Rod light response augmented by active phosphodiesterase.

Light activates rod outer segment (ROS) phosphodiesterase (PDEase), as shown by previous biochemical and physiological studies. We have further investigated the role of PDEase in this system by injecting trypsin-activated PDEase, purified from bovine ROS, into single ROS of the isolated retina of the toad Bufo marinus. Injection of about 300 molecules of activated PDEase in darkness is without immediate detectable effect, as measured by intracellular membrane-voltage recording. The effect of the activated PDEase injections only becomes evident after illumination. The light response is augmented; kinetics of repolarization are slowed. We conclude that this augmentation of the light-dependent hyperpolarization results from the hydrolysis of endogenous cyclic GMP caused by injected PDEase. These results provide evidence that PDEase affects light-dependent channels of the vertebrate scotopic photoreceptors but do not specify whether the effects are exercised for the initial hyperpolarizing phase of the receptor potential and for the recovery phase or only for the recovery phase.