ROMK inhibitor actions in the nephron probed with diuretics

Diuretics acting on specific nephron segments to inhibit Na⁺ reabsorption have been used clinically for decades; however, drug interactions, tolerance, and derangements in serum K⁺ complicate their use to achieve target blood pressure. ROMK is an attractive diuretic target, in part, because its inhibition is postulated to indirectly inhibit the bumetanide-sensitive Na⁺-K⁺-2Cl^- cotransporter (NKCC2) and the amiloride- and benzamil-sensitive epithelial Na⁺ channel (ENaC). The development of small-molecule ROMK inhibitors has created opportunities for exploring the physiological responses to ROMK inhibition. The present study evaluated how inhibition of ROMK alone or in combination with NKCC2, ENaC, or the hydrochlorothiazide (HCTZ) target NCC alter fluid and electrolyte transport in the nephron. The ROMK inhibitor VU591 failed to induce diuresis when administered orally to rats. However, another ROMK inhibitor, termed compound A, induced a robust natriuretic diuresis without kaliuresis. Compound A produced additive effects on urine output and Na⁺ excretion when combined with HCTZ, amiloride, or benzamil, but not when coadministered with bumetanide, suggesting that the major diuretic target site is the thick ascending limb (TAL). Interestingly, compound A inhibited the kaliuretic response induced by bumetanide and HCTZ, an effect we attribute to inhibition of ROMK-mediated K⁺ secretion in the TAL and CD. Compound A had no effect on heterologously expressed flow-sensitive large-conductance Ca²⁺-activated K⁺ channels (Slo1/β1). In conclusion, compound A represents an important new pharmacological tool for investigating the renal consequences of ROMK inhibition and therapeutic potential of ROMK as a diuretic target.