Establishment and characterization of the transformants stably-expressing MDR1 derived from various animal species in LLC-PK1

PURPOSE

Stable transformants expressing human multidrug resistance 1 (MDR1), monkey MDR1, canine MDR1, rat MDR1a, rat MDR1b, mouse mdr1a, and mouse mdr1b in LLC-PK1 were established to investigate species differences in P-glycoprotein (P-gp, ABCB1) mediated efflux activity.

METHODS

The seven cDNAs of MDR1 from five animals were cloned, and their transformants stably expressing the series of MDR1 in LLC-PK1 were established. Transport studies of clarithromycin, daunorubicin, digoxin, erythromycin, etoposide, paclitaxel, propranolol, quinidine, ritonavir, saquinavir, verapamil, and vinblastine were performed by using these cells, and efflux activity was compared among the species.

RESULTS

Except for propranolol, all compounds showed efflux activity in all transformants, and were judged to be substrates of P-gp. There were slight interspecies and interisoforms differences in the substrate recognition. However, the efflux ratio among the series of the MDR1 stably expressing cells showed good correlation as represented between human and monkey MDR1, and poor correlation as represented between human MDR1 and mouse mdr1a, and human and canine MDR1.

CONCLUSIONS

Results in the present study indicate that all MDR1 stably expressing cells have efflux activity for various P-gp substrates, and that interspecies differences and similarities of the P-gp substrate efflux activity may exist.

Inhibition of P-glycoprotein-mediated transport by extracts of and monoterpenoids contained in Zanthoxyli fructus

Citrus (rutaceous) herbs are often used in traditional medicine and Japanese cuisine and can be taken concomitantly with conventional medicine. In this study, the effect of various citrus-herb extracts on P-glycoprotein (P-gp)-mediated transport was examined in vitro to investigate a possible interaction with P-gp substrates. Component monoterpenoids of the essential oil in Zanthoxyli fructus was screened to find novel P-gp inhibitors. LLC-GA5-COL150 cells transfected with human MDR1 cDNA encoding P-gp were used. Cellular accumulation of [3H]digoxin was measured in the presence or absence of P-gp inhibitors or test samples. Aurantii fructus, Evodiae fructus, Aurantii fructus immaturus, Aurantii nobilis pericarpium, Phellodendri cortex, and Zanthoxyli fructus were extracted with hot water (decocted) and then fractionated with ethyl acetate. The cell to medium ratio of [3H]digoxin accumulation increased significantly in the presence of the decoction of Evodiae fructus, Aurantii nobilis pericarpium, and Zanthoxyli fructus, and the ethyl acetate fraction of all citrus herbs used. The ethyl acetate fraction of Zanthoxyli fructus exhibited the strongest inhibition of P-gp among tested samples with an IC50 value of 166 microg/mL. Then its component monoterpenoids, geraniol, geranyl acetate, (R)-(+)-limonene, (R)-(+)-linalool, citronellal, (R)-(+)-citronellal, DL-citronellol, (S)-(-)-beta-citronellol, and cineole, were screened. (R)-(+)-citronellal and (S)-(-)-beta-citronellol inhibited P-gp with IC50 values of 167 microM and 504 microM, respectively. These findings suggest that Zanthoxyli fructus may interact with P-gp substrates and that some monoterpenoids with the relatively lower molecular weight of about 150 such as (R)-(+)-citronellal can be potent inhibitors of P-gp.

Effect of milk thistle (Silybum marianum) and black cohosh (Cimicifuga racemosa) supplementation on digoxin pharmacokinetics in humans

Phytochemical-mediated modulation of P-glycoprotein (P-gp) and other drug transporters may underlie many herb-drug interactions. Serial serum concentration-time profiles of the P-gp substrate, digoxin, were used to determine whether supplementation with milk thistle or black cohosh modified P-gp activity in vivo. Sixteen healthy volunteers were randomly assigned to receive a standardized milk thistle (900 mg daily) or black cohosh (40 mg daily) supplement for 14 days, followed by a 30-day washout period. Subjects were also randomized to receive rifampin (600 mg daily, 7 days) and clarithromycin (1000 mg daily, 7 days) as positive controls for P-gp induction and inhibition, respectively. Digoxin (Lanoxicaps, 0.4 mg) was administered orally before and at the end of each supplementation and control period. Serial digoxin serum concentrations were obtained over 24 h and analyzed by chemiluminescent immunoassay. Comparisons of area under the serum concentration time curves from 0 to 3 h (AUC(0-3)), AUC(0-24), Cmax, apparent oral clearance of digoxin (CL/F), and elimination half-life were used to assess the effects of milk thistle, black cohosh, rifampin, and clarithromycin on digoxin pharmacokinetics. Rifampin produced significant reductions (p < 0.01) in AUC(0-3), AUC(0-24), and Cmax, whereas clarithromycin increased these parameters significantly (p < 0.01). Significant changes in digoxin half-life and CL/F were also observed with clarithromycin. No statistically significant effects on digoxin pharmacokinetics were observed following supplementation with either milk thistle or black cohosh, although digoxin AUC(0-3) and AUC(0-24) approached significance (p = 0.06) following milk thistle administration. When compared with rifampin and clarithromycin, supplementation with these specific formulations of milk thistle or black cohosh did not appear to affect digoxin pharmacokinetics, suggesting that these supplements are not potent modulators of P-gp in vivo.

