Evidence for absorption of ionic calcium and soluble calcium complexes by the duodenum and cecum in the rat

The absorption of dietary calcium (Ca) may in part be determined by the formation in the intestinal lumen of soluble Ca complexes and insoluble Ca salts. This study was undertaken to test the assumption that ionic Ca concentration (Ca2+) is the only species of Ca that is available for absorption. Bidirectional steady-state Ca fluxes were measured in vitro under short-circuit conditions across segments of the proximal duodenum and the cecum in the presence and absence of varying concentrations of soluble Ca citrate complexes. The presence of 5.0 mmol/L medium citrate reduced medium Ca2+ and cecal Ca mucosal-to-serosal fluxes (Jms) (29 +/- 18 versus 108 +/- 7 nmol Ca/cm2/h, P <.001), but did not reduce duodenal Ca Jms (31 +/- 5 versus 23 +/- 9, P not significant). Duodenal Ca Jms increased 106% as medium Ca citrate complex increased to 1.018 mmol/L and Ca2+ remained constant; cecal Jms increased by 48% under the same conditions. The formation of soluble Ca organic anion complexes with lactate, malate, and fumarate reduced medium Ca2+ and cecal Ca Jms decreased with the reduction of medium Ca2+. The results of this study indicate that Ca2+ is the form of Ca most readily absorbed by the small intestine and the colon. Soluble Ca citrate complexes are absorbed by the duodenum and, to a much lesser extent, by the cecum. The reduction of Ca Jms by citrate is caused by the reduction of medium Ca2+ through formation of Ca citrate complexes and not caused by a direct interaction of the anion with the intestinal epithelium.