Digitalis: A neuroexcitatory drug

Antibody-powered DNA switches to initiate the hybridization chain reaction for the amplified fluorescence immunoassay

Designing antibody-powered DNA nanodevice switches is crucial and fascinating to perform a variety of functions in response to specific antibodies as regulatory inputs, achieving highly sensitive detection by integration with simple amplified methods. In this work, we report a unique DNA-based conformational switch, powered by a targeted anti-digoxin mouse monoclonal antibody (anti-Dig) as a model, to rationally initiate the hybridization chain reaction (HCR) for enzyme-free signal amplification. As a proof-of-concept, both a fluorophore Cy3-labeled reporter hairpin (RH) in the 3' terminus and a single-stranded helper DNA (HS) were individually hybridized with a recognition single-stranded DNA (RS) modified with Dig hapten, while the unpaired loop of RH was hybridized with the exposed 3'-toehold of HS, isothermally self-assembling an intermediate metastable DNA structure. The introduction of target anti-Dig drove the concurrent conjugation with two tethered Dig haptens, powering the directional switch of this DNA structure into a stable conformation. In this case, the unlocked 3'-stem of RH was implemented to unfold the 5'-stem of the BHQ-2-labeled quench hairpin (QH), rationally initiating the HCR between them by the overlapping complementary hybridization. As a result, numerous pairs of Cy3 and BHQ-2 in the formed long double helix were located in spatial proximity. In response to this, the significant quenching of the fluorescence intensity of Cy3 by BHQ-2 was dependent on the variable concentration of anti-Dig, achieving a highly sensitive quantification down to the picomolar level based on a simplified protocol integrated with enzyme-free amplification.

Why Whip the Starving Horse When There Are Oats for the Starving Myocardium?

Digoxin is the oldest drug for treatment of heart failure still in clinical use. Despite over 200 years of clinical experience with this drug, the optimal serum concentration required for both efficacy and safety remains unknown. It has been suggested that low doses have more favorable effects than higher ones. Cardiac glycosides act on the Na/K-ATPase (NKA). They show an inverted U-shaped dose-response curve with inhibition of pumping at high concentrations while increasing NKA activity at low concentrations. The classical sigmoidal dose-response curve describing an inhibition of the NKA by cardiac glycosides cannot explain this stimulatory effect. Cardiac glycosides are prototypical examples of hormetic substances. Biphasic dose-response curves of cardiac glycosides are also found in their neurohormonal effects. In low concentrations, vagomimetic effects are observed, whereas in high concentrations, sympathomimetic effects dominate. Lipophilic Digitalis glycosides have greater sympathomimetic effects; hydrophilic Strophanthus glycosides have greater vagomimetic effects. For digoxin, as a strong inotrope, there is evidence of only weak modulation of the autonomic nervous system. In ouabain, the modulation of the autonomic nervous system prevails over weak inotropic effects. Vagomimetic and sympatholytic effects characterize the therapeutic effects. In contrast to those of digoxin, the therapeutic effects of ouabain follow exactly the measurable serum concentration. Contrary to common prejudice ouabain is suitable for oral administration. Timely adjustments of dosage to patient therapeutic needs are easy to achieve with orally administered ouabain. Ouabain has the potential to crucially improve our arsenal of heart failure medications. Therefore, a clinical re-evaluation of ouabain is warranted. Randomized double-blind prospective clinical studies with ouabain, which meet today's standards, are worthwhile and necessary.

