Photon pharmacology of an iron‐sulphur cluster nitrosyl compound acting on smooth muscle

Abstract

The mechanisms of action on smooth muscle of the iron‐sulphur cluster nitrosyl compound, heptanitrosyl‐tri‐μ3‐thioxotetraferrate (1‐), (RBS), a photosensitive nitric oxide donor, have been investigated in the guinea‐pig taenia caeci (coli) in vitro. After exposure to RBS (50 μm) for 30 min, and subsequent washout, a sustained contraction was recorded in the absence of light to either the agonist carbachol (50 μm) or a depolarizing concentration of KC1 (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 × 103 to 1.1 × 105 lux), duration (30 s to 20 min) and wavelength (400 to 800 nm), of the incident illumination. Repeated periods of illumination at 1.1 × 104 lux produced a reversible relaxation of both carbachol and KC1‐evoked tone in muscle pretreated with RBS (50 μm). 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 μm). The nitric oxide synthase inhibitor, NG‐nitro‐l‐arginine (60 μm), 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 μm), and haemoglobin (20 μm), 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. 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. 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. 1994 British Pharmacological Society