title: Diffusion coefficient of the cyclic GMP analog 8-(fluoresceinyl)thioguanosine 3',5' cyclic monophosphate in the salamander rod outer segment.
abstract: 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.
authors.affiliation: Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Koutalos
authors.forename: R L
authors.initials: RL
authors.lastname: Brown
authors.forename: J W
authors.initials: JW
authors.lastname: Karpen
authors.forename: K W
authors.initials: KW
authors.lastname: Yau
mesh_terms: D000557:Ambystoma / Q000378:metabolism*; D000818:Animals; D001693:Biological Transport, Active; D055592:Biophysical Phenomena; D001703:Biophysics; D006152:Cyclic GMP / Q000031:analogs & derivatives* / Q000378:metabolism / Q000493:pharmacokinetics; D004058:Diffusion; D005452:Fluoresceins / Q000493:pharmacokinetics*; D066298:In Vitro Techniques; D007700:Kinetics; D012374:Rod Cell Outer Segment / Q000378:metabolism*
chemical_list: D005452:Fluoresceins; C049144:8-(5-thioacetamidofluorescein)cyclic-GMP; D006152:Cyclic GMP