Calcium and Oxalic Acid Kinetics Differ in Rats

Small molecular weight calcium salts, if absorbed intact, could provide a nutritional source of calcium in subjects with impaired absorption of calcium by the saturable pathway. An understanding of the mechanism of absorption of calcium oxalate (as a representative salt) may be important nutritionally and therapeutically. The aim of the present study was to develop models to study absorption, distribution and retention of calcium and oxalate in rats as a basis for studying calcium oxalate absorption. Labeled compounds (45Ca and [14C]-oxalic acid) were administered to separate groups of rats orally (n = 8-11) or intravenously (n = 3-5) and blood was sampled for up to 240 min. Data were analyzed using SAAM/CONSAM. Calcium kinetics were fitted by a model with three compartments in the body and one absorption pathway from the intestine. By contrast, oxalic acid kinetics were fitted by two pools in the body and two absorption pathways from the intestine. Calcium and oxalic acid, therefore, demonstrate different absorption and distribution kinetics in rats.