title: Photon pharmacology of an iron-sulphur cluster nitrosyl compound acting on smooth muscle.
abstract: 1. The mechanisms of action on smooth muscle of the iron-sulphur cluster nitrosyl compound, heptanitrosyl-tri-mu 3-thioxotetraferrate (1-), (RBS), a photosensitive nitric oxide donor, have been investigated in the guinea-pig taenia caeci (coli) in vitro. 2. After exposure to RBS (50 microM) for 30 min, and subsequent washout, a sustained contraction was recorded in the absence of light to either the agonist carbachol (50 microM) or a depolarizing concentration of KCl (23.5 mM). Photon irradiation (> 400 nm) caused a prompt relaxation of precontracted RBS-treated muscle, the magnitude of which depended upon the intensity (1.1 x 10(3) to 1.1 x 10(5) lux), duration (30 s to 20 min) and wavelength (400 to 800 nm), of the incident illumination. 3. Repeated periods of illumination at 1.1 x 10(4) lux produced a reversible relaxation of both carbachol and KCl-evoked tone in muscle pretreated with RBS (50 microM). These photorelaxations were reproducible at 10 min intervals for several hours with a maximal relaxation amounting to 80 to 90% that of the tone produced by carbachol (50 microM). 4. The nitric oxide synthase inhibitor, NG-nitro-L-arginine (60 microM), caused no inhibition of the photon-induced relaxation of RBS-treated muscle. In contrast, N-methylhydroxylamine (2 mM), L-cysteine (10 mM), DL-dithiothreitol (2 mM), methylene blue (30 microM), and haemoglobin (20 microM), all reversibly but significantly inhibited (P < 0.001) the photorelaxation response. However, neither the aminothiol N-acetyl-L-cysteine (10 mM) nor the tripeptide glutathione (10 mM) blocked the RBS-induced photorelaxation. 5 The photolytic cleavage of RBS depended on the intensity and duration of illumination; it was accompanied by a corresponding decrease in absorbance and by the liberation of NO as measured by the Griess diazo reaction with sulphanilic acid. L-Cysteine (10 mM) prevented the decrease in absorbance and the photolytic liberation of NO.6 It is concluded that (i) sequestered or bound RBS, when photon-activated, liberates NO by a process which can be controlled by the wavelength, intensity and duration of the incident light, (ii) the photon-released NO rapidly relaxes the smooth muscle cells of the taenia coli primarily via cyclic GMP-dependent pathways which can be blocked by use of appropriate inhibitors, and (iii) the RBS-induced photorelaxation effect does not involve the activation of NO synthase. RBS is therefore a valuable photosensitive NO donor for establishing the functional and pharmacological significance of NO.
authors: Department of Pharmacology, University of Cambridge.
authors: E K
authors: EK
authors: Matthews
authors: E D
authors: ED
authors: Seaton
authors: M J
authors: MJ
authors: Forsyth
authors: P P
authors: PP
authors: Humphrey
mesh_terms: D000818:Animals; D001120:Arginine / Q000031:analogs & derivatives / Q000494:pharmacology; D002217:Carbachol / Q000494:pharmacology; D006168:Guinea Pigs; D006454:Hemoglobins / Q000378:metabolism; D066298:In Vitro Techniques; D007502:Iron Chelating Agents / Q000494:pharmacology*; D058085:Iron Compounds; D008297:Male; D009126:Muscle Relaxation / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects* / Q000528:radiation effects; D009569:Nitric Oxide / Q000378:metabolism; D019335:Nitroarginine; D009603:Nitroso Compounds; D010084:Oxidation-Reduction; D010777:Photochemistry; D017785:Photons; D011189:Potassium Chloride / Q000494:pharmacology; D014665:Vasodilator Agents / Q000494:pharmacology*
chemical_list: D006454:Hemoglobins; D007502:Iron Chelating Agents; D058085:Iron Compounds; D009603:Nitroso Compounds; D014665:Vasodilator Agents; D019335:Nitroarginine; D009569:Nitric Oxide; C078952:Roussin's Black Salt; D011189:Potassium Chloride; D002217:Carbachol; D001120:Arginine