Cellular and paracellular calcium transport in rat ileum: effects of dietary calcium.

The mechanism of calcium transport and its modulation by dietary Ca restriction in rat ileum have been investigated employing an in vitro voltage-clamp technique. Ca fluxes directed from mucosa-to-serosa (J(Ca)ms) and serosa-to-mucosa (J(Ca)sm) exhibit components consistent with both cellular and paracellular pathways. The cellular Ca fluxes are both dependent on medium Na and are abolished at 10 degrees C. In addition, a cellular component of J(Ca)ms displays voltage dependence. A low-calcium diet, which induces the formation of 1,25-dihydroxycholecalciferol, causes a marked increase in both cellular J(Ca)ms and Ca influx from media to cells with little absolute effect on the paracellular pathway. This evidence is consistent with the existence of electrogenic Na-Ca exchange pumps at both brush-border and basolateral membranes, driven in part by the Na electrochemical gradient. Dietary Ca conditioning may control the direction of net Ca transport by modulation of the saturable influx process at the brush border.