The pump, the exchanger, and the Holy Spirit: tracing the 40-year evolution of the Ouabain-Na+ pump endocrine system concept

Abstract

The discovery and subsequent evolution of the Ouabain-Na⁺/K⁺ pump endocrine system have profoundly impacted our understanding of cellular physiology and disease mechanisms. Initially identified as a cardiotonic steroid with potent effects on the Na⁺/K⁺ ATPase, Ouabain has been implicated in various physiological and pathological processes. The Na⁺/K⁺ pump, a crucial component of cellular physiology, maintains electrochemical gradients essential for nerve impulse transmission, muscle contraction, and cellular volume regulation. Since Jens Christian Skou's Nobel Prize-winning discovery in 1957, research has unveiled its broader role in cellular homeostasis and disease. A significant breakthrough was the identification of Ouabain as an endogenous ligand of the Na⁺/K⁺ pump, transforming the pump's role from a mere ion transporter to a receptor within a hormonal signaling pathway. This discovery has linked the Na⁺/K⁺ pump to complex intracellular signaling pathways, with implications for hypertension, heart failure, and cancer. However, emerging evidence suggests that its role extends beyond cardiovascular dysfunction to neurological disorders such as epilepsy, Alzheimer's disease, and Parkinson's disease. In epilepsy, dysregulation of the Na⁺/K⁺ pump contributes to altered ion homeostasis and hyperexcitability. At the same time, in Alzheimer's disease, its dysfunction has been associated with disrupted calcium signaling, oxidative stress, and amyloid-beta accumulation. Similarly, alterations in Na⁺/K⁺ pump activity have been linked to dopaminergic neuron vulnerability in Parkinson's disease. This paradigm shift offers exciting therapeutic possibilities for neurodegenerative and neuropsychiatric disorders, including schizophrenia and depression, redefining the pump's significance across multiple disciplines of medicine.

Keywords: Na+/K+ pump; Ouabain; endocrine system; hypertension; intracellular signaling.

Figure 1.

Early conceptual model of Na⁺-driven Ca²⁺ regulation and vascular tone in arterial smooth muscle (1977). Block diagram (1977 version) illustrating the proposed model of Ca²⁺ regulation in arterial smooth muscle cells. It shows how Na⁺ pumps, Na⁺/Ca²⁺ exchangers (NCX), and an endogenous Ouabain-like compound (OLC) may regulate intracellular Ca²⁺ and vascular tone. Key elements include the Ouabain-sensitive Na⁺ pump, NCX (then thought to mediate Ca²⁺ extrusion), and the SR Ca²⁺ pump (SERCA). Na⁺ and Ca²⁺ primarily enter via voltage-gated channels (k1, k2), and Ca²⁺ is released from the SR via ryanodine or IP₃ receptors (k3). This model predates the identification of Na⁺ pump isoforms and PLasmERosomes. Source: Blaustein^[1].

Figure 2.

Structural dynamics and conformational transitions during the sodium pump catalytic cycle. Illustration of the conformational transitions during the Na⁺/K⁺-ATPase catalytic cycle. The diagram features three experimentally determined structures and one homology model, each aligned with the corresponding steps of the pump cycle, depicted below through both schematic cartoons and a reaction sequence. Eight labeled insets (a–h) emphasize key structural features at various stages. The homology model of the pig α1 subunit, based on the SERCA E1-ATP state (PDB ID: 4H1W), demonstrates an inward-facing conformation granting access to the ion-binding sites, highlighted by residues D804 and E327 (a). At this stage, the ATP analog AMPPCP binds with only the β- and γ-phosphates resolved, indicating a non-reactive orientation relative to D369 (b). Upon sodium binding, TM1 undergoes conformational rearrangement that obstructs the cytoplasmic entry path (c), while the cytoplasmic domains close in on the nucleotide, enabling interaction with D369 (d). Subsequent occlusion of sodium and ADP release facilitates the opening of an extracellular pathway for sodium ion exit. The outward-facing conformation, modeled after the Ouabain-bound structure (PDB ID: 4HYT, shown here without Ouabain), reveals three ion-coordinating residues accessible from the extracellular space €, with complete encasement of phosphorylated D369 by the cytoplasmic domains (f). Binding of two potassium ions at the extracellular side (g) promotes gate closure and initiates dephosphorylation of D369 (h). A narrow cytoplasmic pathway illustrated in the cartoon representation reflects a proposed C-terminal proton channel responsible for maintaining charge balance. Structural color scheme corresponds to Fig. 8. Source: Adapted from Clausen MV, Hilbers F, Poulsen H. Front Physiol. 2017;8:371. Doi:10.3389/fphys.2017.00371.

Figure 3.

