X-537A ionophore-mediated calcium transport and calcium phosphate formation in Pressman cells

The present study examined precipitate development induced in phosphate solutions by the ionophoric translocation of Ca2+ across a bulk organic solvent barrier. Experiments were conducted at pH 7.4 and 25 or 37 degrees in a three-compartment Pressman cell. The aqueous reaction (0, 2.2 or 22 mM phosphate, 100 mM K+) and donor (1.3 or 13 mM Ca2+, 0.8 mM Mg2+, 100 mM Na+) compartments were separated by a CHCl3 compartment rendered permeable to cations by the addition of the carboxylic ionophore X-537A (2 or 20 mM). The resultant cation movements increased Ca2+, Mg2+, and Na+ concentrations in the reaction compartment at the expense of K+ loss to the donor compartment. The magnitude of the K+ counterflow and the efficiency of the ionophore-mediated Ca2+ in equilibrium 2K+ exchange reaction resulted in appreciable Ca2+ overshoots in the reaction compartment. In the absence of phosphate, Ca2+ increases exceeded donor levels by several-fold. With phosphate present, the Ca2+ flux was sufficient to induced precipitation. Generally, the first solid formed was amorphous. The amorphous precipitate, however, was unstable and converted to an apatite-like crystalline phase. Both carbonate (26 mM) and Mg2+ (0.8 mM) in the reaction solution delayed but did not prevent the conversion. The possible relevance of these findings to matrix vesicle calcification is discussed.