Influence of fraction unbound upon the renal clearance of furosemide in the isolated perfused rat kidney.

The influence of fraction furosemide unbound (varied over a 28-fold range) upon its renal clearance was studied in an isolated perfused rat kidney, while renal function was maintained. Renal clearance and unbound fraction of furosemide were found to be related linearly, the average unbound renal clearance being 3.3 ml min-1. Urine pH and urine flow rate did not influence furosemide renal clearance in the ranges experienced in this study. Our results are consistent with those predicted from a consideration of the kinetics of renal excretion.