The role of endothelium-derived relaxing factor (EDRF) in the action of vasodilator (acetylcholine, histamine, nitroprusside) and vasoconstrictor (noradrenaline, vasopressin) drugs on vascular resistance in the isolated perfused kidney and mesentery of the rat was studied. 2. Acetylcholine (EC50 = 0.18 +/- 0.05 nmol and 3.1 +/- 0.06 nmol, n = 8) and histamine (EC50 = 31.2 +/- 4.9 nmol and 46.2 +/- 3.9 nmol, n = 8) produced dose-related vasodilatation in noradrenaline-preconstricted (i.e. 'high tone') rat renal and mesenteric blood vessels. The response to both vasodilators (but not nitroprusside) was abolished by infusion of CHAPS (4.7 mg ml-1, 30 s). By use of an immunocytochemical staining procedure CHAPS was demonstrated to remove vascular endothelial cells lining intrarenal blood vessels. 3. Gossypol (3 microM), metyrapone (10 microM) and nordihydroguaiaretic acid, (NDGA, 30 microM), presumed inhibitors of EDRF biosynthesis, reduced or abolished the response to acetylcholine and histamine in perfused kidney and mesentery of the rat without affecting vasodilatation due to nitroprusside. Mepacrine (10 microM) similarly abolished the response to acetylcholine and histamine but in addition, reduced the response to nitroprusside in both preparations. 4. Methylene blue (100 microM), a presumed antagonist of the effect of EDRF, abolished vasodilatation due to acetylcholine and histamine and reduced the response to nitroprusside in perfused rat kidney and mesentery. Superoxide dismutase, SOD (15 u ml-1), was without effect. 5. While CHAPS treatment significantly augmented the vasoconstrictor response to both noradrenaline and vasopressin in perfused renal and mesenteric vessels this effect was not mimicked by metyrapone or gossypol suggesting that the enhanced effect of vasopressor agents in CHAPSperfused rat organs is due to the removal of a permeability barrier rather than impaired EDRF formation. 6. Responses to vasoconstrictor and vasodilator drugs in the perfused kidney and mesentery were obtained in the presence of indomethacin (8 microM) which produced in excess of 90% inhibition of prostacyclin (PGI2) release as measured by radioimmunoassay of 6-oxo-prostaglandin F1 alpha,. (6-oxo- PGF1 alpha) in the Krebs effluent. 7. We provide evidence that EDRF mediates the vasodilator response to acetylcholine and histamine in resistance blood vessels in perfused rat kidney and mesentery. The possibility that EDRF has a physiological role to play in regulating the calibre of resistance blood vessels is discussed.
