Cellular and paracellular calcium transport in the rat ileum and the influence of 1 alpha, 25-dihydroxyvitamin D3 and dexamethasone.

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)