ROLE OF TRANSCELLULAR PATHWAY IN ILEAL CA2- STIMULATION BY LOW-CA2+ DIET( ABSORPTION )

The present study was performed to determine the respective involvemen t of the cellular and paracellular routes in ileal Ca2+ transport. Two groups of rats were either fed a normal Ca2+ diet (1.0%) or a Ca2+-de ficient diet (0.02%) for 14 days. Ileal Ca2+ absorption was determined using both an in situ method of continuous luminal perfusion and an i n vitro method (Ussing chamber model). The low-Ca2+ diet stimulated ne t Ca2+ flux in the ileum twofold, associated with a twofold increase o f the mucosal-to-serosal Ca2+ flux in both models. This effect was obs erved in the absence of concomitant changes in Na+ or water flux in th e in situ model or mannitol flux in the in vitro model, excluding the participation of the paracellular pathway in Ca2+ transport. Thus only cellular Ca2+ flux was stimulated. These data suggest that the ileum plays a major role in the adaptation to low dietary Ca2+. Whereas unde r physiological conditions with usual Ca2+ intakes the transcellular p athway of Ca2+ transport is negligible, it becomes of major importance in the case of Ca2+ deficiency, at least under the present conditions of severe Ca2+ deprivation.