The changing paradigm of outflow resistance generation: towards synergistic models of the JCT and inner wall endothelium.
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.