Urinary osmolality as a function of flow rates in several diuretic states

¹ Department of Physiology, The University of Michigan, Ann Arbor, Michigan

The effect of different solute loads on the renal concentrating mechanism was evaluated at different glomerular filtration rates (GFR). GFR was varied by partial ureteral or arterial occlusion in dogs undergoing osmotic diuresis. A Ringer's solution was infused to which the following solutes were added so that the final concentrations of the solutes were as follows: a) 383 mm/liter mannitol + 255–285 mm/liter NaCl; b) 500 mm/liter urea; c) 193 mm/liter mannitol; d) 300 mm/liter urea + 385 mm/ liter mannitol. Steady-state values for GFR, urine flow, and urine osmolalities were obtained for the occluded and control kidneys and expressed as the ratio occluded-to-control. Changing the sodium chloride load did not alter the correlation between flow rate and urine osmolality as found in previous experiments with dogs which were not infused with additional sodium. However, variations in the urea and mannitol load did alter this relationship. The greater the mannitol load the lower was the osmolality ratio at any given GFR. The results indicate that both urea trapping and urine flow rates are important determinants of urinary concentrating ability.