Water removal and solute additions determining increases in renal medullary osmolality.

Osmolality and solute concentrations of the mammalian renal medulla increase and decrease with changing urine osmolality. These changes are brought about by addition or removal of solute or water to or from the renal medullary tissue. In Munich-Wistar rats and Syrian hamsters, males and females, actual amounts of and the various solutes involved in these changes were determined. Kidneys were removed from animals killed in different stages of water diuresis and antidiuresis. The renal medulla was analyzed by a new method that permits determination of water and solutes on the same piece of tissue. Removal of water and addition of urea were the two most important factors in raising inner medullary osmolality. Papillary water content was inversely related to the papillary osmolality and was 50% lower in extreme antidiuresis compared with water diuresis. Rats had higher papillary water content than hamsters. In the outer medulla, water removal was significant in the hamsters but not in the rats. Addition of urea to the papillary tissue exceeded the osmotic equivalent of NaCl by a factor of 2.8 in both rats and hamsters. Females of both species showed greater changes than males in amounts of urea in the inner medulla.