2000 Global Researcher Conference Proceeding
March 10 - 12, 2000
| Conference: | 2000 Global Researcher Conference |
|---|---|
| Title: | Contribution of chloride channels in urine concentrating ability |
| Authors: | Sasaki, Sei; Uchida, Shinichi; Matsumura, Yoshihiro; Akizuki, Norikazu; Marumo, MD, Fumiaki |
| Institutions: | Tokyo Medical & Dental University, Graduate School, Tokyo Medical and Dental University School of Medicine, Tokyo Medical and Dental University |
CLC-K1 and CLC-K2 are members of the CLC chloride channel family, and selectively express in the kidney. CLC-K1 localizes at the apical and basolateral membrane of thin ascending limb of Henle and CLC-K2 expresses at the basolateral membrane of thick ascending limb of Henle and distal nephron segments. Evidence is accumulating that these two channels play important roles in urine concentrating ability. We recently generated the knockout mice lacking CLC-K1. Absence of CLC-K1 protein and lack of chloride permeability in thin ascending limb of Henle were confirmed by immunohistochemical and electrophysiological studies. Clearance studies showed a 5-fold increase in urine volume and decrease of urine osmolality to 1/3 in the knockout mice in euhydrated condition. Plasma osmolality, and Na, K, and Cl concentrations were not affected. Water deprivation for 24-h caused a 30% body weight reduction with an increase of plasma osmolality to 370 mOsm. Urine osmolality was kept low, less than 1000 mOsm even after administration of vasopressin, demonstrating a phenotype of nephrogenic diabetes insipidus in this animal model. To further examine the underlying mechanisms of this defect, solutes content in the kidney innermedulla was measured. Tissue osmolality and contents of Na, Cl, and urea were about half in the knockout mice compared to those of wild mice. The low tissue osmolality in the medulla did not increase even after water deprivation. Expression and localization of AQP2 water channel were not altered in the knockout mice.
In situ hybridization and immunohistochemical studies showed the expression of CLC-K2 in the medullary thick ascending limb, distal convoluted tubules, connecting tubules, and cortical collecting ducts. It was recently reported that human mutations in this gene cause a hereditary disease, Bartter syndrome. In addition to typical manifestation of hypokalemia and metabolic alkalosis, the defect in urine concentrating ability is known in this disease. Thus, CLC-K2 also contributes to urine concentration. These molecular and genetic data clearly showed that chloride channels play an indispensable role in accumulation of solutes in the kidney medulla and in formation of concentrated urine.
CLC-K1 and CLC-K2 are proteins that allow chloride to travel through cell membranes. They are formed in specific parts of a section of the kidney called the loop of Henle. Sasaki, et al., generated a line of mice lacking CLC-K1. These mice were incapable of concentrating urine and demonstrated symptoms of NDI, even after being injected with vasopressin.
Mutations in the CLC-K2 gene can result in Bartter syndrome. One symptom of Bartter syndrome is an inability to properly concentrate urine. This data indicates that CLC-K1 and CLC-K2 play a vital role in the kidney's ability to concentrate urine.