Rethinking Heart Failure

An increasing body of clinical observations and experimental evidence suggests that cardiac dysfunction results from autonomic dysregulation of the contractile output of the heart. Excessive activation of the sympathetic nervous system and a decrease in parasympathetic tone are associated with increased mortality. Elevated levels of circulating catecholamines closely correlate with the severity and poor prognosis in heart failure. Sympathetic over-stimulation causes increased levels of catecholamines, which induce excessive aerobic metabolism leading to excessive cardiac oxygen consumption. Resulting impaired mitochondrial function causes acidosis, which results in reduction in blood flow by impairment of contractility. To the extent that the excessive aerobic metabolism resulting from adrenergic stimulation comes to a halt the energy deficit has to be compensated for by anaerobic metabolism. Glucose and glycogen become the essential nutrients. Beta-adrenergic blockade is used successfully to decrease hyperadrenergic drive. Neurohumoral antagonists block adrenergic over-stimulation but do not provide the heart with fuel for compensatory anaerobic metabolism. The endogenous hormone ouabain reduces catecholamine levels in healthy volunteers, promotes the secretion of insulin, induces release of acetylcholine from synaptosomes and potentiates the stimulation of glucose metabolism by insulin and acetylcholine. Ouabain stimulates glycogen synthesis and increases lactate utilisation by the myocardium. Decades of clinical experience with ouabain confirm the cardioprotective effects of this endogenous hormone. The so far neglected sympatholytic and vagotonic effects of ouabain on myocardial metabolism clearly make a clinical re-evaluation of this endogenous hormone necessary. Clinical studies with ouabain that correspond to current standards are warranted.

Heart rate variability in left ventricular dysfunction and heart failure: effects and implications of drug treatment

OBJECTIVE

To review the importance of heart rate variability analysis in left ventricular dysfunction and heart failure and to assess the effects of drug treatment. In patients with left ventricular dysfunction or heart failure, a low heart rate variability is a strong predictor of a low probability of survival. Because drug treatment in these patients has rapidly changed over the past two decades, the effect of these drugs on heart rate variability needs special attention.

DESIGN

A study of published reports to give an overview of heart rate variability in patients with left ventricular dysfunction or heart failure and how it is affected by drug treatment.

RESULTS

Analysis of heart rate variability provides an easily obtained early marker for progression of disease. It seems to be more closely related to the degree of neurohumoral activation than to haemodynamic variables. Cardiovascular drugs may either stimulate or inhibit the degree of neurohumoral activation, and the effects of pharmacological intervention can be closely monitored with this method.

CONCLUSIONS

The analysis of heart rate variability, including spectral analysis, is a novel non-invasive way to obtain potentially useful clinical information in patients with reduced left ventricular function. The effects of drug treatment on heart rate variability are in general consistent with their long-term effects in left ventricular dysfunction and heart failure.

Effects of digoxin and enalapril on heart period variability and response to head-up tilt in normal subjects

To test the effects of digitalis and angiotensin-converting enzyme inhibition on the RR interval variability in an electrocardiogram, 20 normal subjects were given digoxin 0.25 mg, enalapril 10 mg, and placebo twice daily in a randomized, double-blind, crossover study. Continuous 24-hour electrocardiographic recordings obtained on day 5 of each treatment were analyzed and several time domain and power spectral measures of heart period variability were calculated. Digoxin markedly increased (up to 51%) indexes of vagal modulation of heart period without changing mean RR interval. Enalapril did not change any measure of heart period variability despite a modest hypotensive effect. To determine the effect of each treatment on the response to orthostatic stress, 10 subjects also underwent 15 minutes of 60 degrees head-up tilt; power spectra were calculated for 15 minutes at 0 degree and at 60 degrees of tilt. Neither active treatment affected the response to head-up tilt.

Failure of long-term digitalization to prevent rapid ventricular response in patients with paroxysmal atrial fibrillation

Digitalis is frequently prescribed to patients with paroxysmal atrial fibrillation to reduce the ventricular rate during subsequent paroxysms. To verify the validity of this assumption, we determined the ventricular rate during paroxysmal atrial fibrillation in 13 patients receiving long-term digoxin therapy (mean plasma digoxin level + 1.28 +/- 0.4 ng/ml) and compared it with that of a group of 14 patients who had not taken digoxin or beta-adrenergic and calcium-blocking agents before the attack. The treated and the untreated groups were similar statistically. The mean ventricular rate of the digitalized patients was 121 +/- 15 beats per minute, while that of the patients in the control group was 118 +/- 16 beats per minute. It is concluded that long-term digoxin therapy is not effective in reducing the ventricular response in patients with paroxysmal atrial fibrillation despite adequate therapeutic levels.

