Occurrence of renal pelvic refluxes during rising urine flow rate in rats and hamsters.

Retrograde movements of urine into the renal pelvic space (pelvic refluxes) were studied in anesthetized Munich Wistar rats and hamsters. The urine was made green by a continuous i.v. infusion of lissamine green in saline, and the experimental kidney was either placed on a shallow trough or left in situ. The renal pelvis was exposed and illuminated with a fiber optic light, and urine movements were observed through the transparent but intact pelvic wall. Urine was collected from both kidneys in the rats. In both rats and hamsters, the inner medulla of the kidney was analyzed for solutes at the end of the experiment. The experimental procedures did not interfere with the normal function of the experimental kidney, and the results were the same in rats and hamsters. During constant urine flow, full refluxes did not occur. Urine either moved straight down the ureter after it exited from the ducts of Bellini or it briefly bathed the papillary tip. In rats, full pelvic refluxes started approximately 0.8 min after a bolus injection (0.5 ml of isosmotic saline, i.v.), at a time corresponding to a steep rise in urine flow (2 microliter.min-2.100 g body wt-1). Following increased infusion rate, full refluxes were associated with an increase in urine flow of 0.05 g microliter.min-2.100 g body wt-1. Full refluxes were also seen in the hamsters following a bolus injection or increased infusion rate. Increasing intrapelvic pressure by 1 cm H2O also caused full pelvic refluxes. When full refluxes occurred, urine came into contact with all areas of the renal pelvis. Because full pelvic refluxes occur only during rising urine flow, this mechanism would bring urine with decreasing osmolality into contact with the outer medullary areas facing the pelvic space.