PS-BPB05-3: SERINE PROTEASE INHIBITION EXERTS AQUARETIC EFFECT AND DECREASES URINARY EXOSOMAL AQP2 LEVELS

Abstract

Objective: Serine proteases (SPs) play pivotal roles in various physiological processes. In the kidney, it is reported that the epithelial sodium channel (ENaC) is activated via the proteolytic cleavage of α and γ subunits by SPs. We previously reported that a synthetic SP inhibitor camostat mesilate (CM) suppressed the cleavage of γENaC, thereby decreasing sodium reabsorption and blood pressure in aldosterone-infused rats. These results suggest the important roles of SPs within the kidney in the regulation of sodium homeostasis and blood pressure. Here, we aimed to identify novel physiological roles of SPs in the renal tubule and understand the mechanism of actions of SP inhibitors by administering CM into healthy rats. Design and method: Six-week-old male Sprague-Dawley rats were divided into control and CM groups. CM group rats were subcutaneously implanted with sustained-release pellets of CM (14 mg/day). After 24 h urine collection was performed, rats were sacrificed at day 7 to obtain blood and kidney samples. Urine volume was measured during the experimental period, and blood and urinary electrolytes, blood vasopressin, and renal tissue osmolality were measured. Protein expression of SPs and aquaporin-2 (AQP2) was analyzed by Western blotting. Results: CM significantly increased urine volume by approximately two-fold throughout the experimental period [urine volume (mL/day): control, 15.5 ± 2.4; CM, 30.6 ± 5.8], in a urinary sodium and osmolyte excretion-independent manner, which indicated that free water excretion had occurred. The vasopressin, potassium, and calcium levels in the blood and osmolality in the inner medulla, which affect free water reabsorption in the renal tubule, remained unchanged after CM administration. CM decreased urinary exosomal AQP2 excretion, suggesting that the AQP2 activity was suppressed in the collecting duct. These changes were reversed by the infusion of desmopressin. Such water diuretic action caused by CM was independent of its action on prostasin and TMPRSS4, which are serine proteases associated with sodium and water homeostasis, respectively. Conclusion: Our results revealed the association of SP inhibition with free water diuresis in the kidney and demonstrated that CM administration resulted in the downregulation of AQP2. The current findings suggest that SP inhibitors could become a new class of aquaretic drugs.