Stable Isotope Approaches to the Understanding of Potassium Homeostasis

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Project Summary Potassium (K+) homeostasis is critical for normal cardiovascular and neuromuscular function, and disturbances in K+ homeostasis (e.g., hyperkalemia) can lead to life-threatening cardiovascular events. Extracellular K+ homeostasis is maintained by renal and extrarenal mechanisms. The kidneys have a remarkable capacity to regulate K+ excretion to match K+ intake, playing the major role in maintaining chronic K+ balance. In addition, extrarenal tissues (mainly skeletal muscle, the major K+ store) provide K+ buffering capacity by shifting K+ between the extracellular and intracellular fluids. Furthermore, gut sensing of dietary K+ appears to generate signals for regulating renal K+ excretion and extrarenal K+ shift. Thus, extracellular K+ homeostasis involves multiple organs and tissues, their adaptation to dietary K+ intake, and crosstalk among them. Despite marked progress in this field in recent decades, many critical issues concerning K+ homeostatic mechanisms remain unresolved due to a lack of appropriate methodology. A main limitation in studies of extracellular K+ homeostasis is an inability to quantify K+ fluxes in vivo. Although previous studies quantified K+ fluxes using 86Rb, a radioactive tracer for K+, radioactivity exposure and short half-life of 86Rb have limited its use to in vitro studies. Stable isotopes of a wide range of elements have been used to quantify fluxes of various substrates or blood constituents in vivo. However, in spite of the existence of two stable isotopes of K+ in nature (39K, 93.3% and 41K, 6.7%), this approach has not been applied to the study of K+ homeostasis in vivo due to analytical limitations. Dr. John Higgins, a geochemist at Princeton University, developed analytical methods for determining the ratio of stable K+ isotopes (41K/39K) in natural samples using inductively coupled plasma mass spectrometry. This state-of-the-art technology provides the opportunity to determine K+ fluxes in vivo using stable isotopes (i.e., without using radioactive tracers). The objective of the current proposal is to develop and validate this new approach for estimating whole-body K+ fluxes in vivo and extend it to assess K+ transport activities in individual tissues. If successfully developed, these new cutting-edge approaches will open new doors to answering important questions and filling gaps in K+ homeostatic mechanisms in vivo. These stable isotope approaches will be highly innovative, as they allow whole-body and in vitro K+ flux analyses that were infeasible with any conventional approaches, including those with 86Rb.