The cells of the renal papilla are subject to widely varying extracellular osmolalities. In antidiuresis these cells are surrounded by hypertonic fluids, while in diuresis interstitial solute concentrations decline to levels comparable to those of the renal cortex. Papillary cells adapt to these extreme variations in extracellular tonicity by modulating the intracellular concentrations of small organic osmoeffectors (osmolytes), such as trimethylamines (glycerophosphorylcholine, betaine) and polyols (sorbitol, inositol). Variations in the intracellular concentrations of these osmolytes are accomplished by transmembrane net movement of water (cell swelling or shrinkage), by regulation of release and/or uptake of osmoeffectors via specific transmembrane transport pathways, and by changes in the intracellular synthesis of these organic compounds. These adaptive mechanisms allow intracellular electrolyte concentrations to remain relatively constant, despite extreme fluctuations in extracellular salinities. Accumulation of nonperturbing, organic osmolytes rather than inorganic electrolytes at high extracellular tonicities avoids the deleterious effects of elevated electrolyte concentrations on intracellular macromolecules. In addition, some of these osmolytes (trimethylamines) are assumed to counteract the adverse effects of high urea concentrations on the structure and function of cell proteins.
KeywordsAldose Reductase Renal Medulla Outer Medulla Medullary Cell Organic Osmolytes
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