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B. G. Kennedy, S. F. Echtenkamp, T. Kostrominova, M. Alloosh, N. J. Mangini; Calcium Transport in Retinal Pigment Epithelium of the Ossabaw Swine. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2535.
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The metabolic syndrome is a distinct disease entity that includes central obesity, dyslipidemia, hyperglycemia, insulin resistance and hypertension. A recent clinical study has documented an increased incidence of retinopathy in individuals with metabolic syndrome. Ossabaw swine provide a novel animal model of the metabolic syndrome. Since ocular tissue has not been extensively studied in this model and since the retinal pigment epithelium (RPE) is involved in the etiology of diabetic retinopathy, our overall goal is to examine RPE function in Ossabaw swine. The purpose of the present study is to elucidate the mechanisms involved in calcium transport in swine RPE.
This study employed lean Ossabaw swine fed a normal diet, Ossabaw swine maintained on a high calorie, high fat diet and domestic swine. Expression of calcium transport proteins in the RPE was assessed by Western blotting and subcellular localization was determined by immunofluorescence. Transepithelial calcium transport, assessed as 45-Ca flux across voltage clamped RPE/choroid explants, was used as a functional measure of transport activity.
The plasma membrane calcium-ATPase (PMCA), a p-type ATPase which actively extrudes calcium from the cell, exists as four isoforms (PMCA1 - 4). Strikingly, by Western blot analysis, RPE isolated from Ossabaw swine express all four isoforms, with PMCA2 and PMCA4 expressed most strongly. By immunofluorescence, PMCA is expressed on both apical and basolateral cell membranes. PMCA1 is found primarily on the basal membrane while PMCA4 is present both apically and basally. Transepithelial calcium transport was measured in RPE/choroid explants from domestic swine. Swine RPE catalyzed net, active calcium transport in the basal to apical direction.
The RPE of Ossabaw swine expresses all four PMCA isoforms. Isoforms exhibit differential plasma membrane localization. Swine RPE explants catalyze active calcium transport. Active transport is oriented in the choroid to retina direction.
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