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.

Cardiac glycoside toxicity in small laboratory animals

Cardiac glycosides are frequently administered to laboratory animals for research purposes. The effects achieved depend not only upon the particular glycoside and dose administered, but also upon an entire array of variables from the species of animal to the temperature of the animal housing facility. We review a number of these factors and their influence upon the effects achieved by the administration of cardiac glycosides to laboratory animals.

Effects of acetyl strophanthidin on duration of atrial fibrillation in the neurally-intact and blockaded dog

Although the inotropic and dromotropic effects of cardiac glycosides in atrial fibrillation (AF) are well recognized, their action on AF itself is not clear. Accordingly, to determine whether cardiac glycosides prolong AF, the duration of electrically induced AF, atrioventricular conduction, and left ventricular function were assessed for 30 minutes before and for 30 minutes following intravenous administration of acetyl strophanthidin (AS), 20 micrograms/kg, in neurally intact, beta-blocked, and beta-blocked and vagotomized dogs. In the intact dog, AS, 20 micrograms/kg, increased peak dp/dt by 132 +/- 35 mmHg.sec-1, p less than 0.05, and slowed ventricular response by 16 +/- 7 min-1, p less than 0.05, but had a variable effect on AF duration. While the increased left ventricular peak dp/dt persisted for 15 minutes after AS, an increased duration of AF was evident only at 20 minutes, when the effects of AS on left ventricular (LV) inotropy were no longer apparent. Moreover, the subset of dogs that did not demonstrate prolongation of average duration of AF after AS had a greater increment of peak dp/dt than those that showed prolongation, 237 +/- 52 versus 53 +/- 31 mmHg.sec-1, p less than 0.05. An additional 20 micrograms/kg, which produced ventricular extrasystoles, prolonged AF duration when compared to both control and 30-minute measurements. Acetyl strophanthidin, 20 micrograms/kg, had a variable effect on duration of AF with beta-blockade but prolonged duration by 114 +/- 34%, p less than 0.05, with both vagotomy and beta-blockade.(ABSTRACT TRUNCATED AT 250 WORDS)

Refractory bradycardia during retrograde pyelography and spinal anesthesia

Bradycardia commonly occurs during spinal anesthesia as a result of vagally mediated slowing of sinoatrial (SA) node firing. Peritoneal traction or urinary bladder distension, for example, may serve as a stimulus for SA node inhibition, particularly when thoracic levels of spinal anesthesia interrupt function of the sympathetic limb of the autonomic nervous system. Although atropine dependably reverses such heart rate slowing, we recently encountered, during spinal anesthesia for cystoscopy and retrograde pyelography, bradycardia that was unexpected and refractory to treatment with atropine.

Reduction in digitalis-associated postinfarction mortality with nadolol in conscious dogs

Previously, we have demonstrated an increased incidence of lethal ischemic arrhythmias in postinfarction dogs with clinically observable serum digoxin concentrations, and a significant reduction in digitalis-related lethal ischemic arrhythmias after subacute left stellectomy. In the present study, the protective actions of acute beta-adrenoceptor blockade with nadolol, 1.0 mg/kg administered intravenously immediately preceding the induction of posterolateral myocardial ischemia, were assessed in conscious dogs with recent, small anterior myocardial infarctions pretreated with digoxin, 0.0125 mg/kg/day intravenously, for 5 to 7 consecutive days (total n = 11). A cohort of postinfarction dogs pretreated with digoxin alone served as a control group (total n = 26). Pre vs postdigoxin electrophysiologic testing indicated reductions in myocardial refractoriness in ventricular noninfarct and infarct zones in both treatment groups, whereas the administration of nadolol tended to reverse the reductions in ventricular refractoriness. Arrhythmia-related deaths in response to posterolateral myocardial ischemia were reduced from 12 of 20 (60%) in the digoxin control group to 2 of 10 (20%) in the digoxin + nadolol group (p = 0.039). Serum digoxin concentrations (1.29 +/- 0.14 ng/ml vs 1.39 +/- 0.24 ng/ml), underlying anterior myocardial infarct size (6.9 +/- 1.5% vs 4.6 +/- 0.9% of left ventricle), and developing posterolateral myocardial infarct size (22.8 +/- 2.5% vs 17.5 +/- 3.6% of left ventricle) did not differ significantly between the digoxin and digoxin + nadolol groups. Acute beta-adrenoceptor blockade with nadolol appears to reduce digitalis-mediated ischemic postinfarction mortality, possibly because of a salutary increase in ventricular refractoriness.

