The plasma membrane sodium-potassium pumps that regulate intracellular sodium in most animal cells have specific, high-affinity receptors for the digitalis glycosides and their aglycones. This has fostered speculation that there is an endogenous ligand. We have purified and structurally identified by mass spectroscopy an endogenous substance from human plasma that binds with high affinity to this receptor and that is indistinguishable from the cardenolide ouabain. This human ouabain-like compound (OLC) displaces [3H]ouabain from its receptor, inhibits Na,K-ATPase and ouabain-sensitive 86Rb+ uptake, and has cardiotonic actions quantitatively similar to commercial ouabain. Immunoreactive OLC was detected in the plasma of many mammals, and high concentrations were found in the adrenals. The circulating OLC may modulate intracellular Na+ and affect numerous Na+ gradient-dependent processes including intracellular Ca2+ and pH homeostasis in many tissues. Furthermore, altered circulating levels of OLC may be associated with the pathogenesis of certain forms of hypertension.

https://www.pnas.org/content/pnas/88/14/6259.full.pdf

Identification and characterization of a ouabain-like compound from human plasma

THE SEARCH for natriuretic hormones or factors by studies of negative pressure breathing, atrial distension experiments, head-out water immersion, expansion of blood volume, Na+/K+-ATPase inhibitors and parabiosis experiments in Dahl rats has led to the finding that the atria are a peptide-secreting endocrine gland. This new natriuretic hormone has now been purified, sequenced and synthetized, and its cDNA and gene have been cloned. The native and synthetic hormones exert identical wide ranging effects (possibly through particulate guanylate cyclase stimulation and adenylate cyclase inhibition) on the kidney, blood vessels, adrenal cortex, and pituitary. Physiopathologic implications of the hormone in experimental hypertension, congestive heart failure, and expansion of blood volume are beginning to emerge.

The Heart and the Atrial Natriuretic Factor

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Hypoxic pulmonary vasoconstriction.

The plasma membranes of most if not all vertebrate cells contain a transport system that mediates the transmembrane exchange of sodium for hydrogen. The kinetic properties of this transport system include a 1:1 stoichiometry, affinity for lithium and ammonium ion in addition to sodium and hydrogen, the ability to function in multiple 1:1 exchange modes involving these four cations, sensitivity to inhibition by amiloride and its analogues, and allosteric regulation by intracellular protons. The plasma membrane sodium-hydrogen exchanger plays a physiological role in the regulation of intracellular pH, the control of cell growth and proliferation, stimulus-response coupling in white cells and platelets, the metabolic response to hormones such as insulin and glucocorticoids, the regulation of cell volume, and the transepithelial absorption and secretion of sodium, hydrogen, bicarbonate and chloride ions, and organic anions. Preliminary evidence raises the possibility that the sodium-hydrogen exchanger may play a pathophysiological role in such diverse conditions as renal acid-base disorders, essential hypertension, cancer, and tissue or organ hypertrophy. Thus, future research on cellular acid-base homeostasis in general, and on plasma membrane sodium-hydrogen exchange in particular, will enhance our understanding of a great variety of physiological and pathophysiological processes.

The plasma membrane sodium-hydrogen exchanger and its role in physiological and pathophysiological processes.

Endogenous cardiotonic steroids (CTS), also called digitalis-like factors, have been postulated to play important roles in health and disease for nearly half a century. Recent discoveries, which include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of an alternative mechanism by which CTS can signal through the Na+/K+-ATPase, have increased the interest in this field substantially. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the regulation of cell growth, differentiation, apoptosis, and fibrosis, the modulation of immunity and of carbohydrate metabolism, and the control of various central nervous functions and even behavior. This review focuses on the physiological interactions between CTS and other regulatory systems that may be important in the pathophysiology of essential hypertension, preeclampsia, end-stage renal disease, congestive heart failure, and diabetes mellitus. Based on our increasing understanding of the regulation of CTS as well as the molecular mechanisms of these hormone increases, we also discuss potential therapeutic strategies.

