Expression and localization of carbonic anhydrase in bovine mammary gland and secretion in milk

Little attention has been paid to carbonic anhydrase VI (CA VI), a secretory type isozyme, in the bovine mammary gland, although the gland is an important exocrine gland and CA VI is known to localize in exocrine glands such as salivary and lacrimal glands in various animal species. In the present study mRNA expression and protein localization of CA VI in isolated gland tissues and in cloned epithelial cells from the mammary gland of Holstein cows (Bos taurus) were observed by reverse transcript polymerase chain reaction and immunocytochemistry. Also, changes of CA VI concentrations in milk were measured for 2 months postpartum by an enzyme-linked immunosorbent assay. CA VI gene expression was detected in the gland tissues and epithelial cells, and CA VI protein was localized in the cytoplasm of the epithelial cells. Colostrum contained the highest concentration of CA VI protein (100 ng/ml), decreasing in an exponential manner (P<0.001). We conclude that bovine mammary epithelial cells synthesize and secrete CA VI in colostrum at higher concentration than in normal milk, implying its role to compensate for low CA VI secretion in neonatal calves.