Effects of Melatonin Administration on Vasomotor Activity and Histological Structure of Isolated Thoracic Aorta in Rats Treated with Thyroxine

Objective: The goal of this study was to examine the effect of in vivo melatonin (MEL) administration on isolated thoracic aorta in rats with thyroxine treatment and its duty in aortic response to contractile agents, such as potassium chloride (KCl) and phenylephrine (PE). In addition, immunohistological alterations were also examined. Methods: Experimental groups were as follows: control group (n= 5), thyroxine group (n= 5), melatonin group (n= 6), and thyroxine + melatonin group (n= 6). L-thyroxine was given by intraperitoneal (i.p) administration at 0.3 mg/kg/day for 14 days. MEL was administered i.p., at 3 mg/kg/day for 14 days. The thoracic aorta was isolated from rats euthanized by cervical dislocation. Then, vascular rings were prepared. Concentration-response curves for KCl and PE applications were recorded in an isolated organ bath. Tissue samples were fixed in 10% formalin for histopathological and immunohistological evaluation. Results: KCl and PE-induced contractions were reduced significantly in the thoracic aortic rings of the thyroxine-treated rats. MEL administration partially attenuated the reduction in the contraction responses due to thyroxine treatment. Immunohistological findings showed that MEL inhibits the thickening of the vessel wall by probably suppressing collagen formation due to thyroxine treatment in the aortic tissue. Conclusion: Our results suggest that MEL may attenuate the decrease in vascular resistance caused by thyroxine treatment.