Ex vivo porcine iris stiffening due to drug stimulation

• Type of Material: Article
• Authors: Julie E Whitcomb, Emory University; Vincent A Barnett, Emory University; Timothy Olsen, Emory University; Victor H Barocas, University of Minnesota, Minneapolis
• Language: English
• Date: 2009-10-01
• Publisher: ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD
• Publication Version: Author Accepted Manuscript (After Peer Review)
• Copyright Statement: © 2009 Published by Elsevier Ltd.
• License: Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International
• Final Published Version (URL): http://dx.doi.org/10.1016/j.exer.2009.04.014
• Title of Journal or Parent Work: EXPERIMENTAL EYE RESEARCH
• Volume: 89
• Issue: 4
• Start Page: 456
• End Page: 461
• Grant/Funding Information: This work was supported by the National Institute of Health (EY015795), an unrestricted grant from Research to Prevent Blindness (RPB), the Minnesota Medical Foundation, and the Minnesota Lions Macular Degeneration Center.
• Keywords: ULTRASOUND BIOMICROSCOPY; PIGMENTARY GLAUCOMA; phenylephrine; eye; DISPERSION SYNDROME; SMOOTH-MUSCLE; Ophthalmology; angle-closure glaucoma; modulus; Life Sciences & Biomedicine; Science & Technology; pilocarpine; pigmentary glaucoma

Abstract

The purpose of this study was to quantify how the elastic modulus of the ex vivo iris changes following stimulation by pilocarpine (PILO), phenylephrine (PE), and tropicamide (TROP). Irides (n = 20) were dissected from porcine eyes within 4 h post-mortem and tested uniaxially. Either the entire iris or sector thereof was used. The samples were stretched up to 40% Green strain. The radial modulus was calculated from the linear portion of the stress-strain curve, and the azimuthal modulus was fitted to a model treating the iris as a collection of circular elastic bands. One of the three drugs (n = 6 or 7) of interest was added (80 μg/ml) to the bath surrounding the tissue, and the test was repeated. Changes in pupil diameter of free-floating samples and isometric force of mounted samples confirmed that the tissue was responsive to the drugs. The untreated iris modulus for cut sections in radial extension was 4.0 ± 0.9 kPa (mean ± s.d., n = 20), and treated iris modulus was 7.7 ± 2.0 kPa (PILO, n = 7), 6.9 ± 2.2 kPa (PE, n = 6), and 8.4 ± 1.7 kPa (TROP, n = 7). Intact irides (n = 10) gave similar trends but values approximately 25% higher, presumably due to support from the nominally unloaded tissue. The azimuthal modulus of the untreated iris was 2.97 ± 1.3 kPa (n = 5), and that of the treated iris (PILO) was 5.34 ± 2.1 kPa. Although PILO, PE, and TROP work by different mechanisms, all three had similar results - an increase of modulus by a factor of two. These results suggest that in most normal situations the iris remains compliant at all pupil diameters. © 2009.