VASODILATOR RESPONSES OF RAT ISOLATED TAIL ARTERY ENHANCED BY OXYGEN-DEPENDENT, PHOTOCHEMICAL RELEASE OF NITRIC-OXIDE FROM IRON-SULFUR-NITROSYLS

1 The vasodilator properties and photochemical decomposition of two sy nthetic iron-sulphur-nitrosyl clusters (cluster A: [Fe4S4(NO)(4)], itr osyl-tetra-mu(3)-sulphido-tetrahedro-tetrairon; and B:[Fe4S3 (NO)(7)]( -1), heptanitrosyl-tri-mu(3)-thioxotetraferrate(-1)) have been investi gated. Experiments were carried out on isolated, internally-perfused s egments of rat tail artery. 2 Bolus injections (10 mu l) of A or B (>0 .25 mM) delivered into the internal perfusate generated sustained (or S-type) vasodilator responses, characterized by a persistent plateau o f 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 solu tion, while passing through a glass tube en route to the artery; or (b ) when retained within the endothelium, by illuminating the artery dir ectly during the plateau of an S-type response. Irradiation produced a n additional vasodilator response, the magnitude of which depended upo n wavelength and laser beam energy. 3 The nitric oxide synthase inhibi tor, N-G-monomethyl-L-arginine (100 mu M), had no effect on light-indu ced vasodilator responses. However, they were (a) blocked entirely by adding oxyhaemoglobin (5 mu M) to the internal perfusate; and (b) grea tly enhanced by the enzyme superoxide dismutase (150 u ml(-1)). 4 Phot olysis of cluster B was measured by absorption spectroscopy and by det ecting NO released with an electrochemical sensor. The photochemical r eaction was found to be oxygen-dependent. The half-lime for inactivati on 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 resp onses of the tail artery also decreased exponentially by increasing th e time delay (t(1/2)=58 s). Superoxide dismutase (150 u ml(-1)) preven ted this From happening, showing that 'inactivation' of cluster-derive d NO was caused by reaction with superoxide anions formed during photo lysis. 5 We conclude that potentiation of vasodilator responses to iro n-sulphur-nitrosyl clusters by visible light is due to an oxygen-depen dent photochemical reaction which accelerates the release of ligated n itrosyl groups as free NO. Based on our measurements, we estimate that ca 100 pM NO is sufficient to produce a just-detectable additional va sodilatation and that the ED(50) dose is ca 3.7 nM.