THE CECUM IS THE SITE WITH THE HIGHEST CALCIUM-ABSORPTION IN RAT INTESTINE

In the absence of electrochemical gradients, mucosa-to-serosa Ca transport across the rat cecum is about six times higher than the serosa-to-mucosa flux, resulting in a marked Ca absorption, which is considerably higher when compared with Ca absorption reported for other intestinal segments. The voltage-clamp experiments reveal that 45% of the total mucosa-to-serosa Ca transport measured across the short-circuited tissue is cellular whereas 55% is paracellular. The serosa-to-mucosa Ca flux, however, is purely paracellular. Dexamethasone or 1,25(OH)2D3 has no effect on the Ca transport across the cecum. Diphosphonate, known to inhibit 1,25(OH)2D3 synthesis, abolishes cellular mucosa-to-serosa Ca transport but this effect can be restored by simultaneous exogenous 1,25(OH)2D3 application. It is concluded that the cecum is the site with the highest calcium absorption in the rat intestine. The cellular mucosa-to-serosa Ca transport is dependent on 1,25(OH)2D3 but cannot be increased by exogenous 1,25(OH)2D3 application, suggesting that the Ca carrier in this segment already under normal nutritional conditions works at the maximal level. Ca absorption across the proximal colon therefore seems to be of physiological importance in the regulation of intestinal Ca homeostasis.