Toxic Effects Of Cardiac Glycosides Research Articles

New ways to acquire resistance: imperfect convergence in insect adaptations to a potent plant toxin.

Evolution of insensitivity to the toxic effects of cardiac glycosides has become a model in the study of convergent evolution, as five taxonomic orders of insects use the same few similar amino acid substitutions in the otherwise highly conserved Na,K-ATPase α. We show here that insensitivity in pyrgomorphid grasshoppers evolved along a slightly divergent path. As in other lineages, duplication of the Na,K-ATPase α gene paved the way for subfunctionalization: one copy maintains the ancestral, sensitive state, while the other copy is resistant. Nonetheless, in contrast with all other investigated insects, the grasshoppers' resistant copy shows length variation by two amino acids in the first extracellular loop, the main part of the cardiac glycoside-binding pocket. RT-qPCR analyses confirmed that this copy is predominantly expressed in tissues exposed to the toxins, while the ancestral copy predominates in the nervous tissue. Functional tests with genetically engineered Drosophila Na,K-ATPases bearing the first extracellular loop of the pyrgomorphid genes showed the derived form to be highly resistant, while the ancestral state is sensitive. Thus, we report convergence in gene duplication and in the gene targets for toxin insensitivity; however, the means to the phenotypic end have been novel in pyrgomorphid grasshoppers.

Cardiotoxicity of Plants in Iran: a Review

In this review an attempt has been made to review the cardiotoxic effects of poisonous plants which are found in Iran. Among various plant species growing in different regions of Iran, a large number of plants contain chemical compounds which are toxic to animals and humans. Among those poisonous plants, cardiotoxic plants are important due to the acute nature of their toxicity and frequent lethal livestock and human intoxications. Cardiotoxic plants of Apocynaceae family, Nerium oleander, N. indicum and Thevetia peruviana contain cardiac glycosides including oleandrin, oleandroside and thevetin A-C. Plants of Taxaceae family, Taxus baccata and T. brevifolia contain taxine alkaloids, including taxine A and B. The toxic effects of cardiac glycosides are primarily attributed to inhibition of plasmalemmal Na+/ K+-ATPase which results into the accumulation of intracellular Ca2+ and, depending to its severity, inotropic or arrhythmic effects are seen. Taxine B, the prominent alkaloid in Taxus spp. block sodium and calcium channels preferentially in cardiac myocytes, thus causing conduction abnormalities. Various cardiac arrhythmias in acute cases of poisoning with aforementioned plants result in acute heart failure and death. Post mortem findings are non-diagnostic and toxicological analysis of gastrointestinal tract content or body fluids is used for detection of cardiac glycosides and taxines.

Ouabain increases sarcoplasmic reticulum calcium release in cardiac myocytes.

The inotropic and toxic effects of cardiac glycosides are thought to be related to their ability to inhibit the Na,K-ATPase. We examined the effects of ouabain and its analogs on sarcoplasmic reticulum (SR) Ca(2+) release in intact cat ventricular myocytes under Na(+)-free conditions and in myocytes in which the sarcolemma was permeabilized using saponin so that cytoplasmic ionic composition was fixed by the bath solutions. We also compared ouabain actions in cat myocytes to those in rat myocytes because the latter is considered to be a glycoside-insensitive species. In intact cat myocytes (Na(+)-free conditions), spontaneous Ca(2+) sparks were prolonged and frequency, amplitude and width were reduced by exposure to ouabain (3 microM). Nearly identical results were obtained with its analogs dihydroouabain or ouabagenin (10 microM). The frequency of spontaneous Ca(2+) waves was also reduced by ouabain. In contrast, ouabain (100 microM) had negligible effects on sparks and waves in rat myocytes in Na(+)-free conditions, consistent with the decreased sensitivity to cardiac glycosides observed in this species. In cat myocytes permeabilized with saponin (0.01%), ouabain (>or=50 nM) decreased spark frequency and increased background SR Ca(2+) leak only when the SR was well loaded (free [Ca(2+)] = 275 nM) and not when SR load was low (free [Ca(2+)] = 50 nM). Similar effects were observed in rat myocytes only when ouabain concentration was 1 microM. These results suggest that the cellular actions of cardiac glycosides may include a direct effect on SR Ca(2+) release, possibly through activation of SR Ca(2+) release channels (ryanodine receptors). In addition, these results are consistent with the idea that direct activation of SR Ca(2+) release is dependent on the extent of SR Ca(2+) load, with elevated load increasing sensitivity of the channel release mechanism to activation by glycoside.

