Objective: To investigate the effects of hyperosmotic stress on rabbit ocular surface and mucin 5AC (MUC5AC) expression.
Methods: Experimental study. Eighteen New Zealand white rabbits were randomly divided into three groups with equal number as hyperosmolar saline solution (HOSS, 500 mmol/L) group, normal saline (NS, 308 mmol/L) group and blank control group respectively. In HOSS and NS groups, the HOSS and NS eye drops were instilled on bilateral eyes six times every day for 14 days. On day 0, 7 and 14, Schirmer I test and tear break-up time (BUT) were measured and conjunctival impression cytology specimens were collected. On day 7 and 14, cornea and conjunctiva were harvested for Hematoxylin and Eosin (HE) staining, scanning and transmission electron microscopy observation and conjunctival TUNEL examination. On day 14, the conjunctiva were also harvested for immunology histological staining and western blot to evaluate the expression of MUC5AC.
Results: In HOSS group, the BUT on day 7 and 14 was (7.6 ± 2.5) and (7.0 ± 2.3) s respectively which was significantly shorter than the (10.3 ± 2.5) s on day 0(t = 5.800, 4.950; P < 0.01), and also significantly shorter than the BUT in NS and control groups (F = 8.030, P < 0.01). But the Schirmer I test value did not change obviously in and between all those three groups. The mean conjunctival goblet cell (GC) density in HOSS group on day 7 and 14 was (19.5 ± 16.6) and (32.3 ± 18.2) cells/mm(2) respectively which was also significantly lower than the (75.7 ± 43.4) cells/mm(2) on day 0 (t = 5.319, 2.970; P < 0.05). However the GC density did not change obviously in other two groups with time. After instillation of HOSS for 14 days, subepithelial inflammatory cell infiltration was showed on conjunctival tissue specimens and decreased epithelial layers and evident desquamation were found in the cornea specimens by the HE staining. Under the electron microscope, decreased microvilli and loosened intercellular junction in the superficial epithelium and increased autophagic vesicles in basal epithelium were observed in the cornea in HOSS group; and decreased microvilli and mucous granules were found in the conjunctiva in HOSS group. Obvious TUNEL positive staining was showed in the conjunctiva in the HOSS group. Also the MUC5AC immunology histological staining and western blot indicated decreased MUC5AC protein expression in HOSS group.
Conclusion: Hyperosmotic stress destroyed the structure of ocular surface epithelium, induced the decrease of conjunctival goblet cell density and MUC5AC expression, and led to the decreased tear film stability.