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It has been postulated that changes in the availability of partially reduced O2 species, such as O2 radicals, could serve as a link between PO2 in the alveolus and pulmonary vascular tone (Herz 11: 127-141, 1986). To assess this hypothesis, the hemodynamic effects of acute changes in the balance between the production of O2 radicals and availability of antioxidant enzymes were studied in the isolated perfused rat lung. Intravascular generation of O2 radicals, by administration of xanthine-xanthine oxidase, decreased the pulmonary vascular pressor response to alveolar hypoxia (-55 +/- 5%) and angiotensin II (-58 +/- 10%, P less than 0.01 for each) in isolated perfused rat lungs without increasing the lung wet-to-dry weight ratio. Decreases in pulmonary vascular reactivity were inhibited by pretreatment of the lung with desferrioxamine or a mixture of catalase and superoxide dismutase. Catalase and superoxide dismutase preserved the hypoxic pressor response whether given in liposomes or in dissolved form. Superoxide dismutase administered free in solution, or combined with catalase in liposomes, increased the normoxic pulmonary arterial pressure and enhanced vascular reactivity to angiotensin II and hypoxia. Lungs treated with antioxidant enzymes in liposomes had 50% higher lung catalase levels than control lungs (P less than 0.05). These findings demonstrate that exogenous partially reduced O2 species can decrease pulmonary vascular reactivity and suggest that endogenous radicals, superoxide radical in particular, might be important in modulating pulmonary vascular tone.

Crystals of K,S5 are orthorhombic, space group P2,2,2, with Z = 4 in a unit cell of dimensions: a = 6.494(1), b = 6.600(2), c = 17.414(5) 8. The structure has been elucidated by vector methods from 656 independent diffracted intensities measured on a four-circle diffractometer and refined to R 0.045. The [S,I2anion exists as a contorted but unbranched chain in which successive S-S bond lengths are 2.037, 2.074, 2.075, and 2.050(5) 8, and the bond angles are close to tetrahedral. The torsions in the chain around the S(2)-S(3) and S(3)-S(4) bonds are 74 and 69" respectively. The potassium ions are evenly distributed between the S5 units with minimum K... S 3.21 5(4) A, and a nearest-neighbour K -K of just under 4 8. THE only precise structural study of a polysulphide is of Na,S4,1 although earlier work has established the crystal arrangements in Na2S,,2 BaS,-H,O (since redetermined), and CS,S,.~All polysulphide ions so far investigated comprise unbranched non-linear chains of S atoms. The pentasulphides are of potential industrial interest because the [S5I2ion is believed to be the favoured species during the sulphur-rich stages of discharge of the sodium-sulphur battery, and hitherto there has been no structural study of a pentasulphide.? EXPERIMENTAL Crystals of K2S5 were prepared by first dissolving potassium metal in ethanol and then saturating the solution of potassium ethoxide so formed with hydrogen sulphide gas to give KHS; addition of the stoicheiometric quantity oi sulphur indicated by reaction (1) gave the required product, ZKHS + 4 s _+ K,S5 + H,S(1) which could be recrystallised from ethanol. All reactions were performed in a Schlenck tube under dry nitrogen.t After this work was complete, a structural study of T12S5 was published by B. Leclerc and T. S. Kabrd (ref. 12). The two crystals K,S, and T1,S5 have closely similar structures; in order to convert parameters for T12S, to the corresponding ones for K,S, the transformation a -+ b, b -+ c, c 3 a should be carried out.Crystals of K,S5 are red-orange prisms which decompose rapidly in air. Diffracted intensities were collected from a crystal of dimensions 0.17 x 0.12 x 0.10 mm mounted in nitrogen in a Lindemann capillary on a Syntex P2, fourcircle diffractometer, according t o methods described earlier.s Of the total 806 independent reflections for 3.7" < 28 < 50.0", 656 satisfied the criterion I > 2.50(1), and only these were used in the solution and refinement of the structure. RESULTS CrystalData.-K,S,, M = 238.5, Orthorhombic, a = 6.494(1), b = 6.600(2), c = 17.414(5) A, D, = 2.08 g ~r n -~, Z = 4, 2.98 > D, > 1.33 g ~m -~, F(000) = 472. Space group P2,2,2,. Mo-K, X-radiation (graphite monochromator), h = 0.710 69 A; ~(Mo-K,) = 5.2 cm-l.The structure was solved by vector methods and refined by full-matrix least-squares with anisotropic thermal parameters for all atoms. Weights were applied according to the scheme l/w = x . y, where x = b/sin 8 if sin 8 < b,