1 Photorelaxation of vascular smooth muscle (VSM) is caused by the release of nitric oxide (NO) from a ®nite molecular store that can be depleted by irradiating pre-contracted arteries with visible light. The ability of an`exhausted' vessel to respond to a further period of illumination is lost temporarily but then recovers slowly as the photosensitive store is reconstituted in the dark. The recovery process, termed repriming, displays an absolute requirement for endothelium-derived NO and is inhibited by pre-treating arteries with ethacrynic acid, a thiol-alkylating agent. Here we demonstrate that agents that up-or down-regulate glutathione (GSH) biosynthesis in¯uence the extent to which the store is regenerated in the dark. 2 Isolated rat tail arteries (RTAs) were perfused internally with Krebs solution containing phenylephrine (PE; mean [PE]+s.e.mean: 5.78+0.46 mM) and periodically exposed to laser light (l=514.5 nm, 6.3 mW cm 72 for 6 min). Photorelaxations of control RTAs were compared with those from either (a) vessels taken from animals previously injected i.p. with buthionine sulphoximine (BSO), an inhibitor of g-glutamylcysteine synthetase (three injections, 100 mg kg 71 at 8 h intervals); or (b) isolated RTAs that were perfused ex vivo with oxothiazolidine (OXO), a precursor of cysteine (10 74 M OXO for 60 min). RTAs from BSO-treated animals exhibited attenuated photorelaxations: the mean (+s.e.mean) amplitude of the response recorded after 72 min recovery in the dark was 12.4+1.6% versus 21.4+2.9% for control arteries (n=5; P50.01). Conversely RTAs treated with OXO and allowed to recover for a similar period showed enhanced photorelaxations, 32.6+6.3% as compared to 21.4+2.9% for control arteries (n=5; P50.01). A hyperbolic curve ®t to repriming curves for BSO-treated and control arteries returned asymptote values (maximum photorelaxations) of (mean+s.e.mean) 24.2+3.2% and 55.2+8.5%, respectively. 3 The level of GSH in RTA extracts was measured by high-pressure liquid chromatography (HPLC). Injecting animals with BSO decreased GSH to 85% of control levels (P50.05) while treatment of isolated vessels with OXO resulted in a 31% increase above control levels (P50.05). Thus, drug-induced changes in RTA GSH levels were positively correlated with altered photorelaxations. 4 The results lead us to postulate that the photosensitive store in VSM is generated, at least in part, from intracellular GSH which becomes converted to S-nitrosoglutathione (GSNO) by nitrosating species that are formed ultimately from endothelium-derived NO. The possible physiological signi®cance of a photolabile store of NO in VSM is discussed brie¯y.

The role of endogenous nitric oxide (NO) in the growth and vascularization of a rat carcinosarcoma (P22) has been investigated. Tumor-bearing animals were treated with (i) nitric oxide synthase (NOS) inhibitors, administered via the drinking water, including N G -nitro-L-arginine methyl ester (L-NAME), a nonisoform-selective inhibitor, and 2 others that target the inducible (NOS II) enzyme preferentially, namely 1-amino-2-hydroxyguanidine or N-[3-(aminomethyl)benzyl]acetamidine hydrochloride; or (ii) daily injections (intraperitoneally) of 2 Ru(III) polyaminocarboxylates, AMD6221 and AMD6245, both of which are effective NO scavengers. L-NAME, AMD6221, and AMD6245 reduced tumor growth by approximately 60% to 75% of control rates. Tumor sections stained with abs to CD-31/platelet endothelial cell adhesion molecule-1 or NOS III showed that this was associated with a marked reduction (60%-77%) of tumor microvascular densitiy (MVD). Tumors resumed growing promptly when treatment was discontinued, accompanied by partial or complete restoration of MVDs. In contrast, NOS-II selective inhibitors had no effect on tumor growth or vascularization, indicating that both responses require complete blockade of NO production. The results corroborate the view that endogenous NO facilitates tumor development. We suggest that NO deprivation causes tumor feeder vessels to constrict, reducing tumor blood flow. The delivery of oxygen and essential nutrients to the developing tumor is impaired as a consequence, hampering further growth. Normalizing NO levels by withholding treatment causes tumor feeder vessels to dilate, increasing tumor perfusion and reestablishing conditions that allow tumors to begin growing again.