Koutalos Y, Brown R L, Karpen J W, Yau K W
Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
Biophys J. 1995 Nov;69(5):2163-7. doi: 10.1016/S0006-3495(95)80090-1.
Cyclic GMP (cGMP) is the intracellular messenger mediating phototransduction in retinal rods, with its longitudinal diffusion in the rod outer segment (ROS) likely to be a factor in determining light sensitivity. From the kinetics of cGMP-activated currents in the truncated ROS of the salamander (Ambystoma tigrinum), the cGMP diffusion coefficient was previously estimated to be approximately 60 x 10(-8) cm2 s-1. On the other hand, fluorescence measurements in intact salamander ROS using 8-(fluoresceinyl)thioguanosine 3',5'-cyclic monophosphate (Fl-cGMP) led to a diffusion coefficient for this compound of 1 x 10(-8) cm2 s-1; after corrections for differences in size and in binding to cellular components between cGMP and Fl-cGMP, this gave an upper limit of 11 x 10(-8) cm2 s-1 for the cGMP diffusion coefficient. To properly compare the two sets of measurements, we have examined the diffusion of Fl-cGMP in the truncated ROS. From the kinetics of Fl-cGMP-activated currents, we have obtained a diffusion coefficient of 3 x 10(-8) cm2 s-1 for this analog; the cGMP diffusion coefficient measured from the same truncated ROSs was approximately 80 x 10(-8) cm2 s-1. Thus, a factor of 27 appears appropriate for correcting differences in size and intracellular binding between cGMP and Fl-cGMP. Application of this correction factor to the Fl-cGMP diffusion coefficient measurements by Olson and Pugh (1993) gives a cGMP diffusion coefficient of approximately 30 x 10(-8) cm2 s-1, in reasonable agreement with the value measured from the truncated ROS.
环磷酸鸟苷（cGMP）是介导视网膜视杆细胞光转导的细胞内信使，其在视杆细胞外段（ROS）中的纵向扩散可能是决定光敏感度的一个因素。根据蝾螈（虎纹钝口螈）截短的ROS中cGMP激活电流的动力学，先前估计cGMP的扩散系数约为60×10⁻⁸ cm² s⁻¹。另一方面，使用8-（荧光素基）硫代鸟苷3',5'-环一磷酸（Fl-cGMP）对完整蝾螈ROS进行的荧光测量得出该化合物的扩散系数为1×10⁻⁸ cm² s⁻¹；在对cGMP和Fl-cGMP之间的大小差异以及与细胞成分结合差异进行校正后，这给出了cGMP扩散系数的上限为11×10⁻⁸ cm² s⁻¹。为了正确比较这两组测量结果，我们研究了Fl-cGMP在截短的ROS中的扩散。根据Fl-cGMP激活电流的动力学，我们得到该类似物的扩散系数为3×10⁻⁸ cm² s⁻¹；从相同截短的ROS测量得到的cGMP扩散系数约为80×10⁻⁸ cm² s⁻¹。因此，一个27的系数似乎适合校正cGMP和Fl-cGMP之间的大小差异以及细胞内结合差异。将这个校正系数应用于Olson和Pugh（1993）对Fl-cGMP扩散系数的测量结果，得到的cGMP扩散系数约为30×10⁻⁸ cm² s⁻¹，与从截短的ROS测量得到的值合理相符。
