Furosemide acts on short loop of descending thin limb, but not on long loop

In order to elucidate the tubular sites of action of loop diuretics such as furosemide, bumetanide and ethacrynic-cysteine complex within isolated rat descending thin limbs, cellular ATP was measured by luciferin-luciferase technique. When short descending thin limbs of Henle's loop (SDL) were incubated in the absence of exogenous substrate at 37 degrees C, cellular ATP content was decreased in a time-dependent manner (up to 49% after 60 min). This ATP decrease, however, was retarded significantly in the presence of loop diuretics at 60 min. The mean percentage of change in ATP compared with the control for each loop diuretic in SDL was as follows: 10(-5) M furosemide, 178%; 10(-5) M bumetanide, 189%; and 10(-7) M ethacrynic-cysteine complex, 154%; respectively. To the contrary, cellular ATP in long descending thin limb of Henle's loop (LDL) was not changed by loop diuretics compared with the control. A similar protection against ATP depletion was observed in the medullary thick ascending limb of Henle's loop, in which the mean percentage was as follows: 10(-5) M furosemide, 163%; 10(-5) M bumetanide, 187%; and 10(-7) M ethacrynic-cysteine complex, 134%. Similarly to LDL, the cellular ATP did not change in outer medullary collecting tubule. From these results, we conclude that loop diuretics act on the isolated rat SDL, but not on LDL.