VITAMIN-D AND ENTEROCYTE BRUSH-BORDER MEMBRANE CALCIUM-TRANSPORT AND FLUIDITY IN THE RAT

Prior studies of vitamin D repletion showed a threefold increase in th e maximum rate (Vmax) for calcium uptake by brush border membrane vesi cles, but did not differentiate saturable and nonsaturable uptake comp onents. We studied the calcium uptake and fluidity response of intesti nal brush border vesicles to vitamin D by treatment with 1 alpha,25-di hydroxy-24,24-difluorocholecalciferol (24,24-F-1,25-(OH)(2) D-3) Treat ment responses were measured by effects on (1) saturable and nonsatura ble initial uptake rates of calcium by rat proximal small intestinal b rush border membrane vesicles; (2) transmucosal calcium transport by e verted duodenal sac; and (3) fluorescence anisotropy. Treatment of vit amin D-depleted weanlings increased the Vmax by 50% (P < .05) in vesic les from the proximal 12 cm of small intestine from rats injected with disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP), but there was no response in rats not injected with EHDP or in vesicles from the proxim al 30 cm of small intestine. Vitamin D-depleted weanlings were D-defic ient based on serum 25-hydroxycalciferol (25-OH-D) concentration, but to produce 1 alpha,25-dihydroxycalciferol [1,25-(OH)(2)D] depletion, E HDP injection was required. Treatment of vitamin D-replete adult rats caused a 20% (P < .05) increase in Vmax. Treatment did not affect the calcium concentration at half-Vmax (Kr), the rate constant for nonsatu rable uptake (K-D), or vesicle fluidity measured as fluorescence aniso tropy. Contrasting with these minimal effects of treatment on brush bo rder Vmax, treatment increased transmucosal calcium transport by evert ed duodenal sac almost threefold in vitamin D-depleted weanlings admin istered EHDP. Thus, vitamin D actions on enterocyte calcium transport (1) at the brush border increase saturable but not nonsaturable uptake , and (2) produce the major transport response distal to the brush bor der. Despite previously described changes in membrane lipid, brush bor der fluidity is unaffected by vitamin D treatment. Copyright (C) 1994 by W.B. Saunders Company