Tissue sodium storage: evidence for kidney-like extrarenal countercurrent systems?

Recent evidence from chemical analysis of tissue electrolyte and water composition has shown that body Na+ content in experimental animals is not constant, does not always readily equilibrate with water, and cannot be exclusively controlled by the renal blood purification process. Instead, large amounts of Na+ are stored in the skin and in skeletal muscle. Quantitative non-invasive detection of Na+ reservoirs with sodium magnetic resonance imaging (23NaMRI) suggests that this mysterious Na+ storage is not only an animal research curiosity but also exists in humans. In clinical studies, tissue Na+ storage is closely associated with essential hypertension. In animal experiments, modulation of reservoir tissue Na+ content leads to predictable blood pressure changes. The available evidence thus suggests that the patho(?)-physiological process of Na+ storage might be of relevance for human health and disease.