PHOTON PHARMACOLOGY OF AN IRON-SULFUR CLUSTER NITROSYL COMPOUND ACTING ON SMOOTH-MUSCLE

The mechanisms of action on smooth muscle of the iron-sulphur cluster nitrosyl compound, heptanitrosyl-tri-mu 3-thioxotetraferrate (1-), (RB S), a photosensitive nitric oxide donor, have been investigated in the guinea-pig taenia caeci (coli) in vitro. 2 After exposure to RBS (50 mu M) for 30 min, and subsequent washout, a sustained contraction was recorded in the absence of light to either the agonist carbachol (50 m u M) or a depolarizing concentration of KCl (23.5 mM). Photon irradiat ion (>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 illumi nation at 1.1 x 10(4) lux produced a reversible relaxation of both car bachol and KCl-evoked tone in muscle pretreated with RBS (50 mu M). Th ese photorelaxations were reproducible at 10 min intervals for several hours with a maximal relaxation amounting to 80 to 90% that of the to ne produced by carbachol (50 mu M). 4 The nitric oxide synthase inhibi tor, N-G-nitro-L-arginine (60 mu M), caused no inhibition of the photo n-induced relaxation of RBS-treated muscle. In contrast, N-methylhydro xylamine (2 mM), L-cysteine (10 mM), DL-dithiothreitol (2 mM), methyle ne blue (30 mu M), and haemoglobin (20 mu M), all reversibly but signi ficantly inhibited (P<0.001) the photorelaxation response. However, ne ither the aminothiol N-acetyl-L-cysteine (10 mM) nor the tripeptide gl utathione (10 mM) blocked the RBS-induced photorelaxation. 5 The photo lytic cleavage of RBS depended on the intensity and duration of illumi nation; it was accompanied by a corresponding decrease in absorbance a nd by the liberation of NO as measured by the Griess diazo reaction wi th sulphanilic acid. L-Cysteine (10 mM) prevented the decrease in abso rbance and the photolytic liberation of NO. 6 It is concluded that (i) sequestered or bound RBS, when photon-activated, liberates NO by a pr ocess which can be controlled by the wavelength, intensity and duratio n of the incident light, (ii) the photon-released NO rapidly relaxes t he smooth muscle cells of the taenia coli primarily via cyclic GMP-dep endent 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 signific ance of NO.