The effect of
calcium concentration (0-10 mmol 1(-1)) on
oxalate uptake and transport was investigated in vitro using everted gut segments and sacs. Increase in
calcium concentration in the incubation medium led to an increase in the amounts of precipitated
oxalate on the intestine; however, the net
oxalate flux to the serosal side decreased. The
ions, i.e. Ca2+, Ox2-, H2PO4-, HPO4(2-), present in the incubation medium favoured formation of
hydroxyapatite and
calcium oxalate crystals, as evidenced by Equil II analysis and free energy of the system. The nature of precipitates was confirmed by elemental analysis, X-ray diffraction spectrometry and electron microscopy.
Oxalate precipitated on the intestine following incubation with
calcium could be released into a
calcium- and
oxalate-free medium. Animals fed
oxalate in the absence and presence of
calcium revealed that, during 1 h in the absence of
calcium, oxalate moved down the intestinal tract as a distinct peak of greater than 50% (70-90 cm in the intestine), leaving less than 10% in the stomach and first 50 cm of the intestine. In the case of animals fed
calcium along with
oxalate, 35% of the
oxalate was still present in the stomach, and the amounts of
oxalate in the intestinal segments gradually increased from 4.5% to 21.7% (0-90 cm) and dropped to 2.1% in the next 20 cm. Since oxalaemia and
oxaluria appear to be influenced by intestinal
oxalate absorption, the present observations may help to improve understanding of the pathophysiology of disorders exhibiting altered
oxalate metabolism.