Vasodilator responses of rat isolated tail artery enhanced by oxygen‐dependent, photochemical release of nitric oxide from iron‐sulphur‐nitrosyls

• Published in: British journal of pharmacology Vol. 117; no. 7; pp. 1549 - 1557
• Main Authors: Flitney, F.W., Megson, I.L., Thomson, Joanne L.M., Kennovin, G.D., Butler, A.R.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.04.1996; Nature Publishing
• Subjects: Animals; Arginine - analogs & derivatives; Arginine - pharmacology; Arteries - drug effects; Biological and medical sciences; Cardiovascular system; Enzyme Inhibitors - pharmacology; In Vitro Techniques; Iron Chelating Agents - chemistry; Iron Chelating Agents - pharmacology; iron‐sulphur‐nitrosyl clusters; Lasers; Male; Medical sciences; Muscle, Smooth, Vascular - drug effects; Muscle, Smooth, Vascular - physiology; Nitric oxide; Nitric Oxide - metabolism; Nitric Oxide - pharmacology; Nitric Oxide Synthase - antagonists & inhibitors; Nitric Oxide Synthase - metabolism; Nitroso Compounds - chemistry; Nitroso Compounds - pharmacology; omega-N-Methylarginine; Oxygen - metabolism; Oxyhemoglobins - pharmacology; Pharmacology. Drug treatments; photodecomposition; Photolysis; Rats; Rats, Wistar; Superoxide Dismutase - pharmacology; vascular smooth muscle; Vasodilation - drug effects; vasodilator; Vasodilator Agents - chemistry; Vasodilator Agents - pharmacology; Vasodilator agents. Cerebral vasodilators; Oxygen; Vasodilator agent; Rat; Nitrosyl; Rodentia; Smooth muscle; Iron sulfide; In vitro; Artery; Muscular relaxation; Vertebrata; Mammalia; Animal; Blood vessel; Visible radiation; Nitrogen monoxide; Photochemical degradation
• ISSN: 0007-1188; 1476-5381
• DOI: 10.1111/j.1476-5381.1996.tb15320.x

1 The vasodilator properties and photochemical decomposition of two synthetic iron‐sulphur‐nitrosyl clusters (cluster A: [Fe4S4(NO)4], tetranitrosyl‐tetra‐μ3‐sulphido‐tetrahedro‐tetrairon; and B[Fe4S3 (NO)7]−1, heptanitrosyl‐tri‐μ3‐thioxotetraferrate(−1)) have been investigated. Experiments were carried out on isolated, internally‐perfused segments of rat tail artery. 2 Bolus injections (10 μl) of A or B (>0.25 mM) delivered into the internal perfusate generated sustained (or S‐type) vasodilator responses, characterized by a persistent plateau of reduced tone due to NO released from clusters which enter and become trapped within endothelial cells. Clusters were therefore irradiated with visible laser light (λ = 457.9 or 514.5 nm) either (a) in solution, while passing through a glass tube en route to the artery; or (b) when retained within the endothelium, by illuminating the artery directly during the plateau of an S‐type response. Irradiation produced an additional vasodilator response, the magnitude of which depended upon wavelength and laser beam energy. 3 The nitric oxide synthase inhibitor, NG‐monomethyl‐L‐arginine (100 μm), had no effect on light‐induced vasodilator responses. However, they were (a) blocked entirely by adding oxyhaemoglobin (5 μm) to the internal perfusate; and (b) greatly enhanced by the enzyme superoxide dismutase (150 u ml−1). 4 Photolysis of cluster B was measured by absorption spectroscopy and by detecting NO released with an electrochemical sensor. The photochemical reaction was found to be oxygen‐dependent. The half‐time for inactivation of cluster‐derived NO was measured by interposing different lengths of tubing (i.e time delays) between the photolysis tube and NO sensor. The steady‐state probe current decayed exponentially with increasing delay time, with a t½ of 21 s. The amplitudes of vasodilator responses of the tail artery also decreased exponentially by increasing the time delay (t½ = 58 s). Superoxide dismutase (150 u ml−1) prevented this from happening, showing that ‘inactivation’ of cluster‐derived NO was caused by reaction with superoxide anions formed during photolysis. 5 We conclude that potentiation of vasodilator responses to iron‐sulphur‐nitrosyl clusters by visible light is due to an oxygen‐dependent photochemical reaction which accelerates the release of ligated nitrosyl groups as free NO. Based on our measurements, we estimate that ca 100 pM NO is sufficient to produce a just‐detectable additional vasodilatation and that the ED50 dose is ca 3.7 nM.