Variability in the time course of single photon responses from toad rods: Termination of rhodopsin's activity

We examined the responses of toad rod photoreceptors to single photons of l ight. To minimize the effects of variability in the early rising phase, we selected sets of responses that closely matched the rise of the mean single photon response. Responses selected in this way showed substantial variati ons in kinetics, appearing to peel off from a common time course after diff erent delays. Following incorporation of the calcium buffer BAPTA, the time to peeling off was retarded. Our analysis indicates that it is not necessa ry to invoke a long series of reaction steps to explain the shutoff of rhod opsin activity. Instead, our results suggest that the observed behavior is explicable by the presently known shutoff reactions of activated rhodopsin, modulated by feedback.