IDENTIFICATION OF COMPETITIVE ANTAGONISTS OF THE ROD PHOTORECEPTOR CGMP-GATED CATION CHANNEL : BETA -PHENYL-1,N2-ETHENO-SUBSTITUTED CGMP ANALOGUES AS PROBES OF THE CGMP-BINDING SITE

cGMP is the natural activator of the cyclic nucleotide-gated channel originally isolated from rod photoreceptors but now known to be expressed in a wide variety of neural and non-neural cells. To identify antagonists of cGMP action and to better understand the interaction between cGMP and the channel protein, experimental studies were undertaken using four synthetic cGMP analogues, PET-cGMP, 8-Br-PET-cGMP, Rp-8-Br-PET-cGMPS, and Sp-8-Br-PET-cGMPS. With excised patches from either Xenopus oocytes expressing a cloned rat rod channel α-subunit or from native Xenopus rod photoreceptors, Rp-8-Br-PET-cGMPS competitively suppressed the cGMP-induced current with an IC50 of 25 μM and Sp-8-Br-PET-cGMPS inhibited this current with an IC50 of 105 μM. On the expressed rat rod channel, 8-Br-PET-cGMP behaved as a very weak partial agonist at high concentrations and an antagonist (IC50 = 64 μM) at lower concentrations when coapplied with cGMP. PET-cGMP did not activate channel currents alone but showed synergism when coa...