Light-dependent delay in the falling phase of the retinal rod photoresponse

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Abstract
Using suction electrodes, photocurrent responses to 100-ms saturating flashes were recorded from isolated retinal rods of the larval-stage tiger salamander (Ambystoma tigrinum). The delay period (Te) that preceded recovery of the dark current by a criterion amount (3 pA) was analyzed in relation to the flash intensity (If), and to the corresponding fractional bleach (R*0/Rtot) of the visual pigment;R*0/Rtotwas compared withR*s/Rtotthe fractional bleach at which the peak level of activated transducin approaches saturation. Over an approximately 8 In unit range ofIfthat included the predicted value ofR*s/Rtot, Teincreased linearly with InIf. Within the linear range, the slope of the function yielded an apparent exponential time constant (TC) of 1.7 ± 0.2 s (mean ± S.D.). Background light reduced the value ofTcmeasured at a given flash intensity but preserved a range over whichTcincreased linearly with InIf; the linear-range slope was similar to that measured in the absence of background light. The intensity dependence ofTcresembles that of a delay (Td) seen in light-scattering experiments on bovine retinas, which describes the period of essentially complete activation of transducin following a bright flash; the slope of the function relatingTdand In flash intensity is thought to reflect the lifetime of photoactivated visual pigment (R*) (Pepperberg et al., 1988; Kahlert et al., 1990). The present data suggest that the electrophysiological delay has a similar basis in the deactivation kinetics ofR*, and thatTcrepresentsTR* the lifetime ofR* in the phototransduction process. The results furthermore suggest a preservation of the “dark-adapted” value ofTR* within the investigated range of background intensity.