ACTIVATION OF RETINAL ROD CGMP-GATED CHANNELS - WHAT MAKES FOR AN EFFECTIVE 8-SUBSTITUTED DERIVATIVE OF CGMP

Analogs of cGMP bearing diverse substituents at the C-8 position of the guanine ring system have been shown to activate the cGMP-activated channel of retinal rods at concentrations lower than cGMP itself. In an effort to understand this behavior, we synthesized eight novel C-8-substituted derivatives and tested their ability to activate channels in excised patches from salamander rod outer segments. We express the effectiveness of each analog as a ratio (in brackets) of the concentration required to open half of the channels in a patch to that required of 8-Br-cGMP, previously shown to be about 10 times more effective than cGMP. Five of the derivatives contained a thio substitution at C-8: n-propylthio-cGMP [0.61], sulfoethylthio-cGMP [0.90], carboxyethylthio-cGMP [0.97], aminoethylthio-cGMP [2.8], and (trimethylamino)ethylthio-cGMP [8.5]. Three of the derivatives contained an amino substitution at C-8: carboxyethylamino-cGMP [22], n-propylamino-cGMP [25], and aminoethylamino-cGMP [230]. The results indicate that thio-substitution at C-8 produces more effective analogs than does amino-substitution, regardless of the chemical nature of the terminal functional group. Derivatives containing neutral and apolar tails opened channels at much lower concentrations than their positively-charged counterparts with the same C-8 substituent. Analogs having negatively-charged tails were also more effective than those with positive charge but not quite as effective as those with neutral tails. H-1 NMR measurements of the H2' chemical shift, previously shown to be sensitive to conformation about the N-glycosidic bond, revealed no significant differences in the syn-anti equilibrium between pairs of amino- or thio-substituted derivatives that opened channels at widely differing concentrations. The disparities in analog effectiveness most likely reflect differences in the chemical interactions between the C-8-substitutions or neighboring atoms affected by the substitutions and the channel's binding sites.