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

Abstract

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.

Transport of chloride ion through phospholipid bilayers mediated by synthetic ionophores

“…Their transporter is based on the Crabtree isophthalamide receptor joined to a crown ether. Transport by the cation-anion binding hybrid molecule was studied by using ion selective electrodes and both 23 Na and 35 Cl NMR methods. 45 Likewise, these methods were used in combination to study Cl À transport mediated by steroid-based receptors.…”

“…Measuring transport by using various dynamic methods is not without its problems either. A very useful approach is to use a glass U-tube 35 as done by Pressman and coworkers in their pioneering studies on valinomycin 36 and gramicidin. 37 The principle is simple.…”