REGULATION OF GENE-EXPRESSION BY HYPERTONICITY

Adaptation of cells to hypertonicity often involves changes in gene ex pression. Since the concentration of salt in the interstitial fluid su rrounding renal inner medullary cells varies with operation of the ren al concentrating mechanism and generally is very high, the adaptive me chanisms of these cells are of special interest. Renal medullary cells compensate for hypertonicity by accumulating variable amounts of comp atible organic osmolytes, including sorbitol, myo-inositol, glycine be taine, 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 rec ent developments concerning the molecular basis for osmotic regulation of these genes, including identification and characterization of osmo tic response elements. Although osmotic stresses are much smaller in o ther parts of the body than in the renal medulla, similar mechanisms o perate throughout, yielding important physiological and pathophysiolog ical consequences.