Mammary glands, aquaporins and milk production

The mammary gland is a specialized, enlarged sudoriferous or sweat (apocrine) gland that produces and secretes milk during lactation. Milk consists of simple sugars, lipids, proteins, vitamins and minerals dissolved in water, which accounts for up to 88% per unit volume of milk. The water content of milk will vary depending on the animal species under investigation and the physiological state of the lactating animal. Current knowledge suggests that water is secreted across the mammary epithelium in a transcellular manner, in response to an osmotic gradient produced largely by the lactose content of milk. Milk yield and quality are important criteria for the dairy industry. Despite the economic importance of milk yield little is known about the physiological mechanisms responsible for water transport in the bovine mammary gland. Recent studies suggest that several aquaporin proteins are present in the rodent, bovine and human mammary gland. Aquaporins play fundamental roles in water and small solute transport across epithelial and endothelial barriers. Immunohistochemical techniques have confirmed the presence of AQP1 and AQP3 water channels in rat, mouse, bovine and human mammary glands. Studies from our laboratory suggest that in addition to AQP1 and AQP3 the AQP4, AQP5 and AQP7 proteins are expressed in different epithelial and endothelial locations in the mammary gland. This chapter discusses the potential functional role of aquaporin water channels in the transport of water and small solutes across endothelial and epithelial barriers in the mammary gland and explores how these membrane proteins may be involved in milk production. © 2012 Nova Science Publishers, Inc. All rights reserved