Vasoregulator Effects on Outflow Resistance, Flow Patterns and Distal Vessels in the Conventional Outflow Pathway of Mouse and Human Eyes

Abstract

Purpose : Vessels distal to Schlemm’s canal (SC) play a significant role in outflow resistance generation, contributing up to 50% of total resistance. Regulatory regions within distal vessels and their contribution to IOP are not well understood. We hypothesize that vasomotion of distal vessels are control points for outflow resistance generation.

Methods : We used two models to test our hypothesis. The first was OCT imaging of human anterior segments (hAS) in organ culture under constant flow conditions (2.5µl/min). Distal vessel behavior was monitored post treatment with the vasoconstrictor ET-1, or the vasodilator DETA-NO. The treatment protocol was repeated post-trabeculotomy in the same eyes to visualize collector channels (CCs). In the second model, enucleated eyes from C57BL/6 mice were perfused ex vivo with fluorescein tagged soy bean agglutinin lectin, immediately or 24 hrs post enucleation. One eye was treated with 10nM ET-1, while the paired eye received perfusion media. The outflow pathway was visualized in PECAM1-labeled limbal flat mounts by confocal microscopy. Masked analysis of images was conducted using ImageJ. Data are presented as Mean±SD (hAS) or Mean±CI (mice).

Results : The cross-sectional area (CSA) of intrascleral vessels decreased by 60.3±20.2% in hAS treated with ET-1 compared to baseline (p=0.003), while CSA increased by 79.5±121.4% (p=0.02) with DETA-NO treatment compared to ET-1. The SC lumen CSA increased with ET-1 by 68.7±29.1%, p=0. 0.03 and decreased with DETA-NO by 21.8±11.8%, p=0.01. Post-trabeculotomy, neither ET-1 (3.4±16.3% decrease p=0.98) nor DETA-NO (16.1±22.7% increase p=0.17) had a significant effect on CC CSA. In C57BL/6 mice, ET-1 decreased outflow facility (C) (4.6±1.6 to 2.7±1.5 nl/min/mmHg, p=0.04) in freshly enucleated eyes, and decreased C (4.1±1.8 to 3.0±2.0 nl/min/mmHg, p=0.12) 24 hrs post enucleation. Interestingly, lectin staining was largely located in the SC lumen with less lectin distal to CCs in ET-1 treated eyes compared to control.

Conclusions : Vasomodulator (NO and ET-1)-mediated changes in intrascleral vessel diameter was easily visualized by OCT in perfused human eyes, while ET-1 effects on behavior of CC ostia was better visualized indirectly using lectin as a flow tracer in perfused mouse eyes. Taken together, there are at least two vasoactive control points that regulate distal outflow resistance.

This is a 2021 ARVO Annual Meeting abstract.