Institute of Functional and Clinical Anatomy, Friedrich Alexander University Erlangen-Nürnberg, Erlangen, Germany; Ophthalmic Plastic Surgery Services, LV Prasad Eye Institute, Hyderabad, Telangana, India; Centre for Ocular Regeneration, LV Prasad Eye Institute, Hyderabad, Telangana, India.
Institute of Functional and Clinical Anatomy, Friedrich Alexander University Erlangen-Nürnberg, Erlangen, Germany; University of Fribourg, Faculty of Science and Medicine, Anatomy Unit, Fribourg, Switzerland.
Ocul Surf. 2023 Jan;27:41-47. doi: 10.1016/j.jtos.2022.11.003. Epub 2022 Nov 12.
Urea has been detected in the tear film, aqueous humor, and vitreous of the eye. While most of the urea in the aqueous humor and vitreous is considered to be an ultrafiltrate from the blood vessels, the presence of urea transporters and urea-synthesizing enzymes in the lacrimal gland, meibomian glands, conjunctiva, and cornea suggests ureagenesis occurring at the ocular surface. This review summarizes the distribution and function of urea transporters, urea and its synthesizing enzymes at the ocular surface to analyze their role in the tear film homeostasis. Urea transporters (UT)-A- and UT-B-as well as the enzymes arginase I, II, and agmatinase are located at the ocular surface. Urea concentration on the ocular surface is influenced by blood urea concentration, the amount of urea released by the tear fluid, tear evaporation, and arginase concentration in the tears. There are conflicting reports on the relationship between tear and plasma urea levels though a linear correlation exists between their levels. Urea protects the ocular surface from osmotic stress and is thought to maintain a lipid-water interface in the lamellar phase of the tear film. The reduction of urea levels in the tears of patients with evaporative dry eye suggests its possible role in tear film stability. Other than mitigating osmotic stress, urea has hydrating properties as well. Animal studies have demonstrated the healing effects of urea on the corneal epithelium. Future studies examining the variations in urea content in tears from different ocular surfaces, at different times of day, and under different environmental conditions would further solidify the role of urea in tear film stability.
尿素已在眼的泪膜、房水和玻璃体中被检测到。虽然房水和玻璃体中的大部分尿素被认为是血管的超滤液，但在泪腺、睑板腺、结膜和角膜中存在尿素转运体和尿素合成酶，表明在眼表面发生尿素生成。本综述总结了尿素转运体、尿素及其在眼表面的合成酶的分布和功能，以分析它们在泪膜稳态中的作用。尿素转运体（UT）-A 和 UT-B 以及酶精氨酸酶 I、II 和胍氨酸酶位于眼表面。眼表面的尿素浓度受血液尿素浓度、泪液中释放的尿素量、泪液蒸发和眼泪中精氨酸酶浓度的影响。虽然眼泪和血浆尿素水平之间存在线性相关性，但关于两者之间的关系仍存在相互矛盾的报告。尿素保护眼表面免受渗透胁迫，并被认为维持泪膜层状相中脂质-水界面的稳定。蒸发性干眼症患者眼泪中尿素水平降低表明其可能在泪膜稳定性中起作用。除了减轻渗透胁迫外，尿素还具有保湿特性。动物研究表明，尿素对角膜上皮有愈合作用。未来的研究检查不同眼表面、不同时间和不同环境条件下眼泪中尿素含量的变化，将进一步巩固尿素在泪膜稳定性中的作用。
