The influence of co-administered organic acids on the kinetics and dynamics of frusemide.

1. This study examines the effects of pretreatment with probenecid with and without pyrazinamide on the elimination kinetics and diuretic action of frusemide. 2. Six normal male volunteers received 40 mg frusemide i.v. on three occasions; i.e. once on its own and twice after pretreatment with 2 g probenecid with and without 3 g pyrazinamide. Both these latter drugs were administered orally 3 h before frusemide administration thereby attempting optimal suppression of proximal tubular secretion. Urinary losses were replaced i.v. with isovolumetric amounts of normal saline while insensible losses were compensated for by taking tap water orally. 3. The mean cumulative urinary frusemide excretion was significantly and similarly decreased by pretreatment with probenecid (34.9%) and probenecid plus pyrazinamide (33.6%), but the mean total volume of diuresis and the mean cumulative urinary sodium excretion did not differ significantly between treatments over the 5 h period. 4. The diuretic efficiency of frusemide was significantly increased with probenecid pretreatment during the first 90 min period after frusemide administration. Furthermore, in the first 30 min after administration the percent sodium fractional excretion was higher after pretreatment with probenecid even though the mean frusemide excretion rate was more than three times with frusemide alone than with probenecid-frusemide (374.4 micrograms min-1 vs 119.1 micrograms min-1). Pretreatment with probenecid results in a higher concentration on the peritubular or blood side of the tubules and these results lead us to question the unconditional acceptance of a luminal site of action for the loop diuretics. Alternatively, probenecid may act in some other way to increase the effects of frusemide.