ARTIFICIAL SIMULATION OF THE EARLY STAGES OF RENAL STONE FORMATION

Objective To study the initial steps of calcium oxalate monohydrate (COM) stone formation and the influence of the inhibitor substances citrate and phytic acid, using an in vitro model. Materials and methods Experiments were performed using a device to simulate renal stone formation in conditions similar to those found in humans. Results COM crystal formation took place directly on the solid substrate in contact with the urine but no previous formation occurs in the liquid with subsequent attachment. The first COM crystals appeared after 3 h, in the presence of citrate at a physiological concentration of 1.7 x 10(-3) M. A decrease in the total amount of COM was observed during the next 9 h. After 24 h the quantity of COM crystals was the same irrespective of the presence or absence of citrate. In the presence of phytic acid (6 x 10(-3) M) the appearance of the first COM crystals was delayed by > 24 h. At 78 h a few small COM crystals were detected. Conclusion A fixed solid surface, irrespective of its composition, in contact with normal urine sooner or later develops stone-forming compounds on its surface. The composition of these compounds depends on the supersaturated salts that are present. Under the experimental conditions citrate had only a limited capacity to inhibit COM crystal development. In contrast, phytic acid exhibited a much higher capacity for inhibition even at a considerably lower concentration than that of citric acid.