HYPERCLO

The molecular mechanisms underlying the pathogenesis of hypertension are still poorly understood. What is known is that excessive dietary salt represents a major risk factor for human hypertension, and it is clear that hypertension results from increased sodium absorption within the distal nephron of the kidney. Therein, only two apical membrane proteins are thought to mediate transepithelial Na+ transport: the thiazide-sensitive electroneutral NaCl cotransporter (NCC) expressed all along the distal convoluted tubule, and the amiloride-sensitive epithelial Na+ channel (ENaC) in principal cells of the connecting tubule and collecting duct. Hyperactivity of these proteins promotes hypertension, whereas hydrochlorothiazide (HCTZ), the most widely used therapy against hypertension, is thought to reduce blood pressure by inhibiting NCC. In the meantime, most studies have neglected the impact of chloride, the accompanying anion. Nevertheless, a number of literature reports suggest that chloride plays a significant role in the "pro-hypertensive" effects of sodium. In particular, recent evidence indicates that chloride transport through the intercalated cells, a specific epithelial cell type in the distal nephron, has an important modulating effect on extracellular volume, and hence, on blood pressure. Remarkably, in the connecting tubule and the collecting duct, the transport of chloride through the apical membrane is not strictly coupled to that of Na+, as in the distal convoluted tubule, but is achieved by anion exchangers. We recently identified a novel mechanism of NaCl absorption in the intercalated cells, which couples two chloride/bicarbonate exchangers, one sodium-dependent and one sodium-independent. Importantly, this transport system is also inhibited by thiazide. The basolateral exit of chloride occurs through ClC-K channels, the disruption of which leads to hypotension in humans (Bartter syndrome); conversely, a common polymorphism in ClC-Kb has been linked to a predisposition to hypertension, again pointing to the potential importance of chloride transport systems in the genesis of hypertension. Therefore, we propose to unravel the role of chloride transport by intercalated cells for blood pressure regulation in general, and its involvement in the predisposition of individuals to salt-sensitive hypertension. This project combines several experimental approaches including: generation of transgenic mouse models, in vivo physiology using metabolic cages, in vitro microperfusion of nephron segments, molecular and cell biology, and patch-clamp for studying chloride channels. It also includes the development of a mathematical model that is crucial for our understanding of the complex set of ion transport systems contributing to sodium chloride absorption within the collecting duct.