BLEACHED PIGMENT ACTIVATES TRANSDUCTION IN ISOLATED RODS OF THE SALAMANDER RETINA

1. We have used suction electrode recording together with rapid steps into Li+ solution and 0.5 mM IBMX solution to estimate the rates of th e guanylyl phosphodiesterase (PDE) and guanylyl cyclase in isolated ro ds of the salamander, Ambystoma tigrinum. 2. We show that both the PDE and cyclase velocities are accelerated by steady background light. Th e steady velocities of both enzymes appear to be saturating functions of background intensity. 3. Bleaching also accelerates both the PDE an d cyclase. This effect is maintained long after the bleaching stimulus is removed (up to 2 h) and is reversed only if the photopigment is re generated with exogenous chromophore. 4. The estimated steady-state PD E and cyclase velocities appear to be linear functions of the amount o f pigment bleached, as if each bleached pigment molecule activated the transduction cascade with the same probability and gain. 5. The effec tiveness of bleached pigment in activating transduction is only 10(-6) to 10(-7) times that of activated rhodopsin (Rh), but this is suffic ient after large bleaches to produce an 'equivalent background' excita tion of the rod, which is probably responsible, at least in part, for bleaching desensitization.