GRAVIMETRIC METHOD FOR THE DYNAMIC MEASUREMENT OF URINE FLOW

The rate of urine formation is a primary index of renal function, but no techniques are currently available to accurately measure low rates of urine flow on a continuous basis, such as are normally found in rat s. We developed a gravimetric method for the dynamic measurement of ur ine flow in anesthetized rats. Catheters were inserted directly into t he ureters close to the renal pelves, and a siphon was created to coll ect all of the urine formed as rapidly as it was produced. Urine flow was determined by measuring the weight of the urine using a direct-rea ding analytical balance interfaced to a computer. Basal urine flow was measured at 2-sec intervals for 30 to 60 min. The dynamic response of urine flow to a rapid decrease in arterial pressure produced by a bol us intravenous injection of acetylcholine (0.5 mu g) was also measured . Intrinsic drift, evaporative losses, and the responsiveness of the s ystem to several fixed pump-flows in the low physiologic range were ev aluated in vitro. The gravimetric method described was able to continu ously measure basal urine flows that averaged 37.3 +/- 12.4 mu l/min. Error due to drift and evaporation was negligible, totaling less than 1% of the measured urine flow. Acetylcholine-induced declines in arter ial pressure were followed within 8 sec by a decline in urine flow. Th ese data demonstrate that this new gravimetric method provides a simpl e, inexpensive, dynamic measurement of urine flow in the mu l/min rang e.