Retention of stone crystallites by urothelium is clearly one of the prime requisites for urinary stone disease. Studies in the literature as early as 1937 have highlighted that the initiation of renal calculi followed the formation of subepithelial calcified plaques in the renal pelvis. The renal papilla is one of the primary sites for crystal fixation and stone maturation. We have developed an in vitro model system for the study of kidney stone crystal retention to tubular epithelium using rat renal papillary collecting tubule (RPCT) cells in primary culture. We have qualitatively and quantitatively analyzed the binding of preformed calcium oxalate monohydrate (COM), hydroxyapatite (HA), and uric acid (UA) crystals to RPCT cells. Our goal was to determine if three common urinary stone crystals evidenced different crystal-cell binding characteristics. Also, since these crystals are frequently observed admixed in stones, we have studied the inhibitive binding characteristics of these crystals with RPCT cells. The RPCT cells in culture grow both as the typical polygonal cells in monolayer and as clumps of aggregated cells. The cells in the aggregates are viable epithelial cells that have lost their attachment to the basement membrane, resulting in the exposure of surface molecules that would not normally be present unless the cells were damaged or if there was a loss of intercellular tight junctions. COM, HA, and UA crystals all preferentially bound to the aggregated cells and all exhibited similar saturable binding patterns. If HA crystals were either applied simultaneously or before COM crystals were added to the culture, HA effectively bound to the cell culture and inhibited approximately 66% of the COM binding, suggesting that there are common binding areas on the cell aggregates for both COM and HA crystals and that there may also exist unique binding sites for these crystals. UA crystal binding to RPCT cells exhibited a similar concentration-dependent binding pattern and the binding was inhibitable by prebound COM crystals. We propose that in urolithiasis, it is likely that some form of renal tubular injury or damage to the epithelial cell membranes in the collecting ducts such as the loss of intercellular tight junctions exposes basolateral or basement membrane molecules, and that this event is a necessary requirement for successful crystal retention and calculi development. © 1991, National Kidney Foundation, Inc.. All rights reserved.
