ROMK is Required for the Net K Secretion in the Thick Ascending Limb of Mice on a Low Na High K Diet

Abstract

Understanding the effect of a cardio‐ and reno‐protective low Na, high K diet (LNaHK) on renal K handling is crucial to choosing diuretics and anti‐hypertensive agents for patients on such diets. We previously showed that furosemide, a K‐wasting diuretic that inhibits the Na‐K‐Cl‐cotransporter‐2 (NKCC2) in the thick ascending limb (TAL), increased urinary K clearance in mice on a control diet but decreased that in mice on LNaHK. We hypothesized that there is a net K secretion in the TAL of mice on LNaHK via the renal‐outer‐medullary‐K channel (ROMK) located in the apical membrane of TAL. Wild‐type (WT) and ROMK knockout mice (ROMK KO) were given either a control diet or LNaHK (0.01% Na, 5% K) for 4–7 days. Free‐flow micropuncture was performed using K‐selective microelectrodes to measure the K concentration in the early distal tubule (EDT [K]) before and after intravenous administration of vehicle or furosemide (5mg/kg). Urine was collected via a bladder catheter. Na concentration was measured by a flame photometer. Within 5 minutes of administration, furosemide increased the EDT [K] of WT on a control diet (Δ[K] = 2.33 ± 0.46 mM; n = 6) but decreased that of WT on LNaHK (Δ[K] = −3.07 ± 0.61 mM; n = 11). However, furosemide did not affect the EDT [K] of ROMK KO on LNaHK (Δ[K] = −0.09 ± 0.23 mM; n = 3). Vehicle had no effect on EDT [K] of mice on either diet. The furosemide‐sensitive Na excretion was significantly greater in mice on LNaHK (1298 ± 354 nmol/min; n = 5) than those on a control diet (362 ± 132 nmol/min; n = 5). Furthermore, single‐channel patch clamp analysis was used to evaluate the activity of ROMK in the apical membrane of split‐open TALs from mice on a control diet or LNaHK. The 70‐pS ROMK channel exhibited a higher open probability (Po) in mice on LNaHK than those on a control diet (Po = 0.65 ± 0.11 vs 0.35 ± 0.13, p = 0.07). However, the density of the 70 pS ROMK was not different in LNaHK compared with control diet. No significant difference was found in the number or Po of the 30‐pS K channels between the two groups. These results indicate ROMK‐dependent net K secretion in the TAL of mice on LNaHK, and it is associated with increased NKCC2 and ROMK activities.Support or Funding InformationThis project is supported by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) grants R01‐DK‐092474, R01‐DK‐071014 and F30‐DK‐108456.