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.

Characterisation of (R/S)-propafenone and its metabolites as substrates and inhibitors of P-glycoprotein

Digoxin is a drug with a narrow therapeutic index, which is a substrate of the ATP-dependent efflux pump P-glycoprotein. Increased or decreased digoxin plasma concentrations occur in humans due to the inhibition or induction of this drug transporter in organs with excretory function such as small intestine, liver and kidney. It is well known that serum concentrations of digoxin increase considerably in humans if propafenone is given simultaneously. However, it has not been investigated in detail whether propafenone and its metabolites are substrates and/or inhibitors of human P-glycoprotein. The aim of this study, therefore, was to investigate the P-glycoprotein-mediated transport and inhibition properties of propafenone and its major metabolites 5-hydroxypropafenone and N-desalkylpropafenone in Caco-2 cell monolayers. Inhibition of P-glycoprotein-mediated transport by propafenone and its metabolites was determined using digoxin as a P-glycoprotein substrate. No polarised transport was observed for propafenone and N-desalkylpropafenone in Caco-2 cell monolayers. However, 5-hydroxypropafenone translocation was significantly greater from basal-to-apical compared with apical-to-basal (P(app) basal-apical vs. P(app) apical-basal, 10.21+/-2.63 x 10(-6) vs. 4.34+/-1.84 x 10(-6) cm/s; P<0.01). Moreover, propafenone, 5-hydroxypropafenone and N-desalkylpropafenone inhibited P-glycoprotein-mediated digoxin transport with IC(50) values of 6.8, 19.9, and 21.3 microM, respectively. In summary, whereas propafenone and N-desalkylpropafenone are not substrates of P-glycoprotein, 5-hydroxypropafenone is translocated by human P-glycoprotein across cell monolayers. In addition, propafenone and its two major metabolites 5-hydroxypropafenone and N-desalkylpropafenone are inhibitors of human P-glycoprotein and therefore contribute to the digoxin-propafenone interaction observed in humans.

Effects of antidigoxin antiserum on endoxin levels, apoptosis and the expression of Bax and Bcl-2 protein in ischaemia-reperfusion myocardium

The aim of the present study was to investigate the effects of antidigoxin antiserum (ADA), an endoxin special antagonist, on endoxin levels, apoptosis and the expression of the apoptosis-related protein bcl-2 and bax in myocardial ischaemia-reperfusion (MIR). The left anterior descending coronary artery was subjected to 30 min ischaemia followed by 45 min reperfusion in open-chest anaesthetized rats. The rats were divided randomly into seven groups: a sham-operated group, an MIR group, a vehicle control (normal saline) group, and groups receiving verapamil (5 mg/kg) or ADA (9, 18 and 36 mg/kg). The drugs were injected into rats via the femoral vein before reperfusion was commenced. Myocardial endoxin levels were measured by radioimmunoassay. Apoptotic cells was detected using the terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick end-labeling method. The expression of the apoptosis-related proteins bcl-2 and bax was detected by immunohistochemistry and their semiquantification scores were recorded by a computer image analysis system. Myocardial endoxin levels, the number of apoptotic cells and bax protein expression were increased in the MIR group compared with the sham group. Although bcl-2 protein expression was elevated in the MIR group, there was no significant difference between the MIR and sham groups. However, the ratio of bcl-2/bax was significantly decreased in the MIR group. In the group receiving 36 mg/kg ADA, myocardial endoxin levels, the number of apoptotic cells and bax protein expression were significantly decreased; bcl-2 protein expression was enhanced. The bcl-2/bax ratio was increased. The results suggest that ADA inhibited myocardial apoptosis induced by MIR in rats. The mechanisms involved require further investigation, but the present study may suggest that ADA prevents bax upregulation and enhances bcl-2 upregulation by antagonizing the effects of endoxin.