Mechanistic pathways of Ouabain-Na⁺/K⁺ pump interaction and signal transduction. Here’s a schematic diagram illustrating the mechanistic pathways of Ouabain’s modulation of the Na⁺/K⁺ pump. The diagram shows how Ouabain binding to the Na⁺/K⁺ pump leads to its inhibition, resulting in increased intracellular sodium (Na⁺) and calcium (Ca²⁺) levels, which subsequently activate reactive oxygen species (ROS) production and various intracellular signaling pathways like MAPKs and PI3K. Source: Authors’ creations.

Figure 4.

Structural architecture of the sodium pump bound to digoxin. structural model of the Na⁺/K⁺-ATPase (α1 isoform) with digoxin bound, highlighting key subunits, domains, and ligands including glycosylations, cholesterol, and phosphorylated D369. Adapted from Clausen et al. 2017 (PDB: 4RET).

Figure 5.

Homology models of human Na⁺/K⁺-ATPase alpha isoforms highlighting structural variations. Homology models representing the four human Na⁺/K⁺-ATPase alpha isoforms, constructed based on the potassium-occluded crystal structure 3 KDP. Isoform-specific amino acid variations are highlighted as spheres. To focus on meaningful structural differences, conservative substitutions were excluded – amino acids were considered functionally equivalent within the following groups: (L, I, V), (E, D), (K, R), (Q, N), (S, T), (Y, F), and (M, C). Residues such as H, G, P, A, and W were treated individually and not grouped. Adapted from Clausen MV, Hilbers F, Poulsen H. Front Physiol. 2017;8:371. Doi:10.3389/fphys.2017.00371.

Figure 6.

Historical trends in Ouabain-Na⁺ pump research. This graph can visually represent the evolution of research findings, highlighting fundamental discoveries and shifts in understanding related to the Ouabain-Na⁺ pump concept. Source: authors’ creations.

Figure 7.

Structural comparison of cardiotonic steroids involved in Na⁺/K⁺ pump modulation. Structures of Ouabain, digoxin, marinobufagenin, and rostafuroxin are shown. The sugar components of Ouabain and digoxin are labeled as “Rs” – source: Blaustein MP. The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the Ouabain-Na⁺ pump endocrine system. Am J Physiol Cell Physiol. 2018 Jan 1;314(1):C3-C26. Doi: 10.1152/ajpcell.00196.2017.

Figure 8.

Strong positive correlation between plasma Na⁺/K⁺-ATPase inhibition and mean arterial pressure in normotensive and hypertensive individuals. Mean arterial pressure (MAP) strongly correlates with plasma Na⁺/K⁺-ATPase (NKA) inhibitory activity. The data include 20 normotensive and 26 hypertensive individuals, with samples collected as single draws (green) or integrated over 6 hours (red and blue). A significant linear relationship was observed (r = 0.73, P < 0.0005), suggesting a link between elevated MAP and increased NKA inhibition. Source: Blaustein^[1].

Figure 9.

Regulation of heart and vascular function via sympathetic and neurohumoral pathways in hypertension and heart failure. This diagram outlines how centrally controlled sympathetic and neurohumoral pathways regulate heart and vessel function, contributing to hypertension and heart failure. Angiotensin II and high salt levels activate hypothalamic AT1 receptors, increasing sympathetic output. This triggers vasoconstriction and stronger cardiac contraction. A separate hypothalamic pathway involving aldosterone, ENaC, endogenous Ouabain, and α2 Na⁺ pumps amplifies sympathetic activity and EO release. EO inhibits α2 pumps, raising intracellular Na⁺ and Ca²⁺, enhancing vascular and cardiac tone. Chronically elevated EO also activates kinase pathways that increase NCX and SERCA2 expression, further promoting calcium signaling. In the heart, impaired Na⁺ gradients and altered SERCA2 function disrupt relaxation and contraction, contributing to heart failure. Source: Blaustein MP. The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the Ouabain-Na⁺ pump endocrine system. Am J Physiol Cell Physiol. 2018 Jan 1;314(1):C3-C26. Doi: 10.1152/ajpcell.00196.2017.

Figure 10.

Relative activity levels of endocrine glands in endocrine regulation. Here is a bar graph illustrating the activity levels of various endocrine glands as part of the overall endocrine regulation. Each gland plays a crucial role in maintaining physiological balance, with varying degrees of activity. This graph serves as a conceptual representation of their contribution to endocrine regulation. Source: Authors’ creations.

Figure 11.

Clinical outcomes of Na⁺/K⁺ pump modulation and Ouabain treatment: effectiveness, survival rates, and dose-response relationships. A. Effectiveness over time: a line graph comparing the effectiveness of Ouabain treatment vs. a control group. B. Survival rate over time: a line graph showing survival rates in patients treated with Ouabain compared to a control group. C. Ouabain dose vs. clinical outcome: a scatter plot showing the relationship between Ouabain dose and clinical outcome improvement. Source: Authors’ creations.

Figure 12.

Visualization of ongoing research areas and proposed models for future studies.