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Joseph F Margiotta, Alexandra Fults-Ganey, Samantha Cole; Development and control of iris photosensitivity. Invest. Ophthalmol. Vis. Sci. 2016;57(12):95.
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© ARVO (1962-2015); The Authors (2016-present)
Isolated irides from many species constrict when exposed to blue light, but relevant mechanisms are incompletely understood. We examined this photosensitivity at 8-18 d of chicken embryogenesis (E8-18) a period covering the appearance and maturation of iris smooth and striated muscle.
Light filtered to 360-525 nm bandwidth was delivered @8-15 mW via fiber optic cable to dark-adapted irides. Photosensitivity was assessed from the maximal light-induced constriction of pupil area from images acquired at 1Hz with a CCD camera (see figure).
Between E8-E10 when only smooth muscle is present, light-induced pupil constriction increased from ≈5% to ≈30%. With the appearance and maturation of striated muscle, light-induced constriction increased further from ≈30% (E12) and plateaued at ≈50% (E15-18). Transcripts for melanopsin photoreceptors (OpnM and OpnX) and cryptochrome flavoproteins (CRY1 and CRY2) were identified by PCR in E17 irides. Consistent with an earlier study implicating CRYs, iris photosensitivity at E15-17 was blocked by CRY inhibition with the flavin reductase inhibitor DPI or by oxidation with H2O2, but persisted in the presence of the Opn inhibitor AA92593. Moreover, in accord with reported reciprocal interactions between CRY and cyclic AMP (cAMP) signaling, elevating cAMP with forskolin or 8-Br-cAMP inhibited light-induced iris constriction. Iris photosensitivity was abolished after blocking the sarco/endoplasmic reticulum ATPase and normal Ca2+ store refilling with thapsigargin (TG), or by incubation with 0 Ca2+ solutions, leading us to test the role of Ca2+ stores and their relationship to Trp and Orai plasma membrane channels. Store-operated-Ca2+-entry (SOCE) can evoke iris constriction since irises incubated in darkness with 0 Ca2+ solutions containing TG constricted when external Ca2+ was restored. However, pharmacological intervention with Pyr3, Pyr10, or 2-APB, to inhibit IP3 receptors, Trp and/or store-operated Orai channels, failed to detectably reduce light-induced iris constriction.
Our results indicate that first smooth and then likely both smooth and striated muscle fibers underlie the development of light-induced iris constriction. We also infer a reciprocal dependence of iris photosensitivity on CRY and cAMP signaling. Lastly, while SOCE can evoke iris constriction, we cannot presently link either Orai or Trp plasma membrane channels to Ca2+-dependent, light-induced iris constriction.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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