Kinetics of desensitization induced by saturating flashes in toad and salamander rods

1. The suction pipette technique was used to examine the effect of a conditioning pre-flash on the saturation time (t(sat)) of a bright test flash (intensity 10,000-250,000 isomerizations) delivered to intact salamander or toad rod outer segments. The conditioning flash was delivered 0-60 s before the test flash; its intensity was typically between six and sixty times dimmer than the test flash, and it was sufficient by itself to fully saturate the photocurrent. 2. A saturating pre-flash delivered before a saturating test flash reduced the t(sat) of the test flash. This was equivalent to a reduction in phototransduction gain (Ψ). 3. The pre-flash had little effect on τ(c), the time constant of decay of the rate-limiting species in photoresponse inactivation (activated rhodopsin or the activated G-protein-phosphodiesterase complex). 4. The t(sat) declined exponentially as the separation time between a fixed intensity pre-flash and test flash was increased. The time constant (τ(p)) of decline in t(sat) was approximately 2.4 s. The maximum reduction in t(sat) corresponded to a reduction in the apparent gain of phototransduction to approximately 0.10 of its original level. This exponential decline is consistent with a [Ca²⁺](i)-mediated effect. 5. We conclude that the rate-limiting step in response inactivation and the step responsible for light-induced gain reduction constitute separate and distinct steps of the phototransduction cascade.