The influence of citrate on the duodenal absorption of calcium in the rat.

Using in situ perfusion in the rat, we studied the duodenal bidirectional fluxes (LP, lumen-to-plasma; PL, plasma-to-lumen) and the net absorption of calcium (Ca) at progressive molar citrate:Ca ratios (1:1, 2.1:1, 8.5:1) and three pH levels (pH 5.0, 6.25, 7.0) of the Ca-containing (4 mmol/liter) perfusion solution. Rats perfused with pH-adjusted, citrate-free solutions served as controls. LP-flux and net calcium absorption (net CaA) fell dose-dependently with increasing citrate concentrations. Also, at a given citrate:Ca ratio, LP-flux and net CaA fell with increasing pH of the perfusion solution. PL-fluxes did not change from control values, irrespective of the composition of the perfusion solution. The concomitant serum Ca and parathyroid hormone levels as well as blood pH and bicarbonate, were largely unchanged under any of the citrate-containing perfusion solutions. In conclusion, citrate inhibits duodenal CaA of the rat pH- and dose-dependently, without changing intestinal Ca secretion. This effect is not related to changes in the acid-base status or to suppressed parathyroid gland function of the animals and may be best explained by dose- and pH-dependent intraluminal complex formation between Ca and citrate.