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Calcium transport across the colon ascendens and the influence of 1,25‐dihydroxyvitamin D3 and dexamethasone

Abstract. Concentration dependence of unidirectional calcium fluxes across the rat colon ascendens were measured in a modified Ussing chamber. Both the mucosa (m) to serosa (s) and the s to m calcium flux exhibited saturation kinetics. The maximum transport rates and the affinity to the transporter of calcium was higher in the m to s direction than that from s to m, resulting in a remarkable net calcium absorption. The results obtained from measurements of unidirectional calcium fluxes in dependence on clamped transepithelial potentials showed that: (i) calcium transport in both direction had a voltage‐independent component; (ii) the voltage‐independent, i.e. non diffusive fraction of the m to s calcium flux was 3·2 times greater than that in the opposite direction; (iii) the voltage dependent, i.e. diffusional fraction of the m to s calcium flux, was about two times greater than the voltage‐dependent fraction of the calcium flux in the s to m direction; and (iv) in the m to s direction 62%, and in the s to m direction 73%, of the total unidirectional flux was voltage‐dependent. Dexamethasone, known to enhance sodium and water absorption in the colon, had no significant influence on net calcium absorption but increased the unidirectional calcium fluxes in both directions. The increase in unidirectional calcium fluxes parallel to that of the extracellular marker mannitol suggests that dexamethasone has no influence on the transcellular calcium transport but increases the calcium flux along the paracellular way. Amiloride had no influence on the dexamethasone‐induced changes of the epithelial electrical parameters as distinguished from the colon descendens. The increase in calcium and mannitol fluxes after dexamethasone treatment was also unaffected by amiloride. 1,25(OH)2D3 had no influence on the kinetics of unidirectional calcium fluxes as distinguished from the colon descendens. The electrical parameters of the tissue and unidirectional mannitol fluxes were also unaffected by 1,25(OH)2D3. The mode of calcium transport found in the colon ascendens exhibits characteristics distinct from the transport properties reported for the colon descendens. The heterogeneity of both parts, with respect to the calcium transport, suggests that both colonic segments might have different functions in the final modulation of calcium balance by the intestine.