Endocytosis inhibition protects the isolated guinea pig heart against ouabain toxicity

Ouabain attenuates cardiotoxicity induced by other cardiac steroids

BACKGROUND AND PURPOSE

All cardiac steroids have a similar structure, bind to and inhibit the ubiquitous transmembrane protein Na(+), K(+)-ATPase and increase the force of contraction of heart muscle. However, there are diverse biological responses to different cardiac steroids both at the cellular and at the molecular level. Moreover, we have recently shown that ouabain inhibits digoxin- and bufalin-induced changes in membrane traffic. The present study was designed to test the hypothesis that ouabain also has an inhibitory effect on cardiotoxicity induced by other cardiac steroids.

EXPERIMENTAL APPROACH

The hypothesis was tested in isolated heart muscle preparations and in an in vivo model of cardiotoxicity in guinea pigs.

KEY RESULTS

Ouabain at a low dose attenuated the toxicity induced by bufalin and digoxin in heart muscle preparations. In addition, ouabain at the low dose (91 ng.kg(-1).h(-1)), but not at a higher dose (182 ng.kg(-1).h(-1)), delayed the development of digoxin-induced (500 microg.kg(-1).h(-1)) cardiotoxicity in anaesthetized guinea pigs, as manifested by delayed arrhythmia and terminal ventricular fibrillation, as well as a reduced heart rate. In addition, as observed with ouabain, the phosphoinositide 3-kinase inhibitor wortmannin (100 microg.kg(-1).h(-1)) delayed the digoxin-induced arrhythmia in anaesthetized guinea pigs.

CONCLUSIONS AND IMPLICATIONS

The present study demonstrates the inhibitory effect, probably through signal transduction pathways, of ouabain on digoxin- and bufalin-induced cardiotoxicity in guinea pigs. Further understanding of this phenomenon could be beneficial for increasing the therapeutic window for cardiac steroids in the treatment of chronic heart failure.

Endogenous and exogenous cardiac glycosides: their roles in hypertension, salt metabolism, and cell growth

Cardiotonic steroids (CTS), long used to treat heart failure, are endogenously produced in mammals. Among them are the hydrophilic cardenolide ouabain and the more hydrophobic cardenolide digoxin, as well as the bufadienolides marinobufagenin and telecinobufagin. The physiological effects of endogenous ouabain on blood pressure and cardiac activity are consistent with the "Na(+)-lag" hypothesis. This hypothesis assumes that, in cardiac and arterial myocytes, a CTS-induced local increase of Na(+) concentration due to inhibition of Na(+)/K(+)-ATPase leads to an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) via a backward-running Na(+)/Ca(2+) exchanger. The increase in [Ca(2+)](i) then activates muscle contraction. The Na(+)-lag hypothesis may best explain short-term and inotropic actions of CTS. Yet all data on the CTS-induced alteration of gene expression are consistent with another hypothesis, based on the Na(+)/K(+)-ATPase "signalosome," that describes the interaction of cardiac glycosides with the Na(+) pump as machinery activating various signaling pathways via intramembrane and cytosolic protein-protein interactions. These pathways, which may be activated simultaneously or selectively, elevate [Ca(2+)](i), activate Src and the ERK1/2 kinase pathways, and activate phosphoinositide 3-kinase and protein kinase B (Akt), NF-kappaB, and reactive oxygen species. A recent development indicates that new pharmaceuticals with antihypertensive and anticancer activities may be found among CTS and their derivatives: the antihypertensive rostafuroxin suppresses Na(+) resorption and the Src-epidermal growth factor receptor-ERK pathway in kidney tubule cells. It may be the parent compound of a new principle of antihypertensive therapy. Bufalin and oleandrin or the cardenolide analog UNBS-1450 block tumor cell proliferation and induce apoptosis at low concentrations in tumors with constitutive activation of NF-kappaB.

Endogenous and exogenous cardiac glycosides and their mechanisms of action

Cardiac glycosides have been used for decades to treat congestive heart failure. The recent identification of cardiotonic steroids such as ouabain, digoxin, marinobufagenin, and telocinobufagin in blood plasma, adrenal glands, and hypothalamus of mammals led to exciting new perspectives in the pathology of heart failure and arterial hypertension. Biosynthesis of ouabain and digoxin occurs in adrenal glands and is under the control of angiotensin II, endothelin, and epinephrine released from cells of the midbrain upon stimulation of brain areas sensing cerebrospinal Na(+) concentration and, apparently, the body's K(+) content. Rapid changes of endogenous ouabain upon physical exercise may favor the economy of the heart by a rise of intracellular Ca(2)(+) levels in cardiac and atrial muscle cells. According to the sodium pump lag hypothesis, this may be accomplished by partial inhibition of the sodium pump and Ca(2+) influx via the Na(+)/Ca(2+) exchanger working in reverse mode or via activation of the Na(+)/K(+)-ATPase signalosome complex, generating intracellular calcium oscillations, reactive oxygen species, and gene activation via nuclear factor-kappaB or extracellular signal-regulated kinases 1 and 2. Elevated concentrations of endogenous ouabain and marinobufagenin in the subnanomolar concentration range were found to stimulate proliferation and differentiation of cardiac and smooth muscle cells. They may have a primary role in the development of cardiac dysfunction and failure because (i) offspring of hypertensive patients evidently inherit elevated plasma concentrations of endogenous ouabain; (ii) such elevated concentrations correlate positively with cardiac dysfunction, hypertrophy, and arterial hypertension; (iii) about 40% of Europeans with uncomplicated essential hypertension show increased concentrations of endogenous ouabain associated with reduced heart rate and cardiac hypertrophy; (iv) in patients with advanced arterial hypertension, circulating levels of endogenous ouabain correlate with BP and total peripheral resistance; (v) among patients with idiopathic dilated cardiomyopathy, high circulating levels of endogenous ouabain and marinobufagenin identify those individuals who are predisposed to progressing more rapidly to heart failure, suggesting that endogenous ouabain (and marinobufagenin) may contribute to toxicity upon digoxin therapy. In contrast to endogenous ouabain, endogenous marinobufagenin may act as a natriuretic substance as well. It shows a higher affinity for the ouabain-insensitive alpha(1) isoform of Na(+)/K(+)-ATPase of rat kidney tubular cells and its levels are increased in volume expansion and pre-eclampsia. Digoxin, which is synthesized in adrenal glands, seems to counteract the hypertensinogenic action of ouabain in rats, as do antibodies against ouabain, for example, (Digibind) and rostafuroxin (PST 2238), a selective ouabain antagonist. It lowers BP in ouabain- and adducin-dependent hypertension in rats and is a promising new class of antihypertensive medication in humans.

