BLEACHED PIGMENT ACTIVATES TRANSDUCTION IN SALAMANDER CONES

We have used suction electrode recording together with rapid steps int o 0.5 mM IBMX solution to investigate changes in guanylyl cyclase velo city produced by pigment bleaching in isolated cones of the salamander Ambystoma tigrinum. Both backgrounds and bleaches accelerate the time course of current increase during steps into IBMX. We interpret this as evidence that the velocity of the guanylyl. cyclase is increased in background light or after bleaching. Our results indicate that cyclas e velocity increases nearly linearly with increasing percent pigment b leached but nonlinearly (and may saturate) with increasing background intensity. In cones (as previously demonstrated for rods), light-activ ated pigment and bleached pigment appear to have somewhat different ef fects on the transduction cascade. The effect of bleaching on cyclase rate is maintained for at least 15-20 min after the light is removed, much longer than is required after a bleach for circulating current an d sensitivity to stabilize in an isolated cone. The effect on the cycl ase rate can be completely reversed by treatment with liposomes contai ning 11-cis retinal. The effects of bleaching can also be partially re versed by beta-ionone, an analogue of the chromophore 11-cis-retinal w hich does not form a covalent attachment to opsin. Perfusion of a blea ched cone with beta-ionone produces a rapid increase in circulating cu rrent and sensitivity, which rapidly reverses when the beta-ionone is removed. Perfusion with beta-ionone also causes a partial reversal of the bleach-induced acceleration of cyclase velocity. We conclude that bleaching produces an ''equivalent background'' excitation of the tran sduction cascade in cones, perhaps by a mechanism similar to that in r ods.