Interaction of hydrolysis-resistant analogs of cyclic GMP with the phosphodiesterase and light-sensitive channel of retinal rod outer segments.

Abstract

CGMP opens cation-selective channels when applied to the cytoplasmic side of excised patches of membrane from retinal rod outer segments (ROS). If the light-sensitive channel in intact rods is gated only by cGMP, it should be possible to find a hydrolysis-resistant analog of cGMP that blocks the normal response to light by holding the channel open independent of the degree of illumination. We have studied the interaction of 8-bromo-cGMP (8-Br-cGMP) and the Sp and Rp phosphorothioate derivatives of cGMP }(Sp)-cGMP[S] and (Rp)-cGMP[S]{ with the cGMP phosphodiesterase (PDEase) of ROS, the cGMP-sensitive channel of excised ROS patches, and the light-sensitive channel of intact rods. All three analogs were hydrolyzed by PDEase much more slowly than was cGMP. The maximal rates of hydrolysis of 8-Br-cGMP, (Sp)-cGMP[S], and (Rp)-cGMP[S] were 7.3, 3.7, and < 0.2 s-1, respectively, compared with 4000 s-1 for cGMP. These analogs are effective competitive inhibitors of the PDEase, with Ki values of 48, 25, and 90 .mu.M, respectively. The nucleotide-activated conductances of excised patches were half-maximal at concentrations of 1.6, 210, and 1200 .mu.M, respectively, compared with 17 .mu.M for cGMP. Thus, 8-Br-cGMP is a highly potent channel agonist. The effects of these analogs on the dark current and photoresponse of intact rod cells were also measured. A suction electrode monitored membrane current across the ROS, while a patch electrode sealed on the inner segment was used to introduce a cGMP analog and to control membrane potential. All three analogs increased the dark current and markedly slowed the response to light flashes. 8-Br-cGMP increased the dark current of the outer segment as much as 48-fold. After the concentration of this analog had risen sufficiently, little of the current could be shut off by light, as expected of a direct effect on the light-sensitive channel of the plasma membrane. These results are consistent with the notions that (i) the light-sensitive channel of rods is controlled solely by the instantaneous concentration of cGMP and (ii) the cGMP-sensitive channel of excised patches is identical to the light-sensitive channel of intact rods.