Effects of the Fab fragment of digoxin antibody on the natriuresis and increase in blood pressure induced by intracerebroventricular infusion of hypertonic saline solution in rats

Preeclampsia: Cardiotonic Steroids, Fibrosis, Fli1 and Hint to Carcinogenesis

Despite prophylaxis and attempts to select a therapy, the frequency of preeclampsia does not decrease and it still takes the leading position in the structure of maternal mortality and morbidity worldwide. In this review, we present a new theory of the etiology and pathogenesis of preeclampsia that is based on the interaction of Na/K-ATPase and its endogenous ligands including marinobufagenin. The signaling pathway of marinobufagenin involves an inhibition of transcriptional factor Fli1, a negative regulator of collagen synthesis, followed by the deposition of collagen in the vascular tissues and altered vascular functions. Moreover, in vitro and in vivo neutralization of marinobufagenin is associated with the restoration of Fli1. The inverse relationship between marinobufagenin and Fli1 opens new possibilities in the treatment of cancer; as Fli1 is a proto-oncogene, a hypothesis on the suppression of Fli1 by cardiotonic steroids as a potential anti-tumor therapeutic strategy is discussed as well. We propose a novel therapy of preeclampsia that is based on immunoneutralization of the marinobufagenin by monoclonal antibodies, which is capable of impairing marinobufagenin-Na/K-ATPase interactions.

On the Many Actions of Ouabain: Pro-Cystogenic Effects in Autosomal Dominant Polycystic Kidney Disease

Ouabain and other cardenolides are steroidal compounds originally discovered in plants. Cardenolides were first used as poisons, but after finding their beneficial cardiotonic effects, they were rapidly included in the medical pharmacopeia. The use of cardenolides to treat congestive heart failure remained empirical for centuries and only relatively recently, their mechanisms of action became better understood. A breakthrough came with the discovery that ouabain and other cardenolides exist as endogenous compounds that circulate in the bloodstream of mammals. This elevated these compounds to the category of hormones and opened new lines of investigation directed to further study their biological role. Another important discovery was the finding that the effect of ouabain was mediated not only by inhibition of the activity of the Na,K-ATPase (NKA), but by the unexpected role of NKA as a receptor and a signal transducer, which activates a complex cascade of intracellular second messengers in the cell. This broadened the interest for ouabain and showed that it exerts actions that go beyond its cardiotonic effect. It is now clear that ouabain regulates multiple cell functions, including cell proliferation and hypertrophy, apoptosis, cell adhesion, cell migration, and cell metabolism in a cell and tissue type specific manner. This review article focuses on the cardenolide ouabain and discusses its various in vitro and in vivo effects, its role as an endogenous compound, its mechanisms of action, and its potential use as a therapeutic agent; placing especial emphasis on our findings of ouabain as a pro-cystogenic agent in autosomal dominant polycystic kidney disease (ADPKD).

Relations of vascular calcium channels with blood pressure and endothelium in hypertension and with aging

To investigate the relationships between the activity in potential operated Ca2+ channels (POC), blood pressure, and endothelium in hypertension, we tested the contractile responses to a Ca2+ channel agonist Bay K 8644 (BAY K) in aorta from deoxycorticosterone-acetate-saline (DOCA-S) and reduced renal mass-saline (RRM-S) hypertensive rats. The effects of mechanical rubbing, N omega-Nitro-L-Arginine Methyl Ester (l-NAME) and indomethacin were also examined. Sensitivity to BAY K increased in experimental rats before they became hypertensive and contractile responses were enhanced as hypertension developed. Force development to BAY K was correlated with blood pressure levels. Endothelium removal enhanced the contractile response to BAY K. L-NAME, but not indomethacin, potentiated the response to BAY K. Contractile response to BAY K was negatively correlated with relaxation to acetylcholine. An enhanced contractile response to BAY K was observed also in aged rats. Enhanced activation of vascular POC in hypertension results from elevated blood pressure and partly from diminished inhibitory action of endothelium. Senescence also enhances vascular POC activity.

The endogenous digitalis-like factor

Intensive research into the presence of endogenous digitalis-like factor (EDLF) started shortly after identification of the alpha subunit of the Na,K,-ATPase as being receptor for digitalis glycosides. After years of skepticism, present data testify EDLF really exists. Most probably, the EDLF has chemical structure of either ouabain or of one of its isomers. It is secreted by the adrenal cortex, and, under conditions of stress, it's secretion is regulated differently from the secretion of both gluco- and mineralocorticoids. The physiological role of the EDLF has not been fully understood yet. In the newborn's kidneys, the inhibition of the Na,K-ATPase may assist to increase elimination of surplus sodium from the organism. In individuals of any age, the inhibitory influence of EDLF upon Na,K-ATPase in the arterial wall smooth muscle cells increases peripheral vascular resistance and thus, blood pressure. In the tissue culture, direct positive inotropic influences of EDLF upon rat cardiomyocytes was observed. However, the importance of positive inotropic effect of the EDLF upon the heart in clinical medicine remains to be elucidated.

