SOLUTE COMPOSITION AND HEAT-SHOCK PROTEINS IN RAT RENAL MEDULLA

The high content of heat shock proteins (HSPs) 25 and 72 in the hypero smotic inner medulla of the concentrating kidney has been ascribed to the high NaCl and urea concentrations in this kidney zone. To assess t he effects of variations in the composition of solutes in the renal me dulla on the intrarenal distribution of HSPs, rats were fed either a h igh- or low-Na diet for 3 weeks. These diets result in greatly differi ng urine and inner medullary solute composition. Sodium dodecyl sulpha te polyacrylamide gel electrophoresis and Western blot techniques were used to analyse HSP25 and HSP72 in the cortex, outer medulla and inne r medulla. In addition, the amounts of organic osmolytes (sorbitol, my o-inositol, betaine and glycerophosphorylcholine) and urea in the tiss ue were determined by high-performance liquid chromatography. Intra- a nd extracellular electrolyte concentrations at the papillary tip were measured by electron microprobe analysis. In the high-Na group, urine osmolality was about 1000 mosmol/kg lower than in rats fed a low-Na di et, due to lower urea concentrations. The sum of urine sodium and pota ssium concentrations, however, did not differ between the two groups. Neither in the outer nor in the inner medulla was the sum of the conce ntrations of organic osmolytes affected by the dietary treatment. The sum of sodium, potassium and chloride concentrations did not differ be tween the two experimental groups, neither in the interstitial nor in the intracellular compartments. However, the urea content and the amou nts of HSP25 and HSP72 were significantly lower in the inner medulla o f the group of rats fed a high-Na diet. Our results suggest that urea participates in the regulation of the medullary levels of the HSPs and that both HSP25 and HSP72 are components of mechanisms protecting med ullary cells against the deleterious effects of high urea concentratio ns.