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 C8 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 C8-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 C8: 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 C8: carboxyethylamino-cGMP [22], n-propylamino-cGMP [25], and aminoethylamino-cGMP [230]. The results indicate that thio-substitution at C8 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 C8 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.(ABSTRACT TRUNCATED AT 250 WORDS)