Mannitol enhances intracellular calcium diffusion in the rat ileum - a hypothesis

The addition of 92 or 136 mM mannitol to a modified saline solution that co ntained 1.25 mM Ca2+ led to a mannitol concentration-dependent increase in the amount of calcium absorbed in 1 h from 8 cm long ileal loops prepared f rom fasted male Sprague-Dawley rats, with body weights of 190 +/- 10 g. It is argued that this mannitol-enhanced movement of calcium out of the loop c annot have utilized the paracellular pathway, inasmuch as the luminal calci um concentration of the mannitol instillate decreased during the experiment , with a negative calcium gradient between luminal and body fluids. Instead it is proposed that uncomplexed mannitol and the uncharged calcium complex of mannitol entered the ileal cells. The uncomplexed intracellular mannito l would bind additional calcium that had crossed the brush border down its gradient. The increase in total intracellular calcium will raise the effect ive intracellular gradient and thereby amplify intracellular calcium diffus ion. This in turn increases calcium absorption.