Comparison of the local effects of amiloride hydrochloride on the isotonic fluid absorption in the distal and proximal convoluted tubule.

The isotonic fluid absorption (Jv) was measured under standard conditions in the proximal and distal convolution of the rat kidney. The peritubular blood capillaries were perfused simultaneously. Amiloride was applied either intraluminally or peritubularly. When applied intraluminally, amiloride strongly inhibited Jv in the distal tubule at concentrations up to 10(-6) M. In the proximal tubule similar effects were obtained only after intraluminal application of one thousand-fold greater concentrations of amiloride. In contrast to amphibian epithelia, amiloride also inhibits Jv in the distal tubule when applied peritubularly, but at higher concentrations and less completely than after intraluminal application. Amiloride was found to be generally more effective in the distal tubule than furosemide and mefruside, although in the proximal tubule it was less effective than these diuretics. That amiloride is most effective after intraluminal application in the distal tubule would suggest a dominant action at the luminal membrane of the distal tubule cell, while not excluding a concomitant effect at the peritubular membrane.