Changes of sodium and urea concentrations in the renal papillary interstitial fluid on dehydration of rats.

1. The isolated renal papillae of a rat were centrifuged in a two tube assembly which allowed fluid from the tissue to separate into the lower tube.2. The papillae were centrifuged for 15 min at 300 g and 1500 g consecutively.3. After intraperitoneal injection of Na (131)I-diatrizoate, the activities of urine, and fluid samples obtained from the papilla, were compared. It was found at 1500 g that the median value for papillary fluid activity was 1.52% of the activity of urine. This is evidence that the papillary fluid was virtually free from any contamination from the terminal collecting ducts.4. It is considered that the fluid sample obtained from the papillae by centrifugation at 1500 g is a representative and reproducible sample of interstitial fluid.5. The method was used to demonstrate changes in solute concentrations in the renal papillary fluid, following dehydration of rats.