87-Rubidium NMR: A novel method of measuring cation flux in intact kidney

The relationship between ion flux and cellular energetics is of fundamental biological importance, most obviously in transporting epithelial cells such as those of the kidney tubule. This is also important clinically because of the relationship between sodium transport, Na-K-ATPase activity and disease [1]. Although non-invasive spectroscopic methods like nuclear magnetic resonance (NMR) are presently not able to resolve intra-renal regions or single cell types, studies of function and biochemistry in the intact kidney are an essential first step in defining the in vivo role of the pathways and mechanisms carefully elaborated by micropuncture and other localized techniques.In perfused kidney the study of Na-K-ATPase activity, oxygen consumption and ion transport has focused on Na+ [2]. Intracellular K+ can be measured in intact tissue by 39K Nuclear Magnetic Resonance (NMR) [3–5]. However, since cytosolic K+ activity is maintained within narrow limits, ion fluxes cannot be measured by this technique without perturbing the steady state or by the addition of potentially toxic paramagnetic shift reagents to the extracellular space.Tracer studies with 86Rb+ have established rubidium as a congener of potassium [6–8] and the energy dependence of 86Rb+ transport has been estimated in intact HeLa cells [6]. We have developed 87Rb+ NMR as a noninvasive method for following net cellular Rb+ fluxes on the minute time scale in the isolated perfused rat kidney. The technique does not require the use of shift reagents or perturbation of existing cellular cation gradients