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

Abstract

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 μCi ⁴⁵Ca) in the absence (control) or presence of 100 mM sorbitol, L-xylose, or creatine. Ileal calcium (Ca) transport, measured by plasma ⁴⁵Ca appearance, was found to be similar 30 minutes after fluid instillation in all four instances. However, thereafter, ⁴⁵Ca appearance in plasma did not increase further in control animals whereas it increased 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 effect was already observed during the first 30 minutes. The stimulation of ileal Ca absorption induced by the presence of sorbitol appeared to be due to a cellular effect, associated with a decreased flux across the paracellular pathway, as indicated by ³H-mannitol absorption. The presence 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). Thirty minutes after instillation, no difference in soluble Ca concentration was observed between control and experimental rats. After 60 minutes, Ca concentration was dramatically decreased in control rats but it remained nearly constant in experimental animals. Thus, the presence of substances enhancing ileal Ca transport favored the maintenance of soluble Ca in ileal solution during longer time peroids than their absence. In the ileal enterocyte, these substances induced a twofold increase of ATP content compared with controls. Furthermore, the size of mitochondria and the number of cristae and dense granules was increased 60 minutes after the presence of sorbitol in ileal solution. In face of the concomitant decrease of luminal Na concentration and increase of enterocyte ATP content, we propose that carbohydrates may act to stimulate Ca transport by decreasing Na, K-ATPase activity, thereby favoring an increase in activity of Ca ATPase. Interestingly, enhanced ileal Ca transport was associated with an apparent enhancement of mitochondrial activity of the ileal mucosa, as reflected by an increase in mitochondrial size and the number of cristae and dense granules.