Absorption, secretion, and precipitation of calcium in the small intestine of the dog

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Abstract
Ca transport was studied with the use of in situ segments prepared from the proximal jejunum and terminal ileum. Endogenous Ca secreted into such segments consisted of a dissolved and a precipitated acid-soluble fraction. The precipitate comprised about 1B of the secreted Ca. Precipitation also occurred when an isotonic solution containing 40Ca at a concentration of 10 mg/100 ml, 45Ca, and inulin as a nonabsorbed indicator was introduced into the segments. There was significantly more precipitation of 40Ca, which represents both endogenous and exogenous Ca, than of 45Ca which was exogenous only. Phosphate was secreted and coprecipitated with the Ca, suggesting that calcium phosphate comprised at least part of the precipitate. The low level of 45ca in the precipitate suggests that Ca and phosphate of endogenous origin precipitate near the mucosal surface before coming into complete equilibrium with the luminal contents. Precipitation was prevented by adding EDTA to the absorption solution. The jejunum showed net Ca secretion and the ileum net absorption from this solution. Flux from lumen to blood was 3 times greater in the ileum than in the jejunum. Failure to consider Ca precipitation would have caused underestimation of jejunal secretion by about 30% and overestimation of ileal absorption by an equal amount. In this cycle of Ca secretion by the jejunum and absorption by the ileum, the precipitate is unabsorbable and may have some role in the formation of endogenous fecal Ca.