Structure, function and regulation of Na,K-ATPase in the kidney

All zones of mammalian kidney, except the papilla, are abundant sources of Na,K-ATPase. The inner stripe of the red outer medulla of rabbit or pig kidney served as the starting material for the first successful purification of the Na,K-pump or Na,K-ATPase [1]. In this tissue the basolateral cell membranes of the predominant structure, the medullary thick ascending limb of Henle (MAL), are tightly packed with Na,K-pump sites in a concentration exceeding 40 million sites per cell as estimated by [3H]-ouabain binding [2]. The pure preparation has been an ideal object for studying organization of the protein in the membrane [3–5] and for establishing structure-function relationships [6]. The pump protein retains its assymmetric orientation in the membrane. Formation of two-dimensional crystals suitable for image analysis can be induced by incubation in vanadate solution [6–8]. Immunologically the renal Na,K-ATPase is identical to the Na,K-pump in other mammalian tissues, including that in human red cells. Antibodies to the pure renal Na,K-ATPase are therefore available for immunoprecipitation studies of biosynthesis of the pump and for identification of the protein in other mammalian cell systems.Figure 1 illustrates how the Na,K-pump provides the driving force for widely different active transport processes in discrete nephron segments (compare [9]). This diversity of the transport processes does not reside within the pump per se. Despite the apparent heterogeneity, the function of the Na,K-pump remains the same in all nephron segments, that is, to maintain electrochemical gradients for sodium (ΔµNa). They form the driving force for secondary active transport of nutrients such as glucose and aminoacids, of metabolites such as citrate or succinate, of ions like protons, calcium, phosphate or chloride. Na+-gradients that are maintained by the Na,K-pump also provide energy for secretion of organic acids like PAH and penicillin or of the diuretics furosemide and bumethanide and for the concentration and dilution of the urine. Only in a short segment of the nephron, the collecting ducts, is it the primary purpose of the transcellular transport process to control the excretion of the substrate of the pump, Na+.The concentration of Na,K-pump sites and provision of metabolic energy determines the capacity of the transcellular transport, but the nature and direction of solute transport in discrete segments depend on the Na+-coupled carriers and the membrane conductances. Proper control of the integrated transport systems requires precise coordination of Na,K-pump function with that of a number of other structures. In contrast to the detailed information available about the structure and function of the pure renal Na,K-pump, little is known about the structure of the carriers that mediates Δ µNa+-driven secondary active transport of other solutes. An equally important gap is the lack of information about the mechanisms for cellular or hormonal control of the tubular transport processes.The purpose of this article is to discuss recent information about the structure and function of the Na,K-pump proteins and the control of pump functions in kidney tubules. The nature of E1E2 transitions in the α-subunit and their relation to cation binding, occlusion, and translocation in the reaction cycle will be examined to identify rate-limiting steps and points for regulatory control of the pump turnover rate. Cellular and humoral mechanisms for regulation of Na,K-pumping in kidney tubules will be discussed relative to the results of new methods for determining the concentration of [3H]-ouabain binding sites in isolated tubules