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 (7" c ) 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 /R lol ) of the visual pigment; Rl/R lol was compared with R*/R lol , the fractional bleach at which the peak level of activated transducin approaches saturation. Over an approximately 8 In unit range of If that included the predicted value of R*/R lol , T c increased linearly with In If. Within the linear range, the slope of the function yielded an apparent exponential time constant (T 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 If-, 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 transducin 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 (/?*) (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 / ? ' , and that T C represents T R >, the lifetime of R* in the phototransduction process. The results furthermore suggest a preservation of the "dark-adapted" value of T R * within the investigated range of background intensity.
We have cloned cDNAs for two closely related connexins (Cx), Cx35 and Cx34.7, from a perch retinal cDNA library. Sequencing of PCR products from genomic DNA revealed that both connexins have an intron 71 bp after the translation initiation site; in Cx35, the intron is 900 bp in length, whereas in Cx34.7 it is ϳ20 kb. Southern blots of genomic DNA suggest that the two connexins represent independent single copy genes. In Northern blots, Cx35 and Cx34.7 transcripts were detected in retina and brain; Cx34.7 also showed a weak signal in smooth muscle (gut) RNA. Antibodies against Cx35 labeled a 30 kDa band on a Western blot of retinal membranes, and in histological sections, the pattern of antibody recognition was consistent with labeling of bipolar cells and unidentified processes in the inner plexiform and nerve fiber layers. When expressed in Xenopus oocytes, Cx35 and Cx34.7 formed homotypic gap junctions, but the junctional conductance between paired oocytes expressing Cx35 was 10-fold greater than that recorded for gap junctional channels formed by Cx34.7. The homotypic gap-junctional channels were closed in a voltage-dependent manner but with relatively weak voltage sensitivity. Heterotypic gap junctions formed by Cx35 and Cx34.7 displayed junctional conductances similar to those of Cx34.7 homotypic pairs and showed a slightly asymmetric current-voltage relationship; the side expressing Cx35 exhibited a higher sensitivity to transjunctional potentials. An analysis of the sequence and gene structure of the connexin family revealed that perch Cx35 and Cx34.7, skate Cx35, and mouse Cx36 constitute a novel ␥ subgroup.
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