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 (Tc) 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/Rtot was compared with R*s/Rtot, the fractional bleach at which the peak level of activated transducin approaches saturation. Over an approximately 8 ln unit range of I(f) that included the predicted value of R*s/Rtot, Tc increased linearly with ln 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 Tc measured at a given flash intensity but preserved a range over which Tc increased linearly with ln I(f); the linear-range slope was similar to that measured in the absence of background light. The intensity dependence of Tc resembles 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 relating Td and ln 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 TR*, the lifetime of R* in the phototransduction process. The results furthermore suggest a preservation of the "dark-adapted" value of TR* within the investigated range of background intensity.
Pepperberg
D R
DR
Department of Ophthalmology and Visual Sciences, University of Illinois College of Medicine, Chicago 60612.
Cornwall
M C
MC
Kahlert
M
M
Hofmann
K P
KP
Jin
J
J
Jones
G J
GJ
Ripps
H
H
D000557:Ambystoma; D000818:Animals; D003623:Dark Adaptation; D004594:Electrophysiology; D008027:Light; D008433:Mathematics; D010775:Photic Stimulation; D010786:Photoreceptor Cells / Q000502:physiology*; D012542:Scattering, Radiation; D015398:Signal Transduction / Q000502:physiology*; D015293:Transducin / Q000502:physiology
D015293:Transducin