Gastrointestinal first-pass effect of furosemide in rats.

The first-pass effect of furosemide was investigated in rats. Furosemide intravenous solution (20 mg kg(-1) Lasix), was administered via the jugular vein and the portal vein, orally, and instilled directly into the duodenum of rats. The first-pass effects of furosemide by lung, heart, and liver seemed to be negligible in rats. The absolute bioavailability of furosemide was 28.9 and 48.3% after oral and intraduodenal administration, respectively. Based on the gastrointestinal (GI) recovery study, 68.3 and 69.5% of furosemide were found to have disappeared mainly due to absorption and/or metabolism from rat GI tract after oral and intraduodenal administration, respectively. The results indicate that gastrointestinal and intestinal first-pass effects of furosemide were approximately 40% (68.3-28.9%) and 20% (69.5-48.3%) of the dose, respectively.