Inhibition of ROMK blocks macula densa tubuloglomerular feedback yet causes renal vasoconstriction in anesthetized rats

The Na+-K+-2Cl(-) cotransporter (NKCC2) on the loop of Henle is the site of action of furosemide. Because outer medullary potassium channel (ROMK) inhibitors prevent reabsorption by NKCC2, we tested the hypothesis that ROMK inhibition with a novel selective ROMK inhibitor (compound C) blocks tubuloglomerular feedback (TGF) and reduces vascular resistance. Loop perfusion of either ROMK inhibitor or furosemide caused dose-dependent blunting of TGF, but the response to furosemide was 10-fold more sensitive (IC50 = 10(-6) M for furosemide and IC50 = 10(-5) M for compound C). During systemic infusion, both diuretics inhibited TGF, but ROMK inhibitor was 10-fold more sensitive (compound C: 63% inhibition; furosemide: 32% inhibition). Despite blockade of TGF, 1 h of constant systemic infusion of both diuretics reduced the glomerular filtration rate (GFR) and renal blood flow (RBF) by 40-60% and increased renal vascular resistance (RVR) by 100-200%. Neither diuretic altered blood pressure or hematocrit. Proximal tubule hydrostatic pressures (PPT) increased transiently with both diuretics (compound C: 56% increase; furosemide: 70% increase) but returned to baseline. ROMK inhibitor caused more natriuresis (3,400 vs. 1,600% increase) and calciuresis (1,200 vs. 800% increase) but less kaliuresis (33 vs. 167% increase) than furosemide. In conclusion, blockade of ROMK or Na+-K+-2Cl(-) transport inhibits TGF yet increases renal vascular resistance. The renal vasoconstriction was independent of volume depletion, blood pressure, TGF, or P-PT.