The saturated response of vertebrate rods and its relation to cGMP metabolism.

Strong light flashes saturate the response of a rod, and increasing flash intensities prolong the period of saturation. We have found that the period of saturation lengthens from base to tip of the outer segment and we have shown that free cGMP is replenished locally to terminate saturation and not in the G-cyclase rich axonemal region near the inner segment which contains the cell mitochondria. Our results also show that each section of outer segment acts as an independent module in this phase of transduction.