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S. Luck, V. Choh; Contractile Microfilaments in the Lens and Their Role in Accommodation. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4384.
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A previous study in chickens revealed that caldesmon, myosin light chain kinase (MLCK), f-actin and myosin are found on the crystalline lens. Their polygonal arrangement at the posterior surface resembles a "two-dimensional muscle", which suggests that these proteins may have a contractile role in accommodation (Bassnett et al., 1999). Unlike the human model, the ciliary muscle in chickens is skeletal in nature and therefore chickens are the ideal model to test any "smooth muscle properties" of the lens without affecting the ciliary muscle function. To test the hypothesis that contractile microfilaments play a role in accommodation, ciliary nerve-induced accommodation was measured in the presence of a myosin light chain kinase inhibitor, 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-7).
Eyes of 6-day-old white Leghorn chickens (gallus gallus domesticus) were enucleated in Tyrode’s saline solution while keeping the ciliary nerve intact. One eye was treated with one of ML-7 while the other eye treated with vehicle only. Three concentrations of ML-7 were used, 1µM (n=4), 10 µM (n=5) and 100 µM (n=4). The ciliary nerve was suctioned into the tip of an electrode attached to a Grass S44 stimulator, then stimulated (30 Hz, 0.1 and 0.15 mA). Focal lengths of the control and treated lenses were measured before and during accommodation using an optical laser scanning monitor (Scantox®).
Prior to accommodation, focal lengths for all ML-7-treated eyes averaged to 19.79 ± 1.48 (± sd) mm. Focal lengths decreased with ciliary nerve-induced accommodation, averaging to 17.03 ± 1.75 mm (p<0.0001), which is equivalent to an accommodative change of 10.66 ± 3.90 D. Vehicle-treated eyes showed similar focal lengths to ML-7-treated eyes at rest (mean ± sd: 19.86 ± 1.79 mm) but focal lengths during accommodation were slightly, but not significantly, different from accommodating ML-7-treated eyes (16.63 ± 1.66 mm; p=0.1365). However, the mean accommodative amplitude (13.53 ± 4.54 D) was greater in vehicle-treated eyes (p=0.0355). There was no significant difference in focal lengths or accommodative amplitudes for the three concentrations of ML-7 (p=0.2151 and p=0.4912, respectively).
The MLCK inhibitor appeared to have a subtle effect on accommodation. It is possible that focal length changes of the lens are masked by the larger ciliary muscle-induced accommodative changes. Note that both eyes showed considerable accommodative amplitudes (both >10 D), which could easily hide any smaller contractile changes by the lens.
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