STIMULATION OF ILEAL CALCIUM-ABSORPTION BY SORBITOL, L-XYLOSE, OR CREATINE VIA A DECREASE IN LUMINAL SODIUM CONCENTRATION - RELATION WITH CONCOMITANT CHANGES IN ENTEROCYTE ENERGY-METABOLISM

Ligated ileal loops, 30 cm in length, of 4-month-old male Wistar rats were instilled with 3 ml of a 10 mM CaCl2 solution (added with 0.25 mu Ci Ca-45) in the absence (control) or presence of 100 mM sorbitol, L-x ylose, or creatine. Ileal calcium (Ca) transport, measured by plasma C a-45 appearance, was found to be similar 30 minutes after fluid instil lation in all four instances. However, thereafter, Ca-45 appearance in plasma did not increase further in control animals whereas it increas ed -twice as much during the subsequent 30 minutes in the presence of sorbitol, L-xylose, or creatine. However, when loops of similar length were instilled with only 1.0 ml of such solutions, the sorbitol effec t was already observed during the first 30 minutes. The stimulation of ileal Ca absorption induced by the presence of sorbitol appeared to b e due to a cellular effect, associated with a decreased flux across th e paracellular pathway, as indicated by H-3-mannitol absorption. The p resence of sorbitol in instilled ileal solution induced a significant decrease in luminal Na, K, bicarbonate, and Cl concentrations at each time point studied (30, 60, 120, or 240 minutes after instillation). T hirty minutes after instillation, no difference in soluble Ca concentr ation was observed between control and experimental rats. After 60 min utes, Ca concentration was dramatically decreased in control rats but it remained nearly constant in experimental animals. Thus, the presenc e of substances enhancing ileal Ca transport favored the maintenance o f soluble Ca in ileal solution during longer time periods than their a bsence. In the ileal enterocyte, these substances induced a twofold in crease of ATP content compared with controls. Further-more, the size o f mitochondria and the number of cristae and dense granules was increa sed 60 minutes after the presence of sorbitol in ileal solution. In fa ce of the concomitant decrease of luminal Na concentration and increas e of enterocyte ATP content, we propose that carbohydrates may act to stimulate Ca transport by decreasing Na, K-ATPase activity, thereby fa voring an increase in activity of Ca ATPase. Interestingly, enhanced i leal Ca transport was associated with an apparent enhancement of mitoc hondrial activity of the ileal mucosa, as reflected by an increase in mitochondrial size and the number of cristae and dense granules.