4-(3‘α15‘β-Dihydroxy-5‘β-estran-17‘β-yl)furan-2-methyl Alcohol: An Anti-Digoxin Agent with a Novel Mechanism of Action

The synthesis and some pharmacological properties of 4-(3'alpha-15'beta-dihydroxy-5beta-estran-17'beta-yl)furan-2-methyl alcohol (16) have been described. The compound was synthesized by reacting a synthetic 3alpha- benzyloxy-5beta-estr-15-en-17-one with the ethylene acetal of 4-bromo-2-furancarboxyaldehyde, followed by hydrolysis of the ethylene acetal and reduction of the aldehyde. Despite its resemblance to the structure of cardiac steroids (CS), 16 does not bind to the CS receptor on Na(+),K(+)-ATPase and does not increase the force of contraction of heart muscle. However, 16 inhibited the digoxin-induced increase in the force of contraction and arrhythmias in guinea pig papillary muscle and human atrial appendages. The steroid also inhibited digoxin-induced alteration in endocytosed membrane traffic, indicating a novel mechanism of action.

Evidence for an intracellular site of action in the heart for two hydrophobic cardiac steroids

Although cardiac steroids (CS) have long been used to treat cardiac insufficiency, the mechanism(s) of action of these agents remain open to question. While many results indicate that inhibition of Na+,K+-ATPase underlies both the therapeutic and toxic actions of CS, other studies suggest that actions on the SR membrane system may be important. We used two experimental approaches and measurements of left ventricular diastolic pressure (LVDP) in isolated guinea pig hearts to test whether CS had an intracellular site of action. In the first approach, we compared the inotropic effects of a hydrophilic CS, ouabain, and a hydrophobic CS, digitoxin, after the activity of the Na+ pump was reduced by perfusing hearts with solutions maintained at 5 degrees C. Under these conditions, exposure of hearts to 1 microM ouabain for 60 min did not increase LVDP above control levels. In contrast, an equi-effective concentration of digitoxin (0.3 microM) increased LVDP by 40 +/- 8.5% (p < 0.01) over pre-drug control levels. In the second experimental approach, we compared the inotropic effects of ouabain and digitoxin in the presence of rapid-cooling contractures (RCC), which result in the release of SR Ca2+. Hearts were perfused with Tyrode solution or Tyrode solution containing either digitoxin (0.3 microM) or ouabain (1 microM) for 180 sec, rapidly cooled and the RCC responses were analyzed. Compared to RCC elicited in Tyrode solution alone, or in Tyrode solution containing ouabain, RCC in the presence of digitoxin reached peak amplitudes more rapidly, but elicited reduced peak amplitude values. Based on these findings, we suggest that: 1) the ability of the hydrophobic CS, digitoxin, but not the hydrophilic CS, ouabain, to produce a positive inotropic effect at 5 degrees C, when the activity of the Na+ pump is markedly reduced, is consistent with a mechanism other than Na+ pump inhibition and involves an intracellular location; and 2) the diminished RCC observed in the presence of the hydrophobic CS, digitoxin, indicate that this alternative mechanism may involve effects on the SR Ca2+ release channel.

Spontaneous cytokine gene expression in normal guinea pig blood and tissues

The authors report, for the first time, the cloning, characterization and sequencing of guinea pig cDNAs for interleukin (IL)-2, IL-10, IL-12p40, and transforming growth factor beta (TGF-beta). Partial cDNAs for two additional cytokines, IL-1alpha and TNF-alpha, whose sequences are present in the GenEMBL database, were also cloned. The IL-10 clone is a full-length cDNA, while the remaining clones are partial cDNAs. The guinea pig cDNA sequences have high identity with their mouse and human counterparts. Northern blot analysis revealed that the guinea pig transcripts range in size from 1.0 kb to 2.2 kb. The constitutive expression of cytokines in two strains of guinea pig (C4D, Albany) that differ in susceptibility to infection with Treponema pallidum was examined. Since susceptibility to T. pallidum is also age dependent, both neonates and adults were examined. Spontaneous cytokine expression was examined in peripheral blood, skin, spleen, lymph node, brain, and peritoneal cells. In skin, lymph node, and peripheral blood, very low levels of IL-1alpha, IL-12p40, tumour necrosis factor alpha (TNF-alpha), and TGF-beta and moderate levels of IL-2 and IL-10 were observed. Cytokine gene expression was not observed in spleen and brain. Peritoneal cells expressed only TGF-beta. Age- and strain-associated differences were not observed, except for IL-12p40, which was elevated in guinea pigs resistant to T. pallidum infection (C4D neonates, Albany adults).