Investigation of the contributions of nitric oxide and prostaglandins to the actions of endothelins and sarafotoxin 6c in rat isolated perfused lungs.
1. The aims of the study were to assess the contribution of prostaglandins and nitric oxide (NO) to the effects of endothelin (ETs) and sarafotoxin 6c (SX6c) in perfused rat lungs. This was carried out by using indomethacin, a cyclo-oxygenase inhibitor and NG-nitro-L-arginine (L-NOARG), a NO synthase inhibitor. Responses were studied under basal perfusion conditions and in other experiments after the elevation of vascular tone with the thromboxane-mimetic, U46619. The sub-types of ET receptors involved were characterized by use of ET receptor antagonists and cross-tachyphylaxis. 2. Pulmonary perfusion pressure (PPP), lung weight and pulmonary inflation pressure (PIP), were continuously recorded. Although L-NOARG (100 microM) did not alter basal parameters it markedly augmented the vasoconstriction and lung weight increases induced by ET-1 (50-400 pmol) or SX6C (25-200 pmol) while vasoconstrictor responses to phenylephrine were not affected by L-NOARG. 3. L-NOARG markedly potentiated the bronchoconstriction induced by ET-1 or SX6C whereas it had no effect on responses to carbachol. 4. When vascular tone was elevated, low doses (1.25-40 pmol) of ET-1, ET-3 and SX6C produced falls in PPP. The vasodilator potencies were SX6C > ET-1 = ET-3. The ETA receptor antagonist, BQ123, did not affect these depressor responses whereas the mixed ETA/ETB antagonist, bosentan, blocked them. 5. Indomethacin (10 microM) partially inhibited vasodilator response to ET-1, whereas it had no effect on SX6C-induced vasodilation. 6. L-NOARG plus indomethacin completely blocked ET-1 induced vasodilation, whereas responses to SX6C were blocked by L-NOARG alone. 7. Repeated injections of submaximal doses of ET-1 or SX6C caused tachyphylaxis to vasodilator responses. Subsequent injections of SX6C or ET-1 did not elicit depressor responses showing cross tachyphylaxis had occurred. 8. These findings indicate that under basal conditions the pulmonary vasoconstrictor, lung weight and bronchoconstrictor responses to ET-1 and SX6C are attenuated by evoked release of nitric oxide (NO). When vascular tone was elevated, lower doses of ETs and SX6C produced vasodilatation. These vasodilator responses are indirect, those to SX6C being mediated via NO production, whereas those to ET-1 involve both NO and prostanoid(s). Tachyphylaxis and ET antagonist experiments indicate that the same receptor subtype is involved in mediating the vasodilatation and that this is of the ETB type located on the endothelium. However the post-receptor vasodilator events triggered by ET-1 or SX6C appear to be different.