Effects of probenecid on furosemide response.

Furosemide gains access to its intraluminal site of action by active secretion by the organic acid transport system of the proximal tubule. Inhibition of this transport by probenecid would predictably decrease the effect of furosemide. In this study in 8 normal volunteers, however, the opposite occurred; namely, pretreatment with probenecid increased the overall response to furosemide by prolonging its effect. Sodium excretion in 8 hr due to 40 mg of furosemide rose from 262 +/- 16 to 358 +/- 11 mEq after probenecid. Urine volume increased from 3,265 +/- 275 to 4,165 +/- 183 ml after probenecid. Analysis of the time-course of the increased diuresis and natriuresis showed that probenecid actually decreased the response for the first 60 to 90 min after furosemide but increased the subsequent response sufficiently to result in a greater overall effect. Possible explanations include access of furosemide to its active site from the serum, an effect of probenecid on prostaglandin transport, and a changing pharmacokinetic interaction between probenecid and furosemide.