LIGHT-DEPENDENT DELAY IN THE FALLING PHASE OF THE RETINAL ROD PHOTORESPONSE

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 (T(c)) that preceded recovery of the dark current by a criterion amount (3 pA) was analyzed in relation to the flash intensity (I(f)), and to the corresponding fractional bleach (R0*/R(tot)) of the visual pigment; R0*/R(tot) was compared with R(s)*/R(tot)), the fractional bleach at which the peak level of activated transducin approaches saturation. Over an approximately 8 In unit range of I(f) that included the predicted value of R(s)*/R(tot), T(c) increased linearly with In I(f). Within the linear range, the slope of the function yielded an apparent exponential time constant (tau(c)) of 1.7 +/- 0.2 s (mean +/- S.D.). Background light reduced the value of T(c) measured at a given flash intensity but preserved a range over which T(c) increased linearly with In I(f); the linear-range slope was similar to that measured in the absence of background light. The intensity dependence of T(c) resembles that of a delay (T(d)) seen in light-scattering experiments on bovine retinas, which describes the period of essentially complete activation of transduction following a bright flash; the slope of the function relating T(d) and 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 of R*, and that tau(c) represents tau(R)*, the lifetime of R* in the phototransduction process. The results furthermore suggest a preservation of the "dark-adapted" value of tau(R)* within the investigated range of background intensity.