DIFFUSION-COEFFICIENT OF THE CYCLIC-GMP ANALOG 8-(FLUORESCEINYL)THIOGUANOSINE 3',5'-CYCLIC-MONOPHOSPHATE IN THE SALAMANDER ROD OUTER SEGMENT

Cyclic GMP (cGMP) is the intracellular messenger mediating phototransd uction in retinal rods, with its longitudinal diffusion in the rod out er segment (ROS) likely to be a factor in determining light sensitivit y. From the kinetics of cGMP-activated currents in the truncated ROS o f the salamander (Ambystoma tigrinum), the cGMP diffusion coefficient was previously estimated to be similar to 60 x 10(-8) cm(2) s(-1). On the other hand, fluorescence measurements in intact salamander ROS usi ng 8-(fluoresceinyl)thioguanosine 3',5'-cyclic monophosphate (Fl-cGMP) led to a diffusion coefficient for this compound of 1 x 10(-8) cm(2) S-1; after corrections for differences in size and in binding to cellu lar components between cGMP and Fl-cGMP, this gave an upper limit of 1 1 x 10(-8) cm(2) 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-activate d currents, we have obtained a diffusion coefficient of 3 x 10(-8) cm( 2) s(-1) for this analog; the cGMP diffusion coefficient measured from the same truncated ROSs was similar to 80 x 10(-8) cm(2) s(-1). Thus, a factor of 27 appears appropriate for correcting differences in size and intracellular binding between cGMP and Fl-cGMP. Application of th is correction factor to the Fl-cGMP diffusion coefficient measurements by Olson and Pugh (1993) gives a cGMP diffusion coefficient of simila r to 30 x 10(-8) cm(2) s(-1), in reasonable agreement with the value m easured from the truncated ROS.