Fast electric photosignals with R1 and R2 component were measured in an artificial system formed by fragments of bovine rod outer segments. The kinetics of the R2 generating process could not be described simply by a single exponential function. Evidence is presented which shows that the time course of the R2 charge displacement Qd (t) should be described by the reciprocal power law Qd(t)=Qinf[1-(I + kpt/n)^-1] rather than by a sum of exponentials. This time course is interpreted as being due to a distribution of rhodopsin substates with different activation energies. The activation energy of the peak rate constant kp as well as the absolute rate constants are in very good agreement with the time constants obtained for the meta I-meta II transition when treated as an average first order process.
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