Hypertonicity-induced accumulation of organic osmolytes in papillary interstitial cells.

Medullary cells of the concentrating kidney are exposed to high extracellular solute concentrations. It is well established that epithelial cells in this kidney region adapt osmotically to hypertonic stress by accumulating organic osmolytes. Little is known, however, of the adaptive mechanisms of a further medullary cell type, the papillary interstitial cell [renal papillary fibroblast (RPF)]. We therefore compared the responses of primary cultures of RPFs and papillary collecting duct (PCD) cells exposed to hypertonic medium.

In RPFs and PCD cells, organic osmolytes were determined by high-performance liquid chromatography; mRNA expression for organic osmolyte transporters [Na⁺/Cl^--dependent betaine transporter (BGT), Na⁺-dependent myo-inositol transporter (SMIT)], and the sorbitol synthetic and degrading enzymes [aldose reductase (AR) and sorbitol dehydrogenase (SDH), respectively] was determined by Northern blot analysis.

Exposure to hypertonic medium (600 mOsm/kg by NaCl addition) caused intracellular contents of glycerophosphorylcholine, betaine, myo-inositol, and sorbitol, but not free amino acids, to increase significantly in both RPFs and PCD cells. The rise in intracellular contents of these organic osmolytes was accompanied by enhanced expression of mRNAs coding for BGT, SMIT, and AR in both RPFs and PCD cells. SDH mRNA abundance, however, was unchanged. Nonradioactive in situ hybridization studies on sections from formalin-fixed and paraffin-embedded, normally concentrating kidneys showed strong expression of BGT, SMIT, and AR mRNAs in interstitial and collecting duct cells of the papilla, whereas expression of SDH mRNA was much weaker in both cell types.

These results suggest that both RPFs and PCD cells use similar strategies to adapt osmotically to the high interstitial NaCl concentrations characteristic for the inner medulla and papilla of the concentrating kidney.