Ann M. Beshensky

Regulation by macromolecules of calcium oxalate crystal aggregation in stone formers

Based on the structure of kidney stones, it is likely that they form as aggregations of preformed crystals, mostly calcium oxalate monohydrate (COM). In this study, we examined the ability of a macromolecular mixture isolated from the urine of normal individuals and stone formers to inhibit aggregation of preformed COM seed crystals in a simple ionic solution using measurements of changes in the particle size distribution (PSD) of preformed COM crystal aggregates. We also examined the effect in this assay of a number of synthetic homopolymers, naturally occurring urine macromolecules, and binary mixtures thereof. The macromolecular mixtures from urine of normals and most stone formers reduced the degree of aggregation of the seed crystals, whereas 22% of stone former urine macromolecules either did not disaggregate or actually promoted further aggregation. Stone formers within one family shared this property, but a non-stone forming sibling did not. Polyanions, either synthetic or naturally occurring, induced disaggregation to an extent similar to that exhibited by normal urine macromolecules, while polycations had no effect on the PSD. However, mixing a polyanion, either poly-aspartate or osteopontin, with the polycation poly-arginine, changed their behavior from disaggregation to aggregation promotion. The disaggregating behavior of normal urinary macromolecules provides a defense against aggregation, but a minority of stone forming individuals lacks this defense, which may contribute to stone formation.

Acidic polyanion poly(acrylic acid) prevents calcium oxalate crystal deposition

Acidic macromolecules inhibit calcium oxalate nucleation, growth, aggregation and attachment to cells in vitro. To test for such an effect in vivo we used osmotic minipumps to continuously infuse several doses of the 5.1 kDa poly(acrylic acid) (pAA(5.1)) into rats fed a diet which causes renal calcium oxalate crystal deposition. Although kidneys of rats receiving the saline control contained calcium oxalate crystals, measured by polarized light microscopy, those of animals given pAA(5.1) had significantly lower numbers of crystals in various zones of the kidney. Delivery of pAA(5.1) to urine was confirmed by measuring excretion of infused biotinylated pAA(5.1). Both the derivatized and unlabelled pAA(5.1) had the same effects on crystallization in vitro. Our study shows that acidic polymers hold promise as effective therapies for kidney stones likely through prevention of calcium oxalate crystal aggregate formation.