Intratubular crystallization of calcium oxalate in the presence of membrane vesicles: An in vitro study

Background. Since urine spends only a few minutes in the renal tubules and has a low supersaturation with respect to calcium oxalate (CaOx), nucleation of CaOx crystals in the kidneys is most probably heterogeneous. We have proposed that membranes of cellular degradation products are the main substrate for crystal nucleation. The purpose of our study was to determine the site of membrane-mediated crystal nucleation within the renal tubules and the required lag time, factors that determine whether crystallization results in crystalluria or nephrolithiasis. Methods. Nucleation of CaOx was allowed to occur in five different artificial urine solutions with ionic concentrations simulating urine in proximal tubules (PTs), descending (DLH) and ascending (ALH) limbs of the loop of Henle, distal tubules (DTs), and collecting ducts (CDs). A constant composition crystallization system was used. Experiments were run for two hours with or without the renal tubular brush border membrane (BBM) vesicles. Results. The addition of BBM significantly reduced the nucleation lag time and increased the rate of crystallization. The average nucleation lag time decreased from 84.6 +/- 43.4 minutes to 24.5 +/- 19 minutes in PTs, from 143.6 +/- 29 to 70.2 +/- 53.4 minutes in DLH, from 17.6 +/- 8.6 minutes to 0.625 +/- 0.65 minutes in DTs and from 9.54 +/- 3.03 minutes to 0.625 +/- 0.65 minutes in CDs. There was no nucleation in the ALH solution without BBM for two hours. CaOx dihydrate (COD) was common in most solutions. Calcium phosphate (CaP) also nucleated in the DLH and CD solutions. Conclusions. In the absence of membrane vesicles, there was no crystallization in any of the solutions within the time urine spends in the renal tubules. As a result, homogeneous nucleation of crystals anywhere within the nephron appears unlikely. However, BBM-supported nucleation is possible in the DTs as well as CDs. A high crystallization rate in CDs would promote rapid crystal growth and aggregation, resulting in crystal retention within the kidneys and development of nephrolithiasis.