Postnatal expression and induction by pregnenolone-16alpha-carbonitrile of the organic anion-transporting polypeptide 2 in rat liver.

Newborn rats are more sensitive to the toxic effects of cardiac glycosides than are adult rats. This is associated with a decreased ability to remove cardiac glycosides from blood into the liver. Pregnenolone-16alpha-carbonitrile (PCN), a prototypical rodent CYP3A inducer and pregnane-X-receptor (PXR) ligand, stimulates the hepatic clearance of cardiac glycosides in newborn rats, which results in decreased toxicity of the cardiac glycosides. The mechanism responsible for this phenomenon is not clear; however, if elucidated, it would help in understanding and preventing potential drug-drug interactions. The recently cloned rat organic anion-transporting polypeptide 2 (oatp2) (Slc21a5) is a sinusoidal hepatic uptake transporter, with very high affinities for cardiac glycosides, and thus it was hypothesized that rat oatp2 increases during postnatal development and is inducible by PCN. In the present study, livers were removed from Sprague-Dawley rats from postnatal days (pnd) 0 to 45, in 5-day increments; as well as from pnd 10 to 90, in 10-day increments, after PCN (75 mg/kg i.p., for 4 days) or corn oil (vehicle for PCN) treatment. The protein and mRNA levels of rat oatp2 were determined by Western blot analysis and branched DNA signal amplification technique, respectively. Expression of rat oatp2 protein and mRNA increased gradually during postnatal development. PCN treatment increased liver to body weight ratio in both genders, and dramatically accelerated the maturation of hepatic oatp2 protein and mRNA levels. In summary, rat oatp2 undergoes age-dependent and chemical regulation during postnatal development, and is a potential target for drug-drug and age-drug interactions.

On the mechanism and possible therapeutic application of delayed adaptation of the heart to stress situations.

Mild (not harmful) stress may initiate an adaptive mechanism, protecting the heart from harmful consequences of a more severe stress. There are at least three known types of cardiac adaptation to stress, such as: a) the gradually developing, long lasting adaptation to chronic mechanical overload, leading to cardiac hypertrophy, later to cardiomyopathy and heart failure, b) the rapidly developing adaptation to moderate stress initiated by 'preconditioning' brief coronary occlusion(s) or brief periods of rapid cardiac pacing, protecting for less than 1 h against consequences of a subsequent, severe stress, c) the later appearing, more prolonged cardio-protective adaptation, described by us in 1983, induced by various forms of more severe but not injurious stimuli, such as an optimal dose of prostacyclin or its stable analogues; or a series of brief periods of rapid pacings. This form of cardiac adaptation to stress protects for 24-48 h against consequences of a more severe stress such as: 1. myocardial ischaemia; 2. early and late postocclusion and reperfusion arrhythmias; 3. early morphologic changes secondary to ischaemia and reperfusion; 4. ischaemia induced myocardial loss of K+ and accumulation of Na+ and Ca++; 5. it may increase the tolerance to the toxic effects of cardiac glycosides. A reduced response to beta-adrenergic stimuli and a concomitant increase in activity and amount of PDE I and IV was shown by us earlier.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of therapeutic and toxic effects of cardiac glycosides by electromagnetic radiations, alternating magnetic and electric fields

A model of bioassay of the toxic and therapeutic effects of cardiac glycosides (strophanthin K) was used in experiments within situ isolated hearts of laboratory frogs. This model helped reveal effective parameters of modulating the effects of electromagnetic radiations and of alternating magnetic and electric fields.

