Inhibition of NaCl absorption from perfused rat ileum by furosemide

The effect of furosemide on intestinal absorption of water and electrolytes was studied using segments of rat ileum perfused in vivo. Furosemide (1 mM) in the perfusion fluid reduced absorption of Na, Cl, and water by 50% from a balanced electrolyte solution without changing the transepithelial potential difference (PD). This effect was also observed in the absence of luminal glucose and was largely reversible. Substitution of all Na in perfusion fluid with choline produced secretion of Na and water and abolished Cl absorption; substitution of all Cl with SO4 reduced Na absorption to 20% of control values. Under both these conditions, furosemide had only trivial effects on electrolyte absorption and exerted no effect on PD. Measurements of unidirectional fluxes of Na and Cl showed that furosemide decreased net flux by reducing lumen-to-blood flux of these ions rather than increasing blood-to lumen flux. These results resemble those obtained in this tissue following exposure to acetazolamide, and suggest that furosemide inhibits a coupled, neutral process of NaCl transport from lumen to blood. Although this effect could be a result of carbonic anhydrase inhibition it more likely occurs from a separate action of furosemide on ileal transport.