What happens to tears inside the efferent lacrimal passage? An animal experimental study

Background: To determine whether absorption of protein components of the te ar fluid occurs in normal efferent tear ducts, an animal experiment was car ried out. Methods: Iodinated albumin was dropped into eyes of female rats. After 10, 20 or 60 min the rats were killed, blood collected and the heads embedded f or histological examination. Serum was obtained from the clotted blood and the radioactivity in a protein sediment and the combined supernatants count ed. In a second approach, serum was fractionated by molecular mass and radi oactivity in the fractions measured. Furthermore, autoradiographs of rat he ad sections were performed. Results: Uptake of radioactivity into the serum was low, but increased with time. After 60 min maximal incorporation of th e applied radioactivity into the blood was 0.13%; most (70-80%) of the inco rporated radioactivity was not protein bound. Gel chromatographic separatio n according to molecular mass yielded fractionated peaks of radioactivity c orresponding to albumin with maximal 4.8 Bq/ml serum, iodinated tyrosine (5 .5 Bq/ml), and free iodine (237 Bq/ml; each after 60 min). Histologically t he rat efferent lacrimal tear ducts showed a multilayered lining epithelium with integrated goblet cells in a characteristic arrangement of several ce lls. In autoradiographs of rat head sections no transport of radioactivity could be visualized. Conclusion: In rats only traces of iodinated albumin a re incorporated from the efferent lacrimal tear ducts into the blood. A hig her proportion of the radioactivity is taken up as the proteolytic degradat ion product of bovine serum albumin to free amino acids, and 96% of the rad ioactivity incorporated was free iodine, probably as a contaminant of the i odinated preparation.