Role of Thyroid State in Age-Dependent Cardiac Effects of Ouabain in Guinea Pigs

We tested the hypothesis that the decrease in the thyroid state, with age, contributes to the age-related increase in myocardial responsiveness to cardiac glycosides. Thyroid hormone levels (reflecting the thyroid state): total T4 (microgram/dl) and total T3 (ng/dl) in the 3 groups of guinea pigs were (mean +/- SEM): adults (3 months old): less than 1.0 and 22.6 +/- 1.1; euthyroid newborns (0-5 days old): 3.9 +/- 0.4 and 56.5 +/- 11.9; hypothyroid newborns, (0-5 days old): 1.5 +/- 0.3 and 26.5 +/- 9.8. In euthyroid newborns, T4 and T3 levels were significantly higher than in adults (p less than 0.01 for T4 and p less than 0.05 for T3) and in hypothyroid newborns (p less than 0.05). Isometric twitch was recorded from right ventricular papillary muscles by means of a force transducer. Ouabain 10(-6) M increased isometric twitch tension in adults (tension = 0.66 +/- 0.18 g/mm2) by 123.6 +/- 18.2%, in euthyroid newborns (tension = 0.19 +/- 0.04 g/mm2) by 83.6 +/- 14.5%, and in hypothyroid newborns (tension = 0.12 +/- 0.01 g/mm2) by 170.9 +/- 33.8% (p less than 0.01). Ouabain dose-response curve in the range of 10(-7) M-0.5 x 10(-5) M was significantly different (compared by two-way ANOVA) between euthyroid newborns and hypothyroid newborns (p less than 0.01), and between euthyroid newborns and adults (p less than 0.01). Toxic effects of ouabain reflected by the generation of aftercontractions were also age related and were augmented by hypothyroidism in newborns.(ABSTRACT TRUNCATED AT 250 WORDS)

Ouabain injected into the hypothalamus elicits pressor responses in anaesthetized rats: A mapping study

Cardiac glycosides are known to have a narrow therapeutic index, due in part to their effects on the brain. Injections of cardiac glycosides into the ventricles of the brain elicit activation of the autonomic nervous system, and may even elicit cardiac arrhythmias. However, the specific brain regions responsible for such action are unknown. The hypothalamus receives chemo- and baro-receptive innervation from the cardiovascular system. In turn, there are both direct and indirect effector pathways from the hypothalamus accessing the sympathetic preganglionic and parasympathetic ganglia regions. This suggests that the hypothalamus may be a prime candidate for the central toxic effects of cardiac glycosides. The purpose of this experiment was to map the rostral diencephalon to determine sites at which injections of a low dose of the cardiac glycoside, ouabain, resulted in altered cardiovascular responses in the anaesthetized rat. Microinjections of 20 ng of ouabain in 200 nl were made into sites throughout the rostral diencephalon of urethane (1.2 g/kg) anaesthetized rats while monitoring heart rate and blood pressure. Injections into the nucleus medianus, paraventricular, anterior and posterior hypothalamic nuclei produced increases in pressure of from 5 to 25 mmHg. These data suggest that part of the toxicity resulting from the cardiac glycoside administration may be due to the direct action of the glycosides on these hypothalamic structures. The paraventricular region has the greatest sensitivity and may be a primary target due to its direct connections with the preganglionic sympathetic regions in the spinal cord.

The Role of Intracellular Ca Ions in the Therapeutic and Toxic Effects of Cardiac Glycosides and Catecholamines

Many inotropic maneuvers act by increasing the intracellular calcium concentration [( Ca2+]i). The present report illustrates this with respect to the positive inotropic effects of cardiac glycosides and catecholamines. It is shown that the increased contractility produced by cardiac glycosides is accompanied by an increase in intracellular Na concentration and, furthermore, that the relationship between contraction and Na is very steep. This steep dependence, which may result from a Na-Ca exchange which exchanges several Na ions per Ca, means that maneuvers that have only small effects on Na will have significant effects on contraction. Cardiac glycosides also produce abnormal pacemaker activity and cardiac arrhythmias. These originate from a transient inward current activated by oscillations of [Ca2+]i, which result from spontaneous oscillatory release of Ca ions from the sarcoplasmic reticulum. The local anesthetic group of antiarrhythmic agents abolishes the transient inward current. Catecholamines also increase systolic [Ca2+]i and, in high enough concentrations, can produce oscillations of [Ca2+]i. This tendency of glycosides and catecholamines to produce arrhythmogenic oscillations of [Ca2+]i is a major limitation to their use. Therefore, inotropic agents that act by means other than increasing [Ca2+]i may be of great efficacy.

Suppression of positive inotropic and toxic effects of cardiac glycosides by amiloride

