Organic osmolytes in human and other mammalian kidneys

Osmotically-active organic solutes, or osmolytes, have been found in high concentration in the renal inner medulla of a wide variety of mammalian species, but their existence in human kidneys has not yet been shown. The aim of this study was to demonstrate the presence of osmolytes in the human kidney. Human tissues were obtained from kidneys removed surgically for diseases which involved only one pole of the kidney; in most cases this was a tumor. Animal kidneys analyzed were from dogs, pigs and rabbits. Inner medulla and cortex tissue samples were analyzed and found to contain the organic osmolytes glycine betaine, myo-inositol, sorbitol and glycerophosphorylcholine. The levels were much higher in the medulla than in the cortex. Further dissection of the human kidneys showed that sorbitol, glycerophosphorylcholine and glycine betaine were maximally concentrated at the papillary tip, while myo-inositol was found in highest concentration at the papillary base. Osmolytes were in low concentrations or undetectable in rabbit skeletal muscle, ureter and bladder. The organic osmolytes detected are likely to be physiologically important in humans. Studies in other mammals can be used as models for the investigation of the osmolyte system in human kidney function.