The presence of nitric oxide synthase in the mammary glands of lactating rats.

Our objective was to show the presence of nitric oxide synthase (NOS) in the mammary glands of lactating rats and to determine the role that nitric oxide plays in lactation of humans. We used the nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase reaction and immunostaining on fresh frozen breast tissue obtained from lactating Wistar rats (13-18 wk old, 320-380 g, 3-4 d after parturition, first pregnancy). The breast tissues positive for NADPH diaphorase staining were the mammary gland, muscle, sebaceous gland, epidermis, and endothelium of the lactiferous ducts and blood vessels. These findings corresponded to tissues that were NOS-immunoreactive. The NOS detected in the mammary glands was the constitutive NOS. The finding of positive anti-NOS immunoreactivity and of positive NADPH diaphorase activity in the cells of the rat mammary glands leads us to conclude that nitric oxide is synthesized in mammary glands of rats. The high nitric oxide activity in the human breast was previously reported by measuring the total concentration of nitrite plus nitrate, biopterin, and neopterin in human milk. These results suggest that nitric oxide plays a role in the secretion of human breast milk.