6 research outputs found
Estimating the rate constant of cyclic GMP hydrolysis by activated phosphodiesterase in photoreceptors
The early steps of light response occur in the outer segment of rod and cone
photoreceptor. They involve the hydrolysis of cGMP, a soluble cyclic
nucleotide, that gates ionic channels located in the outer segment membrane. We
shall study here the rate by which cGMP is hydrolyzed by activated
phosphodiesterase (PDE). This process has been characterized experimentally by
two different rate constants and : accounts
for the effect of all spontaneously active PDE in the outer segment, and
characterizes cGMP hydrolysis induced by a single light-activated
PDE. So far, no attempt has been made to derive the experimental values of
and from a theoretical model, which is the goal of this
work. Using a model of diffusion in the confined rod geometry, we derive
analytical expressions for and by calculating the flux
of cGMP molecules to an activated PDE site. We obtain the dependency of these
rate constants as a function of the outer segment geometry, the PDE activation
and deactivation rates and the aqueous cGMP diffusion constant. Our formulas
show good agreement with experimental measurements. Finally, we use our
derivation to model the time course of the cGMP concentration in a
transversally well stirred outer segment.Comment: 20 pages, revtex4, 5 figure
A kinetic analysis of mouse rod and cone photoreceptor responses
International audienceMost vertebrate eyes have rods for dim-light vision and cones for brighter light and higher temporal sensitivity. r Rods evolved from cone-like precursors through expression of different transduction genes or the same genes at different expression levels, but we do not know which molecular differences were most important. r We approached this problem by analysing rod and cone responses with the same model but with different values for model parameters. We showed that, in addition to outer-segment volume, the most important differences between rods and cones are: (1) decreased transduction gain, reflecting smaller amplification in the G-protein cascade; (2) a faster rate of turnover of the second messenger cGMP in darkness; and (3) an accelerated rate of decay of the effector enzyme phosphodiesterase and perhaps also of activated visual pigment. r We believe our analysis has identified the principal alterations during evolution responsible for the duplex retina