Characterization of lacrimal gland carbonic anhydrase VI

We have previously demonstrated by immunohistochemistry the presence of secreted carbonic anhydrase (CA VI) in the acinar cells of the rat lacrimal glands. In this study we purified the sheep lacrimal gland CA VI to homogeneity and demonstrated by Western analysis that it has the same apparent subunit molecular weight (45 kD) as the enzyme isolated from saliva. RT-PCR analysis showed that CA VI mRNA from the lacrimal gland was identical to that of the parotid gland CA VI mRNA. An RIA specific for sheep CA VI showed the lacrimal gland tissue concentration of the enzyme to be 4.20 ± 2.60 ng/mg protein, or about 1/7000 of the level found in the parotid gland. Immunohistochemistry (IHC) and in situ hybridization (ISH) showed that lacrimal acinar cells expressed both immunoreactivity and mRNA for CA VI. Moreover, CA VI immunoreactivity was occasionally observed in the lumen of the ducts. Unlike the parotid gland, in which all acinar cells expressed CA VI immunoreactivity and mRNA, only some of the acinar cells of the lacrimal gland showed expression. These results indicate that the lacrimal gland synthesizes and secretes a very small amount of salivary CA VI. In tear fluid, CA VI is presumed to have a role in the maintenance of acid/base balance on the surface of the eye, akin to its role in the oral cavity.