PRETREATMENT WITH HYPERTONIC NACL PROTECTS MDCK CELLS AGAINST HIGH UREA CONCENTRATIONS

In antidiuresis, the cells of the renal medulla are exposed to high ex tracellular concentrations of NaCl and urea. Since urea equilibrates w ith the intracellular compartment and is known to perturb intracellula r macromolecules, high urea concentrations may well disturb the struct ure and function of cell proteins. Two types of organic substances are believed to counteract the adverse effects of high intracellular urea concentrations: specific organic osmolytes of the trimethylamine fami ly [betaine and glycerophosphorylcholine (GPC)], which accumulate in r enal medullary cells during prolonged periods of antidiuresis and cyto protective heat shock proteins (HSPs), the tissue content of two of wh ich (HSPs 27 and 72) is much higher in the inner medulla than in the i so-osmotic renal cortex. To evaluate the contribution of trimethylamin es and HSPs to cytoprotection in the presence of high urea concentrati ons, the effect of HSP induction and osmolyte accumulation prior to ex posure to high urea concentrations was examined in Madin-Darby canine kidney (MDCK) cells. Accumulation of organic osmolytes and synthesis o f HSP27 and HSP72 was initiated by hypertonic stress (increasing the o smolality of the medium from 290 to 600 mosmol/kg H2O by NaCl addition ). Control, non-conditioned cells remained in the isotonic medium for the same period. Upon subsequent exposure to an additional 600 mM urea in the medium for 24 h, 90% of the osmotically conditioned cells but only 15% of non-conditioned cells survived. The HSP72 and trimethylami ne contents of the NaCl-conditioned MDCK cells, but not HSP27 content, correlated positively with cell survival. To separate the effects of organic osmolytes and HSP72, chronically NaCl-adapted MDCK cells were returned to isotonic medium for 1 or 2 days, so depleting them of trim ethylamine osmolytes. HSP72, with its longer half life, remained eleva ted. Subsequent exposure of these cells to 600 mM urea in the medium r esulted in about 80% survival. These results suggest that in MDCK cell s and probably in the renal medulla, HSP72 and perhaps additional prot ective factors contribute substantially to the resistance against high urea concentrations.