Clinical use of digitalis materials

Digitalis is a drug widely used in modern medicine for the control of ventricular response in atrial fibrillation and the treatment of congestive heart failure (CHF). Recently, the use of digitalis for the treatment of CHF in patients in sinus rhythm has become quite controversial. The findings of several clinical studies suggest a small but definite hemodynamic or clinical improvement in patients treated with digitalis. These effects are limited by the onset of toxicity, which is at least partially mediated via the central nervous system. If the inotropic effect of the drug could be separated from the central nervous system effect, much higher doses of digitalis could be tolerated and presumably a greater therapeutic effect could be obtained.

Independent and interactive effects of digoxin and quinidine on the atrial fibrillation threshold in dogs

To assess the effects of digoxin as single therapy and in combination with quinidine in the treatment of atrial fibrillation, the atrial fibrillation threshold was determined from the right atrial appendage and Bachmann's bundle in 11 open chest dogs. In group 1 (six dogs), the atrial fibrillation threshold was determined at baseline, post-quinidine (10 mg/kg intravenously) and then post-digoxin (50 micrograms/kg intravenously). In group 2 (five dogs), the order of drug administration was reversed. The results of this study were: 1) Digoxin had no significant effect on the atrial fibrillation threshold when given alone. 2) Quinidine significantly increased the atrial fibrillation threshold (p less than 0.002) and the addition of digoxin resulted in a further increase in threshold (p less than 0.002). 3) Quinidine produced greater suppression of atrial fibrillation induction at the right atrial site than at the Bachmann's bundle site, suggesting differential effects of quinidine on atrial fibers.

Basic understanding of the electrophysiologic actions of antiarrhythmic drugs. Sources, sinks, and matrices of information

The author creates an intellectual framework consisting of key electrophysiologic principles, basic mechanisms of arrhythmogenesis, and important drug reactions that will allow the rational use of antiarrhythmic drugs. Basic principles have been emphasized because current understanding requires it.

The effect of digoxin on the response to lithium therapy in mania

Patients suffering from manic-depressive psychosis, manic type (ICD 296.0), were treated with lithium carbonate and randomly allocated to two groups, one received digoxin and the other matching placebo for 7 days. Severity of mania was rated by psychiatrists on the Manic Rating Scale and Analogue Line on days 0 and 7 and by nurses daily on the Hargreaves Rating Scale, Psychotic Rating. Fourteen patients received digoxin and lithium carbonate and 14 patients received placebo and lithium carbonate. Improvement in the placebo lithium group was significantly greater than that in the digoxin lithium group. This trial suggests, therefore, that the effect of inhibition of membrane cation carrier is to reduce the response to lithium. This result is in keeping with our hypothesis that an increase in Na-K ATPase is essential to the therapeutic effect of lithium carbonate. It does not, however, exclude the possibility that the observations resulted from the inhibition by digoxin of lithium entry into the brain.

