“The following possible explanations for the significant increases in the oral bioavailability and … furosemide observed when ascorbic acid was coadministered to dogs were investigated: ascorbic acid … furosemide , might inhibit GI wall metabolism of furosemide, might enhance the reabsorption … might increase the unionized fraction of furosemide at the receptor sites.”
“The significant increases in the diuretic and natriuretic effects of furosemide with ascorbic acid could be the result of increases in the reabsorption of furosemide from renal tubules and increases in the unionized fraction of furosemide at the renal tubular receptor sites.”