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Paul J. Donaldson, Haruna Suzuki-Kerr, Kerry L. Walker, Julie C. Lim; Hypotonic Stress Induces Localised Release of ATP in Cultured Rat Lenses Via Hemi-channels: Implications for Diabetic Lens Cataract. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3409.
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Since purinergic signalling via both the P2Y and P2X receptor sub families appears to be a feature of lens physiology we wanted to determine whether the lens itself can release ATP and if so how is this release mediated and where in the lens is it occurring.
Rat lenses were organ cultured for 18 hours in artificial aqueous humour (AAH) of differing osmolarities (175m to 425mOsm), and extracellular Ca2+ concentrations. Extracellular ATP release was measured using a luciferin/luciferase assay kit and cell damage monitored by measuring lactate dehydrogenase (LDH) release into the culture media. ATP release was determined by culturing lenses in fluorescent extracellular marker dyes of varying molecular weight which following tissue fixation and processing imaged using confocal microscopy to localise dye uptake.
Lenses cultured in hypotonic AAH for a period of 1 hour exhibited a significant increase in ATP release relative to lenses cultured in either isotonic or hypertonic AAH. During this incubation period no changes in LDH levels were observed indicating that ATP was not due to non-specific cell damage. Under isotonic conditions significant ATP release was induced by lowering AAH Ca2+ concentrations, a procedure known to activate hemi-channels. Lenses cultured in hypotonic and low Ca2+ conditions all exhibited a localised zone of uptake of the hemi-channel permeable tracer Lucifer Yellow but not the larger dye Texas Red-dextran (MW 10KDa).
Rat lenses exposed to hypotonic stress release ATP via a mechanism that appears to involve activation of a hemi-channel. The area of dye uptake and therefore ATP release is the same region where P2X receptors are dynamically inserted into the membrane of fibre cells. This is the same region of the lens that is damaged in diabetic cataract suggesting a mechanistic link between localised cell swelling in this zone and subsequent ATP release, P2X cation channel activation, Ca2+ influx, and activation of Ca2+-dependent proteases known to cause the localised tissue liquefaction characteristic of diabetic cataract.
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