Mechanisms of melatonin (MT) effects on blood vessels have not been elucidated until now. The aim of this study was to investigate effect of MT on reactivity of rabbit main pulmonary artery (MPA) and its extralobular branches (PA), mesenteric artery (MA) and thoracic aorta (TA). Isometric contractions of vascular rings were measured.

MT in concentrations of 10^-7 - 3x10^-5 mol/l did not change basal vascular tone. Phenylephrine-precontracted vessels (10^-6 mol/l) relaxed after application of MT. The highest concentration of MT used in experiments (3x10^-5 mol/l) caused relaxation of PA (42.33 ± 6.76%), MPA (15.43 ± 3.61 %) and TA (20.14 ± 6.62 %). Transmural nerve-induced stimulation (4 Hz) contracted vascular rings. MT (10^-5 mol/l) decreased neurogenic contractions of both MA and MPA to 84.13 ± 5.83 % and 75.34 ± 4.36 %, respectively. Endothelium-dependent relaxation to acetylcholine was not changed with MT. The application of NO-synthase inhibitor L-NAME (10^-6 mol/l) did not affect MT-induced relaxation of MPA.

It can be suggested that melatonin affect smooth muscle reactivity in intact rabbit vessels directly whereas inhibition of NO-synthase did not change melatonin-induced relaxation.