Effects of amiloride on the inotropic and toxic actions of cardiac glycosides were examined using left atrial muscle isolated from guinea pig heart. Preincubation of atrial muscle with amiloride significantly decreased the maximum positive inotropic effect of dihydrodigoxin but failed to reduce that of isoproterenol. Amiloride prevented the contracture and significantly reduced the incidence of arrhythmias induced by 2 μM digoxin. Similar experiments examining 5 μM digoxin-induced arrhythmias showed that amiloride increased both the time required to produce arrhythmias and the fractional occupancy of sarcolemmal Na,K-ATPase by digoxin at the onset of arrhythmias. The antagonism of cardiac glycoside actions was best observed during the decline in developed tension elicited by amiloride subsequent to its initial positive inotropic effect. Amiloride had no effect on binding site concentration for ATP-dependent [ 3H]ouabain binding but decreased affinity of the binding sites for ouabain in membrane preparations obtained from guinea pig heart. Furthermore, amiloride inhibited Na,K-ATPase activity and increased the IC 50 value for ouabain inhibition of the enzyme. These results indicate that amiloride antagonizes the positive inotropic and toxic effects of cardiac glycosides. Possible mechanisms for the antagonism include inhibition of sarcolemmal Na +/Ca 2+ or Na +/H + exchange.

The role of intracellular calcium in the positive inotropic and toxic effects of cardiac glycosides in canine Purkinje fibers

The role of intracellular calcium in the positive inotropic and toxic effects of cardiac glycosides in canine Purkinje fibers

Preferential sensitivity of the left canine purkinje system to cardiac glycosides.

Previous studies have shown that the toxic effects of cardiac glycosides are not manifested uniformly throughout the myocardium. The purpose of our study was to determine whether cardiac glycosides exert different effects on the right vs. left peripheral Purkinje systems and to ascertain mechanisms involved. Control in vitro measurements of paired right and left canine Purkinje fibers showed higher spontaneous rates in left (24.2 +/- 1.75 beats/min) than in right (11.6 +/- 1.55 beats/min, P less than 0.01, n = 81) Purkinje fiber bundles. Following overdrive stimulation, left Purkinje fiber bundles also showed earlier escape beats. After ouabain exposure (2 X 10-7 M), left Purkinje fiber bundles showed earlier signs of toxicity in 20 of 28 experiments, as determined by changes in the maximum diastolic potential, the degree of diastolic depolarization, spontaneous escape intervals, and the magnitude of delayed after-depolarizations. The enhanced sensitivity of left Purkinje fiber bundles was independent of the extracellular potassium concentration and glycoside polarity, and was also observed in situ. We conclude that distal Purkinje fibers are functionally dissimilar and that the left Purkinje system shows greater sensitivity to cardiac glycosides than the right Purkinje system. These data also support the observation that digitalis-induced dysrhythmias arise in the left ventricle.

Ouabain effects on intracellular potassium activity and contractile force in cat papillary muscle.

The relationship between the positive inotropic and toxic effects of cardiac glycosides and their effects on intracellular ionic composition is incompletely defined. We measured intracellular potassium activity (alpha ik), extracellular potassium activity (alpha ek), resting potential, action potential duration, and contractile force at 32 degrees C in paired papillary muscles from feline right ventricles exposed to ouabain. Muscles used for electrophysiological measurements were quiescent except for isolated stimulation to confirm impalement and record action potential duration. Muscles used for contractile force measurements were quiescent except for 4-min periods when force was measured at a cycle length of 1,400 ms. Muscle length was adjusted to achieve 50% of maximal tension at this cycle length before each experiment. In four experiments, alpha ik and contractile force were measured in the same muscle. Alpha iK was measured with single and double-barrel K-sensitive electrodes. At 10 nM ouabain, action potential duration is prolonged. Among the concentrations tested, the threshold for a clear positive inotropic effect is 0.1 microM ouabain. The threshold for decrease in alpha iK, increase in alpha eK, and decrease in membrane potential is 1 microM, at which concentration toxic signs develop, including arrhythmias, aftercontractions, and alteration in the staircase response of contractile force to repetitive stimulation. Ouabain need not change alpha iK to effect positive inotropy in ventricular muscle, a relationship different from that reported between [K]i (intracellular potassium concentration) and positive inotropy. Higher ouabain concentrations, which others have shown to clearly inhibit active Na and K transport, are shown to upset intracellular potassium activity homeostasis and to consistently produce toxicity.

The use of digitalis in patients with pulmonary disease.

A review of available literature suggests that patients with pulmonary disease may be more susceptible to the toxic effects of cardiac glycosides than patients with normal lung function. However, few studies correlating digitalis effects with pulmonary status have characterized the type of respiratory illness present or controlled for concurrent, unrelated left ventricular disease. Indications for digitalis therapy with particular emphasis on its role in the management of cor pulmonale, the evidence for an enhanced sensitivity to cardiac glycosides, and the possible mechanisms underlying such sensitivity in patients with pulmonary disease are considered.

