Inhibition of noradrenaline release in the rat vena cava via prostanoid receptors of the EP3‐subtype

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Abstract
In segments of the rat vena cava preincubated with [3H]‐noradrenaline and superfused with physiological salt solution (containing desipramine and corticosterone), we studied the effects of prostaglandins of the D, E and F series, of a prostacyclin analogue and a thromboxane‐mimetic and of subtype‐selective prostaglandin E‐receptor (EP‐receptor) ligands on the electrically (0.66 Hz)‐evoked tritium overflow. The electrically‐evoked tritium overflow was inhibited by prostaglandin E2 (maximum inhibition by about 80%; pIC40 7.49). The effect of prostaglandin E2 was not affected by rauwolscine, which, by itself, increased the evoked overflow; the α2‐adrenoceptor antagonist was added to the superfusion medium in all subsequent experiments. Indomethacin failed to affect either the evoked tritium overflow or its inhibition by prostaglandin E2. The inhibitory effect of prostaglandin E2 on the electrically‐evoked tritium overflow was not altered by the EP1‐receptor antagonist, AH 6809 (6‐isopropoxy‐9‐oxoxanthene‐2‐carboxylic acid) at a concentration at least 30 fold higher than its pA2 value at EP1‐receptors. The following compounds mimicked the effect of prostaglandin E2 showing the following rank order of potencies: misoprostol (EP2‐/EP3‐receptor agonist) ≃ sulprostone (EP1/EP3‐receptor agonist) ≃ prostaglandin E1 = prostaglandin E2 ≫ iloprost (EP1‐/IP‐receptor agonist) = prostaglandin F2α. The evoked overflow was not affected by high concentrations of prostaglandin D2 or the thromboxane‐mimetic U46619 (9,11‐dideoxy‐11α, 9α‐epoxymethano‐prostaglandin F2α). The present results suggest that the postganglionic sympathetic nerve fibres innervating the rat vena cava are endowed with presynaptic EP3‐receptors. They are not tonically activated by endogenously formed products of cyclo‐oxygenase and do not interact with the presynaptic α2‐adrenoceptors.