Pepperberg D R, Birch D G, Hofmann K P, Hood D C
Department of Opthalmology and Visual Sciences, Lions of Illinois Eye Research Institute, University of Illinois, College of Medicine, Chicago 60612, USA.
J Opt Soc Am A Opt Image Sci Vis. 1996 Mar;13(3):586-600. doi: 10.1364/josaa.13.000586.
Electroretinographic data obtained from human subjects show that bright test flashes of increasing intensity induce progressively longer periods of apparent saturation of the rod-mediated electroretinogram (ERG) alpha wave. A prominent feature of the saturation function [the function that relates the saturation period T with the natural logarithm of flash intensity (ln I(f)] is its two-branched character. At relatively low flash intensities (I(f) below approximately 4 x 10(4) scotopic troland second), T increases approximately in proportion to ln I(f) with a slope [delta T/delta (ln I(f)] of approximately 0.3 s. At higher flash intensities, a different linear relation prevails, in which [deltaT/delta(ln I(f) is approximately 2.3 s [Invest. Ophthalmol. Vis. Sci. 36, 1603 (1995)]. Based on a model for photocurrent recovery in isolated single rods [Vis. Neurosci. 8, 9 (1992)], it was suggested that the upper-branch slope of approximately 2.3 s represents tau R*, the lifetime of photoactivated rhodopsin (R*). Here we show that a modified version of this model provides an explanation for the lower branch of the alpha-wave saturation function. In this model, tau E* is the exponential lifetime of an activated species (E*) within the transducin or guanosine 3', 5'-cyclic monophosphate (cGMP) phosphodiesterase stages of rod phototransduction; the generation of E* by a single R* occurs within temporally defined, elemental domains of disk membrane; and Ex, the immediate product of E* deactivation, is converted only slowly (time constant tau Ex) to E, the form susceptible to reactivation by R*. The model predicts that the decay of flash-activated cGMP phosphodiesterase (PDE*) is largely independent of the deactivation kinetics of R* at early postflash times (i.e., at times preceding or comparable with the lifetime tau E*) and that the lower-branch slope (approximately 0.3s) of the a-wave saturation function represent tau E*. The predicted early-stage independence of PDE* decay and R* deactivation furthermore suggests a basis for the relative constancy of the single-photon response observed in studies of isolated rods. Numerical evaluation of the model yields a value of approximately 6.7s for the time constant tau Ex.
从人类受试者获得的视网膜电图数据表明，强度不断增加的明亮测试闪光会使视杆介导的视网膜电图（ERG）α波的明显饱和期逐渐延长。饱和函数[将饱和期T与闪光强度的自然对数（ln I(f)）相关联的函数]的一个显著特征是其二分支特性。在相对较低的闪光强度下（I(f)低于约4×10⁴暗适应troland秒），T大约与ln I(f)成比例增加，斜率[δT/δ(ln I(f)]约为0.3秒。在较高的闪光强度下，存在不同的线性关系，其中[δT/δ(ln I(f)]约为2.3秒[《Invest. Ophthalmol. Vis. Sci.》36, 1603 (1995)]。基于分离的单个视杆中光电流恢复的模型[《Vis. Neurosci.》8, 9 (1992)]，有人提出上分支斜率约2.3秒代表视紫红质（R*）光激活后的寿命τR*。在此我们表明，该模型的一个修改版本为α波饱和函数的下分支提供了解释。在这个模型中，τE是视杆光转导的转导素或鸟苷3',5'-环磷酸（cGMP）磷酸二酯酶阶段内活化物种（E）的指数寿命；单个R产生E发生在盘膜的时间定义的基本区域内；并且E失活的直接产物Ex仅缓慢（时间常数τEx）转化为E，即易被R重新激活的形式。该模型预测，闪光激活的cGMP磷酸二酯酶（PDE*）的衰减在闪光后早期（即，在寿命τE之前或与之相当的时间）很大程度上与R的失活动力学无关，并且α波饱和函数的下分支斜率（约0.3秒）代表τE*。PDE衰减和R失活在早期阶段的预测独立性进一步为在分离视杆的研究中观察到的单光子反应的相对恒定性提供了基础。该模型的数值评估得出时间常数τEx的值约为6.7秒。