Insensitivity of lepidopteran tissues to ouabain: Physiological mechanisms for protection from cardiac glycosides

Neuronal tissues from Manduca sexta, the tobacco hornworm, Hyalophora cecropia, the silkmoth and Danaus plexippus, the Monarch Butterfly, contain Na +K +-ATPase which is sensitive to cardiac glycoside (ouabain). The K m for K + stimulation of Na +K +-ATPase in M. sexta and D. plexippus is 2.2 mM and for Na + stimulation in D. plexippus, 6.0 mM. In vitro ouabain concentrations of 1.0 × 10 −5 M and 5.0 × 10 −5 M in the presence of 7.5 mM K + inhibited Na +K +-ATPase activity in H. cecropia and M. sexta by 50% respectively. Na +K +-ATPase from D. plexippus was approximately 300 times less sensitive. High concentrations (10 −3 M in haemolymph) of ouabain had no effect on M. sexta in vivo. This is largely explained by haemolymph K + (>; 30 mM) antagonizing the binding of ouabain to Na +K +-ATPase. As demonstrated in vitro, 30 mM K + totally protects Na +K +-ATPase from inhibition by 7.5 × 10 −3 M ouabain in D. plexippus and protects the enzyme by 65% in M. sexta. At least part of the physiological burden incurred in utilization of cardiac glycoside ingestion and storage for protection from predation, however, is probably related to the toxic effects of cardiac glycosides on neuronal Na +K +-ATPase.

Serum digitalis measurements in the assessment of digitalis resistance and sensitivity

Antibodies to digitalis glycosides have been elicited in experimental animals and have been utilized in the development of rapid, sensitive, specific and convenient radioimmunoassay methods for the clinical measurement of digoxin and other cardiac glycosides in man. The use of these assay methods has supplemented earlier studies with radiolabeled digitalis preparations and has made it possible to obtain much new information concerning factors which may contribute to the well known patient to patient variability in digitalis dosage requirements and in sensitivity to the toxic effects of cardiac glycosides. In some patients with a poor clinical response to digitalis, the finding of a serum concentration which is relatively low for the dose prescribed may suggest that true digitalis resistance is not present and may raise questions of poor patient compliance, tablet inadequacies, intestinal malabsorption, increased metabolic degradation or hyperthyroidism; if the cause of the low serum level cannot be identified or corrected, serial serum measurements should enable safe and rational upward adjustment of dosage. In some patients with digitalis toxicity, the finding of a serum level which is relatively high for the dose prescribed may suggest that the patient is not sensitive to digitalis but rather is excreting it slowly; in such instances in elderly patients (with decreased glomerular filtration rates) and in patients with renal disease, serial digitalis measurements are useful adjuncts to clinical observation in determining optimal digitalis dosage schedules. A knowledge of serum digitalis concentrations should enable us to develop sound principles for a more rational approach to the clinical administration of cardiac glycosides, especially in patients with unusually high dosage requirements or with unusual sensitivity to relatively small doses of digitalis.

Digitalis Mechanisms: Effects on Sodium Flux

Measurements were made of human erythrocyte ghost Na flux, Na+K dependent ATPase activity, and intracellular water and electrolyte concentrations in patients receiving cardiac glycosides and in normal subjects to investigate mechanisms of action of cardiac glycosides. The effects of in vitro exposure of normal cells to cardiac glycosides were also studied. The mean erythrocyte ghost Na efflux rate constant in adequately digitalized patients was elevated to 1.19±0.027(SEM). Normal mean efflux rate was 0.62±0.017 and the difference was significat, p<0.01. The increased efflux was due to increased active transport since the passive outward leak of Na was unaffected. Since erythrocyte intracellualar Na concentration was normal in these patients the passive influx rate of Na was also increased. The in vitro exposure of normal ghosts to ouabain 10−9 to 10−10 M. produced similar increases in Na flux. Erythrocyte water and Na+K dependent ATPase activity were both increased in digitalized patients. Patients with digitals toxicity had reduced active transport of Na, reduced Na+K dependent ATPase activity, and increased intracellular Na concentration. Similar changes in flux and ATPase were produced in vitro by ouabain 10−4 to 10−7 M. It is believed that these findings support KLAUS' suggestion that therapeutic doses of cardiac glycosides are associated with increased active transport of Na, decreased intracellular Na concentration and suggest increased cell water. Toxic effects of cardiac glycosides are associated with decreased active Na transport, decreased Na+K dependent ATPase and increased intracellular Na.