SEGMENTAL HETEROGENEITY OF CELLULAR AND PARACELLULAR CALCIUM-TRANSPORT ACROSS THE RAT DUODENUM AND JEJUNUM

Concentration- and voltage-dependent 45Ca transport was measured across the rat duodenum and jejunum. Mucosa-to-serosa calcium transport across the short-circuited tissue exhibits a saturable component, whereas serosa-to-mucosa calcium flux in both segments is linear to the calcium concentration between 0.125 mmol/L and 10 mmol/L. Calcium is absorbed in the duodenum at concentrations between 0.125 mmol/L and 2.5 mmol/L but is secreted in the jejunum at all concentrations. The simultaneously measured paracellular marker [3H]mannitol at all calcium concentrations is secreted in both segments. The study across clamped preparations shows that (a) only mucosa-to-serosa calcium flux has a voltage-independent cellular component; (b) serosa-to-mucosa calcium flux is totally voltage dependent, i.e., diffusive and probably restricted to the paracellular pathway; (c) diffusive calcium flux in the duodeum is equal in both directions; and (d) diffusive serosa-to-mucosa flux in the jejunum is higher than the corresponding flux in the opposite direction, suggesting that calcium passively is secreted in the jejunum as the consequence of a preference of the paracellular serosa-to-mucosa calcium movement caused by "anomalous solvent drag effect." 1α,25-dihydroxyvitamin D3 [1,25(OH)2D3] stimulates serosa-to-mucosa and more pronounced mucosa-to-serosa calcium flux and increases calcium absorption in the duodenum and abolishes calcium secretion in the jejunum. 1,25(OH)2D3 stimulates cellular mucosa-to-serosa calcium flux in the duodenum only but has no effect on cellular calcium in the jejunum. However, the vitamin increases bidirectional diffusive calcium fluxes across both segments. Tissue resistance is decreased and the flux of the paracellular marker mannitol in both directions is increased, suggesting that 1,25(OH)2D3 stimulates diffusive calcium flux across both segments by increasing the paracellular permeability. Dexamethasone inhibits cellular mucosa-to-serosa cal-cium flux and abolishes calcium absorption in the duodenum only but has no effect on cellular calcium flux in the jejunum. The glucosteroid has no influence on diffusive calcium flux in the duodenum but increases voltage-dependent bidirectional calcium flux across the jejunum. Simultaneously, the flux of the paracellular probe mannitol in both directions across the jejunum is increased, whereas mannitol flux in the duodenum is unresponsive to dexamethasone. It is hypothesized that dexamethasone in the jejunum increases paracellular calcium flux by normal and anomalous solvent drag due to an increase in fluid absorption. In the duodenum, however, convective forces have no effect on paracellular calcium flux. It is concluded that calcium in the rat duodenum and jejunum is transported by distinct cellular and paracellular mechanisms. © 1991 by the American Gastroenlerological Association.