Effects of Elevated Intraocular Pressure on Outflow Facility and TIGR/MYOC Expression in Perfused Human Anterior Segments

Purpose: To investigate the effects of high intraocular pressure (h-IOP) on TIGR/MYOC expression, extracellular matrix (ECM) deposition, and outflow facility (C) in perfused human anterior segment cultures.

Methods: Anterior segments of 31 pairs of normal human eyes from postmortem donors were perfused at constant flow (3 microl/min). After reaching stable baseline, the flow of one eye from each of 31 pairs was raised to obtain a continuous pressure of 60 to 70 mm Hg for a period of 1 hour (3 pairs), 6 hours (10 pairs), 24 hours (2 pairs), 48 hours (3 pairs), and 7 days (13 pairs). Sixteen of these pairs were used to study trabecular meshwork expression of TIGR/MYOC and stromelysin by Northern blot analysis hybridization. Nine pairs (1 pair each at h-IOP for 1, 6, and 48 hours and 6 pairs at 7 days) were fixed at pressure for analysis by electron microscopy. Eyes selected for C measurements fulfilled the inclusion criteria of C0 values between 0.06 and 0.4, intact RNA recovery and normal light microscopy morphology. Percent change of facility from the baseline (C/C0) was computed at 6 and 24 hours and 2, 4, and 7 days from the long-term perfusion experiments (n = 9 h-IOP, n = 8 controls).

Results: No induction of TIGR/MYOC expression was observed after h-IOP for 1 and 6 h. A slight induction was seen after 24 and 48 hours. At 7 days, the treated eye from 4 of 5 pairs showed a clear induction, which was very pronounced in one of the pairs. In contrast, stromelysin expression was induced at 6 hours and not at 7 days. Morphometric electron microscopy after 7 days showed no significant difference in the amounts of fine fibrillar material or plaque material in the juxtacanalicular (JCT) region. The percent increase of C of the treated eye at 6 hours was 11.0% +/- 4.6% compared with 3.7% +/- 3.8% in the control eyes (P = 0.26). However, after longer time periods, the facility of the h-IOP eyes increased, whereas that of the contralateral eyes remained unchanged. This difference reached peak, significant values at 4 days (32.9% +/- 8.4% versus 7.4% +/- 7.6%, respectively; P = 0.04) and decreased to 8.9% +/- 7.9% versus 1.1% +/- 12.7% (P = 0.6) at 7 days.

Conclusions: Elevated IOP appears to cause a decrease in outflow pathway resistance at 1 to 4 days, and this effect seems to disappear with further time. In contrast, induction of TIGR/MYOC appears to be strongest at 7 days. We speculate that this induction pattern might indicate a stress-related, rather than a possible homeostatic, role for the TIGR/MYOC protein.