SUBSTITUTED CGMP ANALOGS CAN ACT AS SELECTIVE AGONISTS OF THE ROD PHOTORECEPTOR CGMP-GATED CATION CHANNEL

Cyclic nucleotide-gated (CNG) channels are expressed in many cell type s in both the nervous system and nonexcitable tissues. In order to und erstand the roles of cGMP-gated channels, and to distinguish actions o f cGMP mediated through CNG channels from those through cGMP-dependent protein kinase (G-kinase), several new cGMP analogs were tested for p otency as CNG channel agonists. Using Xenopus oocytes expressing the r at rod cGMP-gated ion channel cx-subunit, we showed that an analog con taining a pCPT group at the 8-position, 8-pCPT-cGMP, was 80 times more potent than cGMP and 14 times more potent than 8-Br-cGMP. 8-pCPT-cGMP is the most potent CNG channel agonist so far described and also has the advantages of much better membrane permeability as well as much hi gher resistance to PDE-hydrolysis, as compared with 8-Br-cGMP. Modific ation of both 8-Br-cGMP and 8-pCPT-cGMP by introduction of a sulphur a tom into the cyclic phosphate group gave smaller changes in agonist ef ficiency. Both Sp-8-Br-cGMPS and Sp-8-pCPT-cGMPS acted as agonists of CNG channels and are also G-kinase activators. In contrast, Rp-8-Br-cG MPS was a channel agonist, with an EC50 of 173.5 mu M, but a G-kinase antagonist with a Ki of 4 mu M. Finally, Rp-8-pCPT-cGMPS was a channel agonist and showed additional noncompetitive antagonist activity at h igher concentrations. The results suggest that 8-pCPT-cGMPS is a highl y potent photoreceptor CNG channel agonist with high membrane permeabi lity and PDE-resistance and furthermore Rp-8-Br-cGMPS can be used to t est whether the actions of cGMP are selectively mediated by CNG channe ls.