Specificity of the volume-sensitive sodium pump inhibitor isolated from human peritoneal dialysate in chronic renal failure

Compromised renal function predisposes to volume-dependent hypertension. Increased plasma levels of a sodium pump inhibitor as a possible pathogenetic factor have been demonstrated by many investigators in such patients, but efforts to identify the responsible agent have led to many, diverse candidates. Our premise in this study is that candidacy must depend on the satisfaction of rigorous criteria, including a specific action of the agent on the sodium pump. These criteria included reversibility, concentration dependence, receptor mediation, and an action at the appropriate step in the enzyme cycle. These criteria were applied to a potent [Na,K]ATPase inhibitor we have identified in the peritoneal dialysate of patients with chronic renal failure, present only during extracellular fluid volume expansion, the levels of which are correlated with the blood pressure rise that results from excessive NaCl and water intake. In microsomes that contained both [Na,K]ATPase and other ATPases, this candidate inhibited only the Na and K dependent, ouabain-sensitive ATPase. It displaced ouabain from the cardioglycoside binding site and its binding was linked to inhibition. Inhibition was produced by slowing the pump's dephosphorylation step, the exact action of all cardioglycosides. Finally, the candidate cross reacted with a digoxin Fab fragment and this Fab reversed its inhibition of [Na,K]ATPase. Together, these experiments demonstrate that the PD candidate specifically, and reversibly, inhibits the sodium pump via the cardioglycoside binding site, and hence, meets this crucial criterion for candidacy.

Modulation of vascular calcium channel activity in response to acute volume expansion in rats

The mechanisms of the increased resistance in hypertension are still unclear. Several studies have indicated that the potential-sensitive Ca2+ channels (PSC) are altered in arteries isolated from hypertensive patients or animals. An expansion of body fluid volume may trigger local autoregulatory responses or may induce the release of humoral factors, either of which could increase systemic vascular resistance and cause volume-dependent forms of hypertension. We tested the hypothesis that volume expansion per se may cause the alterations of PSC similar to those seen in hypertension. For this, we examined the alterations of PSC in aortas from volume-expanded rats with the use of dihydropyridine-type Ca2+ channel activator, BayK 8644, in parallel with the changes in endothelium-dependent relaxation. Volume expansion was produced by a rapid intravenous infusion of saline (10% of body weight) over 30 min in rats. At the end of infusion, rats were killed and aorta removed for in vitro measurement of isometric tension. Relaxation to acetylcholine (10(-7)-10(-5) mol/L, % relaxation to 10(-7) mmol/L norepinephrine contraction) was not significantly changed. In contrast, contractile response to BayK 8644 (10(-9)-10(-6) mol/L, % response to 50 mmol/L KCl) was significantly enhanced in rats with volume expansion (12 control rats: 11.6 +/- 4.9%; 18 volume-expanded rats: 40.9 +/- 10.4% at 10(-6) mol/L, p < 0.05). These findings suggest that acute volume expansion could induce a similar enhanced vascular Ca2+ channel activity to that seen in hypertension in rats.

Role of brain ouabainlike compound in central nervous system-mediated natriuresis in rats

Intracerebroventricular infusion of artificial sodium-rich cerebrospinal fluid induces increases in blood pressure and urinary sodium excretion. To examine the role of brain ouabainlike compound in these central nervous system-mediated responses, we evaluated the effects of prior intracerebroventricular injection of the Fab fragments of digoxin-specific antibody (Digibind, 10 mg/mL, 10 microL) on changes in blood pressure and urinary sodium excretion after intracerebroventricular infusion of high-sodium (323 mmol/L, 150 microL/kg per 15 minutes) cerebrospinal fluid in anesthetized rats. Antiouabain action of Digibind was revealed by the inhibition of a contractile response to ouabain in guinea pig aorta. Similar significant increases in blood pressure were found in rats that received preinjection of Digibind (n = 14) compared with control rats that received injection of saline (n = 5) or normal sheep IgG (n = 8). In rats pretreated with Digibind the natriuretic responses to central high sodium were significantly diminished by 68% (P < .05) or 82% (P < .05) compared with rats treated with saline or normal IgG, respectively. In contrast, Digibind did not affect either pressor or natriuretic responses to intracerebroventricular angiotensin II (600 ng/30 microL per 10 minutes). These data indicate that Digibind significantly inhibits increases in renal sodium excretion in response to high central sodium and suggest that brain ouabainlike compound may be involved in central nervous system-mediated natriuresis with nonpressor mechanisms.