title: Cellular and paracellular calcium transport in the rat ileum and the influence of 1 alpha, 25-dihydroxyvitamin D3 and dexamethasone.
abstract: Concentration dependence of unidirectional calcium fluxes across the rat ileum freed from the serosa and the muscularis externa were measured in a modified Ussing-chamber. Mucosa (m) to serosa (s) calcium flux showed a saturable component, whereas s to m calcium flux was linearly related to the calcium concentration between 0.125 mmol/l and 5 mmol/l. At all calcium concentrations used net secretion of calcium was observed. The s to m flux of the simultaneously measured paracellular marker mannitol at all calcium concentrations was remarkably higher than the m to s flux, resulting in net mannitol secretion. The results obtained from the calcium fluxes when clamping the transepithelial electrical potential agree well with those of the concentration dependence of the calcium fluxes: 1. Only m to s flux has a voltage independent, transcellular component. 2. Calcium s to m flux is totally voltage dependent, i.e. diffusive. 3. Diffusional s to m calcium flux is about 80% greater than the diffusional fraction of the m to s flux. Omitting glucose from the bathing solution effected a decrease of the transepithelial electrical potential and of the short circuit by 91% and 85% respectively; net calcium secretion was almost abolished and net mannitol secretion remarkably reduced. Addition of glucose, which stimulates water absorption in the ileum as a metabolic substrate, activated m to s but significantly more pronounced s to m calcium flux parallel to that of mannitol.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D003907:Dexamethasone / Q000494:pharmacology*; D004100:Dihydroxycholecalciferols / Q000494:pharmacology*; D007082:Ileum / Q000187:drug effects / Q000378:metabolism; D007413:Intestinal Mucosa / Q000187:drug effects / Q000378:metabolism; D008353:Mannitol / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D004100:Dihydroxycholecalciferols; D008353:Mannitol; C023850:1 alpha,24-dihydroxyvitamin D3; D003907:Dexamethasone; D002118:Calcium