Abstract Body: Background: Kallikrein-1 is a serine protease expressed in multiple tissues including the kidney, where it is secreted into the urine. Low urinary kallikrein levels are associated with essential hypertension, whereas high levels are associated with primary hyperaldosteronism in humans. Global kallikrein-1 deletion in mice has been shown to lead to hyperkalemia in response to an acute potassium load as well as defective cleavage of gamma-ENaC, suggesting a direct role in distal K⁺ handling. However, it remains unclear whether effects seen with global kallikrein-1 loss are secondary to loss of extrarenal or renal kallikrein, which is primarily expressed in the connecting tubule.

Methods: To test the hypothesis that renal kallikrein-1 is necessary for increased K⁺ secretion in response to acute dietary K⁺ load, we generated mice with conditional knockout of kallikrein-1 in the distal tubule (Klk1 flox - Calbindin-Cre, KS-Klk1 KO). Mice were treated with a K⁺ deficient diet (0% K⁺, LK) for 9.5 days and then placed in metabolic cages for 12hr overnight collection of urine on control (LK) or high K⁺ (5%, OHK) diets. Metabolic parameters in plasma were measured using iSTAT and kidneys were harvested for downstream western blotting.

Results: Deletion of kallikrein-1 in KS-Klk1 kidney was confirmed by western blotting and urine kallikrein activity assay. On LK diet, both WT and KS-Klk1 KO mice developed marked hypokalemia (WT 2.83+/-0.23 vs KO 2.88+/-0.15 meq/dL). Following OHK challenge, hyperkalemia developed to similar levels in both groups (WT 7.13+/-0.56 vs KO 7.74+/-0.35). However, KS-Klk1 KO mice with HK challenge exhibited a more pronounced drop in serum [Na⁺] compared with WT (WT 142.6+/-1.4 vs KO 136.6+/-1.3, p=0.022) and correspondingly lower serum [Cl^-] (WT 119.0+/-0.62 vs KO 115.9+/-0.96). Gamma-ENaC quantitation by western blot revealed similar ratios of cleaved: uncleaved species between genotypes. No significant changes between genotypes were detected in urine volume or urine Na+/K+ ratios.

Conclusions: KS-Klk1 KO mice do not display defective K+ handling or ENaC processing in response to an acute 12 hr overnight high K⁺ diet. However, they do display relative hyponatremia compared with WT. These data may suggest compensatory mechanisms for potassium transport or a direct a role for renal kallikrein-1 in regulation of renal water transport.