E-ring 8-isoprostanes inhibit ACh release from parasympathetic nerves innervating guinea-pig trachea through agonism of prostanoid receptors of the EP3-subtype

1 In the present study, we examined the effect of E-ring 8-isoprostanes on cholinergic neurotransmission in guinea-pig trachea and identified the receptor(s) involved. As isoprostanes are isomeric with prostaglandins, PGE(2) and sulprostone (a selective EP3-receptor agonist) were examined in parallel. 2 8-Iso-PGE(1), 8-iso-PGE(2) (0.1 nM-1 muM), sulprostone (1 nM-1 muM) and PGE(2) (l muM) suppressed EFS-evoked [H-3]ACh release from guinea-pig trachea in a concentration-dependent manner, producing 39.5, 53.9, 61.2 and 59.9% inhibition, respectively, at 1 muM. It should be noted that an established maximum effective concentration was not determined. 3 Neither SQ 29,548 (1 muM; a TP-receptor antagonist) nor AH 6809 (10 muM; an EP1-/EP2-/DP-receptor antagonist) reversed the inhibitory effect of these compounds. 4 L-798,106, a novel and highly selective EP3-receptor antagonist, produced a parallel shift to the right of the concentration-response curves that described the inhibitory action of sulprostone on EFS-evoked contractile responses in guinea-pig vas deferens (an established EP3-receptor-expressing tissue), from which a mean pA(2) of 7.48 was derived. On guinea-pig trachea, L-798,106 also antagonised sulprostone-induced inhibition of EFS-induced twitch responses, with similar potency (mean pA(2) = 7.82). 5 The inhibitory effects of 8-iso-PGE(1), 8-iso-PGE(2), sulprostone and PGE(2) on EFS-induced [H-3]ACh release was blocked by L-798,106 at a concentration (10 muM) that binds only weakly to human recombinant EP1-, EP2- and EP4-receptor subtypes expressed in HEK 293 cells. 6 These data suggest that E-ring 8-isoprostanes, PGE(2) and sulprostone inhibit EFS-evoked [H-3]ACh release from cholinergic nerves innervating guinea-pig trachea, by interacting with prejunctional prostanoid receptors of the EP3-Subtype.