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Effects of cGMP and the nitric oxide donors glyceryl trinitrate and sodium nitroprusside on contractions in vitro of isolated myometrial tissue from pregnant women

The purpose of this study was to determine the relaxant effects in vitro of two nitric oxide donors, glyceryl trinitrate and sodium nitroprusside, which are currently available for use in vivo, on contractions of non-labouring myometrium from pregnant women. Since nitric oxide also mediates relaxation by increasing the concentration of cGMP, sensitivity to 8-bromo-cGMP (a cGMP analogue) was also determined. The effects of the K⁺-channel opener lemakalim and of the Ca²⁺-channel blocker nifedipine were studied for comparison. After the addition of glyceryl trinitrate (0.1–100 µmol l^–1), sodium nitroprusside (0.1–100 µmol l^–1) or 8-bromo-cGMP (0.001–3 mmol l^–1), the spontaneous rhythmic contractility of myometrial strips was inhibited in a concentrationdependent manner: the maximum inhibition produced by the highest tested concentration of each drug was 40 ± 7%, 53 ± 8% and 39 ± 8% of the original degree of contraction, respectively. Myometrial contractions were completely abolished by lemakalim and by nifedipine and verapamil at concentrations of 10^–5 mol l^–1. The nitric oxide donors, glyceryl trinitrate and sodium nitroprusside, attenuate myometrial contractions and are therefore useful as tocolytic agents. However, at equimolar concentrations in vitro, the ability of glyceryl trinitrate and sodium nitroprusside to attenuate myometrial contractions is less than that of lemakalin, nifedipine and verapamil. Controlled trials are required to determine the side-effects and clinical efficacy of each of these agents in vivo.