Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.

Cells of almost all organisms accumulate organic osmolytes when exposed to hyperosmolality, most often in the form of high salt or urea. In this review, we discuss 1) how the organic osmolytes protect; 2) the identity of osmolytes in Archaea, bacteria, yeast, plants, marine animals, and mammals; 3) the mechanisms by which they are accumulated; 4) sensors of osmolality; 5) the signaling pathways involved; and 6) mutual counteraction by urea and methylamines.The function and regulation of intracellular organic osmolytes were already well described in a classical review 25 years ago (1). The basic concepts, as described then and still valid, are as follows. 1) All water-stressed organisms, except halobacteria, accumulate intracellular organic osmolytes in response to water stress. 2) The major systems of organic osmolytes include polyhydric alcohols, free amino acids and their derivatives, and combinations of urea and methylamines.3) The evolutionary advantage of the organic osmolyte systems is compatibility with macromolecular structure and function at high and variable osmolyte concentrations without modifying cellular proteins to function in concentrated intracellular solutions. 4) Osmolyte compatibility results from non-perturbing or favorable effects of osmolytes on macromolecule-solvent interactions. In this review, we emphasize subsequent findings that expand upon these concepts in various species.

Sorbitol, inositol, GPC, and betaine are the predominant organic osmolytes in renal medullary cells. They protect the cells from harmful effects of the high interstitial NaCl and urea concentrations that occur normally in the renal medulla with operation of the urinary concentrating mechanism. Their levels correlate with extracellular NaCl concentration and, in the case of GPC, also with urea. Sorbitol is synthesized from glucose in a reaction catalyzed by aldose reductase. Inositol and betaine are transported into the cell. Glycerophosphorylcholine synthesis is dependent on choline. The transcription of aldose reductase and the transport of betaine and inositol are regulated, dependent on the degree of hypertonicity. Normal organic osmolyte regulation contributes to the survival and growth of medullary cells in their hyperosmolal environment, and defective regulation can damage them.

Adaptation of cells to hypertonicity often involves changes in gene expression. Since the concentration of salt in the interstitial fluid surrounding renal inner medullary cells varies with operation of the renal concentrating mechanism and generally is very high, the adaptive mechanisms of these cells are of special interest. Renal medullary cells compensate for hypertonicity by accumulating variable amounts of compatible organic osmolytes, including sorbitol, myo-inositol, glycine betaine, and taurine. In this review we consider how these solutes help relieve the stress of hypertonicity and the nature of transporters and enzymes responsible for their variable accumulation. We emphasize recent developments concerning the molecular basis for osmotic regulation of these genes, including identification and characterization of osmotic response elements. Although osmotic stresses are much smaller in other parts of the body than in the renal medulla, similar mechanisms operate throughout, yielding important physiological and pathophysiological consequences.