/pdf/endogenous-cardiotonic-steroids-physiology-pharmacology-and-30jc3aa63c.pdf

Endogenous Cardiotonic Steroids: Physiology, Pharmacology, and Novel Therapeutic Targets

The aetiology of essential hypertension, a disease prevalent in cultured societies, is unknown. However, much evidence suggests that abnormal sodium metabolism has a critical role—this has led to the hypothesis that an increase in the circulating concentration of an inhibitor of (Na+ + K+) ATPase is responsible for the increased peripheral vascular resistance in essential hypertension1. Evidence for relatively high levels of a Na+ pump inhibitor in essential hypertension has come from bioassay and cytochemical assays of plasma and urine from normotensive and hypertensive individuals2,3. There is also evidence for increased plasma levels of a Na+ pump inhibitor in some animal models (for example, renal and deoxycorticosterone acetate (DOCA) models) of hypertension4. Nevertheless, direct biochemical determination of (Na+ + K+) ATPase inhibition by this substance has not yet been reported. We demonstrate here, with a kinetic (Na+ + K+) ATPase assay, a highly significant correlation between levels of a plasma inhibitor of (Na+ + K+)ATPase activity and mean arterial blood pressure (MAP) in normotensive and hypertensive individuals. These data provide evidence for the involvement of a circulating Na+ pump inhibitor in the genesis of essential hypertension. Moreover, our assay methods may be useful for the isolation and characterization of this inhibitor.

A circulating inhibitor of (Na+ + K+) ATPase associated with essential hypertension

A Na/Ca exchange system has been demonstrated in systemic vascular smooth muscle and has been suggested as a mechanism for the entry and extrusion of Ca2+. Using isolated rings of bovine intrapulmonary arteries, we sought to determine whether a Na/Ca exchange mechanism is present in pulmonary vascular smooth muscle, and if so, whether it contributes to the modulation of vascular tone during hypoxia. Under both normoxic (PO2 greater than or equal to 120 mm Hg) and hypoxic (PO2 less than or equal to 40 mm Hg) conditions, the amplitude of the 40 mM K+ contraction was significantly (p less than 0.005) enhanced when most of the external Na+ was replaced with N-methylglucamine (NMG) (Na+ = 1.2 mM). The rate of contraction was also increased by replacing Na+ with NMG during normoxia (1.02 +/- 0.24 to 2.52 +/- 0.57 mg/mg weight/s, p less than 0.005) and hypoxia (1.72 +/- 0.74 to 4.08 +/- 1.63 mg/mg weight/s, p less than 0.05). Although the relaxation rate was slowed in the low Na+ media during normoxia (3.48 +/- 1.07 with Na+ versus 2.58 +/- 0.81 mg/mg weight/s with NMG, p less than 0.02), it was unaltered during hypoxia (3.15 +/- 0.90 with Na+ versus 2.95 +/- 0.83 mg/mg weight/s with NMG, p = 0.3). The ratio of relaxation rates in the normal and low Na+ media correlated with the perfusate PO2 (r = 0.76, p less than 0.001). During normoxia and hypoxia, inhibition of the Na+,K+ pump by strophanthidin, which increases intracellular [Na+], reversibly enhanced the amplitude of the K+ contraction, even when the voltage-gated channels were blocked with verapamil.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of the transmembrane sodium gradient on the responses of pulmonary arteries to decreases in oxygen tension

Furosemide-sensitive sodium and potassium cotransport and intracellular sodium content ([Na]i) were measured in erythrocytes (red blood cells, RBCs) from a population of 90 adult black men with and without essential hypertension (EH). The mean values for sodium cotransport activity, expressed as furosemide-sensitive Na efflux (mmol/liter RBC/hr), were not significantly different among the EH patients and two control groups, normotensive subjects with a positive history (N+) and those with a negative family history (N-) for hypertensive disease (EH: 154 +/- 123, n = 53; N+: 167 +/- 93, n = 12; and N-: 207 +/- 142, n = 20; all values are means +/- SD). The mean [Na]i 9.66 +/- 3.02 mmol/liter RBC (n = 56) for the EH group was greater than the mean value for the N- control group (7.96 +/- 1.97, n = 20; p less than 0.05). The N+ group also displayed a higher mean [Na]i (10.38 +/- 3.18, n = 12; N+ vs N- p less than 0.01). Although there was substantial overlapping of [Na]i values between the groups and no clear dividing line, the distribution curve of the [Na]i values in EH was skewed toward higher concentrations than in N-. Nevertheless, we must conclude that erythrocyte cotransport and [Na]i are not clinically useful in the identification of EH in black men.

Juergen Schaeffer

Papers

A circulating inhibitor of (Na+ + K+) ATPase associated with essential hypertension

The influence of the transmembrane sodium gradient on the responses of pulmonary arteries to decreases in oxygen tension

Red cell cotransport activity and sodium content in black men. Relationship to essential hypertension.

TRPA1 Dependent Signaling in Cardiac Dysfunction and Pulmonary Hypertension