的反转电位表明，视锥细胞cGMP门控通道的通透率比（PCa/PNa）（7.6±0.8）高于视杆细胞（3.1±1.0）。钙离子以电压依赖的方式阻断这两种通道。在任何钙离子浓度下，通道阻断在离子反转电位附近最大。就其程度、电压和钙离子依赖性而言，视锥和视杆通道中的最大阻断基本相同。钙离子阻断可通过电压解除，但视杆和视锥之间这种解除的特征明显不同。虽然玻尔兹曼分布函数描述了超极化电压解除阻断的情况，但任何给定电压在解除视锥细胞中的钙离子阻断方面比视杆细胞更有效。同样，去极化电压在解除视锥细胞中的钙离子阻断方面比视杆细胞更有效。我们的结果表明，通道含有两个钙离子结合位点，其中一个在两种受体类型中相似。第二个位点要么与钙离子的相互作用比第一个更强,要么它在膜中的位置不同，因此对膜电压不太敏感。视杆和视锥中的通道在第二个位点的特征上有所不同。通道之间钙离子通透性的差异可能导致细胞质钙离子浓度的光依赖性变化，视锥细胞中的变化比视杆细胞更大、更快。因此，通道之间的功能差异可能在解释两种光感受器类型的光转导信号差异方面至关重要。