Digitalis and baroreceptor reflexes in man

Data in animals indicate that large amounts of digitalis potentiate arterial baroreflexes and that this factor may be important for the cardiovascular effects of the drug. To determine if arterial baroreflex potentiation also exists after administration of therapeutic doses of digitalis in man, we studied how stimulation and deactivation of arterial baroreceptors by phenylephrine and nitroglycerin injection affect heart rate and how stimulation and deactivation of carotid baroreceptors by neck suction and pressure affects blood pressure and heart rate. The study was performed in 29 normotensive or hypertensive subjects before and after injection of Lanatoside C (0.8 mg i.v.). Baroreceptor stimulation reduced heart rate and blood pressure, while baroreceptor deactivation increased both of these variables. The bradycardic and hypotensive effect of baroreceptor stimulation increased significantly after digitalis both in normotensive and hypertensive subjects. However, the tachycardic and hypertensive responses to baroreceptor deactivation were not affected by digitalis. Thus, therapeutic doses of digitalis in man enhance baroreceptor reflexes, and both the heart rate and the blood pressure reflex effects are involved. However, the enhancement occurs to a marked degree only with baroreceptor stimulation and is not evident with baroreceptor deactivation.

Interactions of norepinephrine and strophanthidin in cardiac Purkinje fibers

The actions and interactions between strophanthidin and norepinephrine on electrical and mechanical events were studied in cardiac canine Purkinje fibers perfused in vitro. The results obtained show that norepinephrine (but not strophanthidin) shifts phase 1 and the beginning of the plateau to a more positive potential, an effect reduced by beta-receptor blockade. Norepinephrine increases the contractile force to the maximal value sooner than strophanthidin. In low Ca solution, the inotropic action of strophanthidin far exceeds that of norepinephrine. Norepinephrine does not cause spontaneous discharge in the driven fibers, whereas strophanthidin eventually induces fast spontaneous rhythms. When norepinephrine is given in the presence of strophanthidin (or vice versa) spontaneous activity is induced which consists either of a slow rhythm (characterized by large action potentials) or of a fast rhythm (characterized by small action potentials). The latter is typically induced by strophanthidin alone but occurs sooner in the presence of both agents. Propranolol prevents the onset of the slow rhythm but not that of the fast rhythm. It is concluded that norepinephrine and strophanthidin increase force by different mechanisms and that the potentiation between these agents in causing spontaneous discharge may involve an enhancement of diastolic depolarization(slow rhythm) or of the oscillatory potential (fast rhythm).

The modifying effect of autonomic blockade on digitoxin-induced changes in monophasic action potential and refractoriness of the right ventricle of the dog heart

The aim of the present investigation was to study the modifying effect of autonomic blockade with atropine plus propranolol and atropine alone on the electrophysiologic response to digitoxin during an observation period of 8 hours. Twenty-five pentobarbital-anaesthetised Labrador dogs were used. Pacing and programmed electrical stimulation were used to determine heart rate independent changes in repolarisation times and the effective (V-ERP) and the functional (V-FRP) refractory periods. The dogs were divided into three groups and all dogs got digitoxin (2.0 mg) intravenously during 5 min. Fourteen dogs were given digitoxin without blockade, while 5 dogs were pretreated with propranolol 0.5 mg/kg plus atropine 0.05 mg/kg every hour for 8 hours and 6 dogs with atropine in the same dose and with the same interval. Digitoxin alone increased 50 and 90% repolarisation times 2-4 hours after injection of the drug. During autonomic blockade digitoxin induced a shortening of action potential duration, and thus digitoxin interacts with autonomic transmitters resulting in prolongation of action potential duration. The "direct" effects of digitoxin, the action potential shortening effect, the decrease in membrane responsiveness (i.e. V-ERP/50% repolarisation time) reached a maximum 2-3 hours after injection of the drug.

The acute changes seen in cardiac glycoside receptor sites, 86rubidium uptake and intracellular sodium concentrations in the erythrocytes of patients during the early phases of digoxin therapy are not found during chronic therapy: pharmacological and therapeutic implications for chronic digoxin therapy

1 Measurements of the binding of 12-alpha-[3H]-digoxin to the membranes of intact erythrocytes, erythrocyte 86rubidium uptake and intraerythrocytic sodium concentrations have been made in the red cells of various groups of patients--those who have not received digoxin, those during the early phases of treatment, those during chronic (greater than 2 months) treatment, and those toxic. 2 The values of those measurements in the patients in the early phases of therapy and in the toxic patients differed significantly from those of the untreated patients. 3 However, the values in the chronically treated patients were not different from those of the untreated patients. 4 The results suggest that the biochemical pharmacological effects of digoxin which occur during the early phases of therapy do not persist in the long-term. 5 The possible clinical significance of these observations is discussed.

