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S. Tamiya, N.A. Delamere; The Role Of Sodium Calcium Exchanger In Maintaining Lens Ion Balance . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4096.
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© ARVO (1962-2015); The Authors (2016-present)
Regulation of calcium is vital for proper cellular function. In the lens, dysregulation of calcium has been implicated in cortical cataract formation. In most tissues, the task of cytosolic calcium export is shared by plasma membrane Ca–ATPase (PMCA) and the sodium calcium exchanger (NCX). Under some circumstances, however, NCX works in reverse mode to effect calcium entry. A previous study from this laboratory implicated a role for NCX in calcium export following G protein–coupled receptor–mediated calcium signaling in cultured lens epithelium (Okafor et al, Cell calcium 34:231, 2003). However, the role of NCX in the whole lens remains ambiguous. In this study, inhibitors of NCX were used to investigate contribution of NCX to ion homeostasis of the intact lens.
Lenses were dissected from rabbit eyes and cultured in M199 supplemented with 0.1% BSA. Following a two day pre–incubation period, lenses were cultured for a further 2 dayswith the following inhibitors: bepridil, KBR7943 or SN–6. At the end of the culture period, lenses were weighed, dried, and weighed again to obtain the wet and dry lens weight. Dried lenses were digested in 30% nitric acid at 60°C and the ion concentration was determined by atomic absorption spectrophotometry. For Western blot, lens epithelial homogenates were subjected to SDS–PAGE, transferred to nitrocellulose membranes, and probed using an antibody directed against NCX1.
NCX1 protein was detected in both the anterior and equatorial lens epithelium. Immunoblot band density was similar in the two regions. Inhibition of forward mode NCX (Ca efflux) by bepridil for 2 days disrupted lens ion balance, significantly increasing the sodium and calcium concentrations and reducing potassium concentration. Bepridil also induced an opacification similar to cortical cataract. In contrast, inhibition of reverse mode NCX (Ca influx) by either KBR7943 or SN–6 had little effect on lens ion homeostasis. SN–6 caused no detectable opacification but KBR7943 caused a slight haziness of the anterior surface.
The results from this study suggest that calcium efflux by the sodium calcium exchanger is important for maintenance of lens ionic balance and transparency.
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