ISSN:
1432-1912
Schlagwort(e):
Ileal mucosa
;
Calcium transport
;
1α,25-Dihydroxyvitamin D3
;
Dexamethasone
Quelle:
Springer Online Journal Archives 1860-2000
Thema:
Medizin
Notizen:
Summary Concentration dependence of unidirectional calcium fluxes across the rat ileum freed from the serosa and the muscularis externa were measured in a modified Ussingchamber. 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/1 and 5 mmol/1. 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 current 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. Dexamethasone, known to stimulate sodium and water absorption in the ileum by activation of the Na,K-ATPase, effected an increase of the transepithelial electrical potential difference and of the short circuit current by about 100% but had no influence on tissue resistance; m to s and more pronounced s to m calcium flux was stimulated after the induction by dexamethasone and net calcium secretion was increased by 70%. After pretreatment with 1α,25-dihydroxyvitamin D3 tissue resistance was decreased by about 42%. The vitamin had no effect on net calcium or mannitol secretion but significantly increased bidirectional calcium and mannitol fluxes. Flux measurements in clamped preparations revealed: 1. 1α,25-dihydroxyvitamin D3 and dexamethasone has no effect on the cellular-mediated m to s calcium transport; 2. diffusive calcium flux in m to s and in the opposite direction, from s to m, is increased by the vitamin and by the hormone. In conclusion the net ileal calcium and mannitol secretion is the consequence of an asymmetry of the paracellular flux with a prevalence of the s to m flux over that in m to s direction. It is hypothesized that this prevalence is caused by an anomalous solvent drag effect. Paracellular calcium flux in both directions is increased by 1α,25-dihydroxyvitamin D3 and dexamethasone by different mechanisms, as indicated by the changes in the electrical parameters of the tissue.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1007/BF00177761
Permalink
