title: Ex vivo porcine iris stiffening due to drug stimulation.
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 microg/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.
authors.affiliation: Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN, USA. whit0802@umn.edu
authors.forename: Julie E
authors.initials: JE
authors.lastname: Whitcomb
authors.forename: Vincent A
authors.initials: VA
authors.lastname: Barnett
authors.forename: Timothy W
authors.initials: TW
authors.lastname: Olsen
authors.forename: Victor H
authors.initials: VH
authors.lastname: Barocas
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D055119:Elastic Modulus; D007498:Iris / Q000502:physiology*; D008916:Miotics / Q000494:pharmacology; D009130:Muscle, Smooth / Q000187:drug effects*; D009184:Mydriatics / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology*; D010862:Pilocarpine / Q000494:pharmacology*; D011680:Pupil / Q000502:physiology*; D013552:Swine; D014331:Tropicamide / Q000494:pharmacology*
chemical_list: D008916:Miotics; D009184:Mydriatics; D010862:Pilocarpine; D010656:Phenylephrine; D014331:Tropicamide