Recovery kinetics of human rod phototransduction inferred from the two-branched a-wave saturation function (original) (raw)
1996, Journal of the Optical Society of America
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) a 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 ϳ4 3 10 4 scotopic troland second), T increases approximately in proportion to ln I f with a slope [DT͞D͑ln I f ͒] of Ӎ 0.3 s. At higher flash intensities, a different linear relation prevails, in which [DT͞D͑ln I f ͒] is Ӎ2.3 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 Ӎ2.3 s represents t R ء , the lifetime of photoactivated rhodopsin (R ء). Here we show that a modified version of this model provides an explanation for the lower branch of the a-wave saturation function. In this model, t E ء is the exponential lifetime of an activated species (E ء) within the transducin or guanosine 3 0 , 5 0-cyclic monophosphate (cGMP) phosphodiesterase stages of rod phototransduction; the generation of E ء by a single R ء occurs within temporally defined, elemental domains of disk membrane; and E x , the immediate product of E ء deactivation, is converted only slowly (time constant t Ex) to E, the form susceptible to reactivation by R ء. The model predicts that the decay of flash-activated cGMP phosphodiesterase (PDE*) is largely independent of the deactivation kinetics of R ء at early postflash times (i.e., at times preceding or comparable with the lifetime t E ء) and that the lower-branch slope (Ӎ0.3 s) of the a-wave saturation function represents t E ء. The predicted early-stage independence of PDE* decay and R ء 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 Ӎ 6.7 s for the time constant t Ex .
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