Vasodilator responses of rat isolated tail artery enhanced by oxygen- dependent, photochemical release of nitric oxide from iron-sulphur-nitrosyls.
1. The vasodilator properties and photochemical decomposition of two synthetic iron-sulphur-nitrosyl clusters (cluster A: [Fe4S4(NO)4], tetranitrosyl-tetra-mu 3-sulphido-tetrahedro-tetrairon; and B:[Fe4S3 (NO)7]-1, heptanitrosyl-tri-mu 3-thioxotetraferrate(-1)) have been investigated. Experiments were carried out on isolated, internally-perfused segments of rat tail artery. 2. Bolus injections (10 microliters) 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 (lambda = 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 microM), had no effect on light-induced vasodilator responses. However, they were (a) blocked entirely by adding oxyhaemoglobin (5 microM) 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 t 1/2 of 21 s. The amplitudes of vasodilator responses of the tail artery also decreased exponentially by increasing the time delay (t 1/2 = 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.
Flitney
F W
FW
Division of Cell & Molecular Biology, School of Biological and Medical Sciences, University of St Andrews, Fife Scotland.
Megson
I L
IL
Thomson
J L
JL
Kennovin
G D
GD
Butler
A R
AR
D000818:Animals; D001120:Arginine / Q000031:analogs & derivatives / Q000494:pharmacology; D001158:Arteries / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D066298:In Vitro Techniques; D007502:Iron Chelating Agents / Q000737:chemistry / Q000494:pharmacology*; D007834:Lasers; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000502:physiology; D009569:Nitric Oxide / Q000378:metabolism / Q000494:pharmacology*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D009603:Nitroso Compounds / Q000737:chemistry / Q000494:pharmacology*; D010100:Oxygen / Q000378:metabolism; D010108:Oxyhemoglobins / Q000494:pharmacology; D010782:Photolysis; D051381:Rats; D017208:Rats, Wistar; D013482:Superoxide Dismutase / Q000494:pharmacology; D014664:Vasodilation / Q000187:drug effects*; D014665:Vasodilator Agents / Q000737:chemistry / Q000494:pharmacology*; D019323:omega-N-Methylarginine
D004791:Enzyme Inhibitors; D007502:Iron Chelating Agents; D009603:Nitroso Compounds; D010108:Oxyhemoglobins; D014665:Vasodilator Agents; D019323:omega-N-Methylarginine; D009569:Nitric Oxide; D001120:Arginine; D019001:Nitric Oxide Synthase; D013482:Superoxide Dismutase; D010100:Oxygen