Localization of the neurally mediated arrhythmogenic properties of digitalis

Available evidence suggests that the propensity of digitalis glycosides to produce cardiac arrhythmias is due in part to their neuroexictatory effects. We have performed experiments in cats which support the existence of a neurogenic component in the etiology of digitalis-induced ventricular arrhythmias. Our data further indicate that the locus of this neural effect lies within an area of the medulla 2 millimeters above to 2 millimeters below the obex. These findings, when considered with the effects of polar cardiac glycosides that do not cross the blood-brain barrier, suggest that the area postrema may be the site of neural activation.

Dosage of digoxin in premature infants

The pharmacokinetics and pharmacodynamics of digoxin in premature infants was studied. During maintenance therapy, after a total digitalizing dose of 30 microgram/kg, the measured digoxin level was related inversely to body weight at birth and to estimated gestational age. The serum digoxin levels found in the immature and smaller infants were two to three times the values usually reported to be toxic in older children. Based on these findings, a second group of premature infants was digitalized with 20 microgram/kg; in this group, the serum digoxin levels were below the toxic range, irrespective of gestational age or birth weight. The cardiac effects of digoxin, i.e., shortened left ventricular pre-ejection period and ejection time as determined by echocardiography, were similar in the two groups. For both groups, the half-life of digoxin in the serum was twice that reported for term infants and children. Since digitalis effect is obtained with lower dose and serum concentration, we recommend that this dose be used in premature infants.

Effects of digitalis on atrial vulnerability

The effects of digitalis on vulnerability to atrial fibrillation and flutter were assessed in man, using the model of repetitive atrial firing initiated by post-drive atrial extrastimulation. Nine patients without heart failure or significant mitral valve disease were tested before and 30 minutes after the administration of 0.01 mg/kg ouabain. When repetitive firing was manifested by flutter, neither the flutter cycle length nor the interval from the initiating beat to the first flutter beat was consistently altered by ouabain. Repetitive firing was found at the atrial site with the shortest functional refractory period. The vulnerable zone bordered this refractory period. The functional refractory period was lengthened after ouabain, from 231 +/- 13 to 246 +/- 15 msec (mean +/- standard error of the mean) (P less than 0.025). Partly because of prolonged refractoriness, the vulnerable zone was curtailed by ouabain, from 32.2 +/- 5.7 to 9.4 +/- 4.6 msec (P less than 0.001). This result suggests a protective effect of digitalis against atrial fibrillation and flutter independent of its hemodynamic actions.

Lack of effect on brain stem and cerebral cortex Na+, K+-ATPase during heart block produced by chronic digoxin treatment

The autonomic nervous system has been shown to play an important role in digitalis toxicity. In order to determine whether the central nervous system could be digitalis' site of action, the effect of chronic treatment with toxic doses of digitalis on brain Na+,K+-ATPase was studied in the dog. After four weeks of digoxin treatment, Na+,K+-ATPase activity of the brainstem or cerebral cortex was unaffected at the time when digitalis toxicity (heart block) was apparent. ATP-dependent (3H)-ouabain binding to these tissues was also unaffected indicating that a significant occupancy of brain Na+,K+-ATPase by digoxin did not occur during chronic drug treatment. In contrast, cardiac Na+,K+-ATPase was markedly inhibited with the concomitant binding of digoxin to the enzyme. Since Na+,K+-ATPase is the most digitalis sensitive system identified to date, it appears that digoxin does not affect neuronal function directly.

Penetration of digoxin into cerebrospinal fluid

The concentration of digoxin in the cerebrospinal fluid (CSF) of ten patients receiving conventional oral doses of this cardiac glycoside has been measured by a radioimmunoassay technique. Digoxin was undetected in eight patients and barely detectable in two, suggesting the presence of a significant blood-CSF barrier for digoxin. The implication of these findings is discussed.