Effects of lactose on calcium absorption and secretion by rat ileum

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Abstract
The direct effects of lactose on net intestinal Ca absorption were determined by measuring unidirectional steady-state Ca fluxes in vitro under short-circuited conditions in segments of rat ileum. The isosmotic mucosal addition of lactose (160 mM) increased net Ca absorption (Jnet) by increasing the absorptive flux from mucosa to serosa (Jm.fwdarw.s) and reducing the secretory flux from serosa to mucosa (Js.fwdarw.m). Lactose also reduced tissue conductance and short-circuit current and reversed tissue polarity. 1,25-Dihydroxyvitamin D3 administration (50 ng/day for 4 days) increased Jnet from secretion to no net flux (Jm.fwdarw.s = Js.fwdarw.m) and lactose increased Jnet further to net absorption. Removal of Na from the medium, like lactose addition, increased Jnet by increasing Jm.fwdarw.s and reducing Js.fwdarw.m. The replacement of medium Na with choline abolished a further increase of Jnet by lactose. Lactose evidently increases net Ca absorption in the absence of transepithelial electrochemical or osmotic gradients. Transcellular Ca transport may be stimulated by lactose by hyperpolarization of the brush border as a result of reduced mucosal Na.

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