A Natriuretic Hormone–Binding Site on the Sodium Pump
Jack H. Kaplan
Department of Biochemistry and Molecular Genetics, University of Illinois at Chicago, Chicago, Illinois
Correspondence: Dr. Jack H. Kaplan, University of Illinois at Chicago, Department of Biochemistry & Molecular Genetics, Molecular Biology Research Building, Room 2072, 900 S Ashland Avenue, Chicago, IL 60607. Phone: 312-355-2732; Fax: 312-355-1765; E-mail: kaplanj{at}uic.edu
The Na+,K+-ATPase, or "sodium pump," is an integral membraneprotein that provides an energetic underpinning to salt andnutrient reabsorption in the nephron, as well as being the centralmodulator of fluid and electrolyte homeostasis in humans.1,2The exclusive basolateral localization of the sodium pump inrenal (and gastrointestinal) epithelia and its functional couplingto apical sodium-dependent transport systems provide the basisfor much of our understanding of renal function. The emergenceof this knowledge has been a major achievement of the reductionistapproach to physiologic function. The recent demonstration ofa physiologic role for the ouabain-binding site in the renalresponse to salt challenge provides an elegant demonstrationof the power of extending knowledge obtained from studies ofthe structure and function of single proteins to the elucidationof renal function in living organisms.
The phrase "ouabain (or digitalis) sensitive" has become a synonymfor sodium pump–mediated processes, which reflects theexquisite selectivity of cardiac glycosides for this protein.Recently, this pharmacologic shorthand has taken on a fascinatingand physiologically significant new dimension. It has been proposedthat the highly conserved sensitivity of the sodium pump tothese naturally occurring plant products mirrors the actionsof endogenous circulating inhibitors of the sodium pump. Theseinhibitors may have an important physiologic role in pump regulation,influencing salt and water homeostasis, and in the regulationof BP. The article in this issue of JASN by Loreaux et al.3provides compelling evidence in support of this hypothesis andpoints to an important regulatory role of an endogenous sodiumpump inhibitor in renal function.
The cloning of the sodium pump proteins in the 1990s led tothe discovery of several isoforms of the two major pump subunitsand the importance of the subunit in ouabain sensitivity.4Subsequent studies provided a rationale for the well-documentedvariation in sensitivity of the pump to cardiac glycosides andthe realization that the relative sensitivity of the renal (andhousekeeping) sodium pump isoform (1) could be rendered resistantby substituting only two specific amino acid residues.5 Couplingthis molecular physiologic strategy with transgenic mouse modelsprovides persuasive evidence that sodium pump molecules engineeredinto mice that differ from the native pumps with respect onlyto their cardiac glycoside sensitivities produce significantand profound physiologic consequences.6–8 The study byLoreaux et al. is an extension of recent work, also using thistype of transgenic mouse model, that provided compelling evidencethat mutation of the ouabain-binding site on the 2 isoform ofthe sodium pump, making it go from ouabain sensitive to ouabainresistant, yields mice that are resistant to ACTH-induced hypertension.9The interpretation of these observations, that the ouabain-bindingsite on the sodium pump is responsive to the ACTH-induced releaseof an endogenous hormone, offers an important link between thesodium pump ouabain-binding site and increased BP.7,10 Alterationsin the ouabain-binding region of the pump that do not affectits active transport function but modulate relevant ouabainsensitivity instead suggest the pump responds physiologicallyto the presence of an endogenous ligand at this receptor site.
In the article by Loreaux et al.,3 the authors modified andreversed the ouabain sensitivity of the mouse 1 and 2 isoformsto examine the role of the 1 ouabain-binding site in the renalresponse to salt challenge. The two major findings are thatmice expressing a more ouabain-sensitive sodium pump respondedto a sodium load with a greater level of natriuresis than didmice expressing the wild-type isoform, and, strikingly, treatmentwith cardiac glycoside-sequestering antibodies equalized thenatriuretic response in these mice. The most plausible explanationfor such observations is that ouabain-binding characteristicsof sodium pumps play a role in the regulation of salt and waterbalance, and the physiologic mechanism occurs through the actionsof a circulating effector of the sodium pump that acts throughthe ouabain-binding site.
More than 20 years ago, de Wardener11 hypothesized the existenceof a natriuretic hormone and its putative involvement in hypertension,and extensive work has been carried out to identify this natriureticfactor.12 The article by Loreaux et al. advances these studiesusing a different approach, by showing that the ouabain sensitivityof the 1 subunit and its manipulation influence renal salt handlingand excretion in a way that is abolished by the presence ofa reagent (the antidigitalis antibody) that removes circulatingdigitalis-like substances. There is a great deal of evidencethat endogenous circulating cardiotonic steroid levels are higherin patients with some forms of hypertension,12–14 andit seems clear their actions are exerted through "receptor"sites on tissue sodium pumps.
The question of how cells respond to occupancy of a fractionof their sodium pumps is central to understanding the basisof many physiologic responses in which pump modulation may playa role. There are several different alternatives; these includecellular responses to changes in sodium concentrations, suchas consequent changes in calcium concentrations that may occurglobally or locally, if a subset of sodium pumps are a partof specialized microdomains in such cells, or they may involvethe postulated actions of the sodium pump as part of a signalingcomplex that responds to ouabain occupancy by activating intracellularsignaling cascades.15 It is likely that all three mechanismsoccur in a variety of physiologic situations.
It is clear today that the sodium pump, the first protein discoveredas an ATP-dependent active ion transporter, whose ion pumpingis central to renal function plays a more complex role in theregulation of salt and electrolyte homeostasis than previouslyrecognized. The modulation of its actions by endogenous inhibitorshas profound effects on cardiovascular and renal function. Identificationof these endogenous modulators will be an exciting next stepin better understanding their role in renal physiology.
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Introduction
DISCLOSURES
subunit in ouabain sensitivity.4 Subsequent studies provided a rationale for the well-documented variation in sensitivity of the pump to cardiac glycosides and the realization that the relative sensitivity of the renal (and housekeeping) sodium pump isoform (