Immediate control of life-threatening digoxin intoxication in a child by use of digoxin-specific antibody fragments (Fab)

Digoxin-immune antibody fragments (Fab) for treatment of digitalis intoxication was introduced in 1976. Many reports have been published concerning this therapy for children, but few have focused on its immediate reversal of cardiac as well as extracardiac life-threatening manifestations of digoxin toxicity. We present a case of life-threatening digitalis intoxication in a child with postoperative renal insufficiency, after a Sennings procedure for transposition of the great arteries. Digoxin administration according to the nationally recommended dosage and intervals unexpectedly resulted in serum levels in the toxic range. Severe cardiac arrhythmias, haemodynamic instability and a rapid-increasing serum potassium level resulted. This report demonstrates how administration of Fab according to the manufacturer's dosage recommendation reversed the tachyarrhythmia immediately and re-established a normal level of serum potassium within minutes.

From bench to bedside: utilization of an in vitro model to predict potential drug-drug interactions in the kidney: the digoxin-mifepristone example

Drug interactions are a common source of drug-induced toxicity. For drugs with narrow therapeutic windows, such as digoxin, an understanding of the potential mechanisms by which drugs might interact is essential to clinical practice. This article describes the utility of a renal tubular cell culture model in the prediction of drug interactions involving P-glycoprotein. Digoxin is a cardiac glycoside that undergoes active secretion in the renal tubules by the MDR1 (P-glycoprotein) drug efflux pump. Mifepristone (RU486) is a recently introduced abortifacient that is largely unstudied in terms of drug-drug interactions. The authors used an in vitro model to study the effects of mifepristone on the renal tubular secretion and cellular uptake of digoxin by Madin-Darby canine kidney (MDCK) cells. Mifepristone significantly inhibited the renal tubular secretion of digoxin (p = 0.0005), without interfering with its ability to enter the renal tubular cell. Similar results were found with the P-glycoprotein substrate vinblastine. The findings suggest that drug interactions may result if mifepristone is administered with P-glycoprotein substrates, highlighting the usefulness of this model in the study of not only common but also rare combinations of drugs.

Digitalis-like factor response to hyperinsulinemia in human pregnancy, a model of insulin resistance

Insulin resistance is strongly associated with hypertension and is postulated to participate in the elevation of blood pressure, although the mechanisms involved are not understood. Recently, we reported that acute increases in plasma insulin levels in normal subjects resulted in increased serum levels of a sodium pump inhibitor, termed the digitalis-like factor (DLF), which has been implicated in both experimental and essential human hypertension. This study looked at the DLF response to hyperinsulinemia, achieved by an oral glucose tolerance test (OGTT), in the setting of a naturally occurring and self-resolving state of human insulin resistance, during third-trimester pregnancy. This model allowed us the further opportunity to compare the DLF response to insulin in the same subjects postpartum, after resolution of their insulin resistance. Administration of an OGTT during pregnancy and postpartum in the same subjects elicited a comparable serum glucose response but a significantly greater insulin response during third-trimester pregnancy, consistent with diminished insulin sensitivity (integrated insulin response during pregnancy: 1611+/-236 vs postpartum: 685+/-101 pmol/l, P=0.004). The time courses of the glucose and insulin responses were identical whether women were pregnant or not. Plasma free fatty acids fell significantly and to a comparable degree during pregnancy and postpartum, but the response was slower during pregnancy. DLF levels increased in response to oral glucose in both pregnant and nonpregnant states. The response was more rapid during pregnancy than after. These findings showed that the increment of insulin induced by oral glucose during pregnancy caused a more rapid rise in circulating DLF levels than it did during the nonpregnant state. At the same time, the response of circulating fatty acids to glucose is retarded during pregnancy. This suggests that the insulin resistance of pregnancy impairs insulin's influence on intermediary metabolism but not its influence on DLF. As a vasoactive substance, DLF might contribute to the hypertension characteristic of insulin-resistant states.

Interaction of omeprazole, lansoprazole and pantoprazole with P-glycoprotein

Proton pump inhibitors are a class of drugs which are widely prescribed for acid-related diseases. They are primarily metabolized by CYP2C19 and CYP3A4. It is unknown so far whether proton pump inhibitors are also substrates of the ATP-dependent efflux transporter P-glycoprotein. Moreover, it is not established whether proton pump inhibitors are also inhibitors of P-glycoprotein function. The aim of our study was therefore to characterize omeprazole, lansoprazole and pantoprazole as P-glycoprotein substrates and inhibitors. Polarized transport of these compounds was assessed in P-glycoprotein-expressing Caco-2 and L-MDR1 cells. Inhibition of P-glycoprotein-mediated transport was determined using the cyclosporine analogue PSC-833 (valspodar) as P-glycoprotein inhibitor. Inhibition of efflux transport by omeprazole, lansoprazole and pantoprazole was assessed using digoxin as P-glycoprotein substrate. At concentrations of 5 microM, basal-to-apical transport of omeprazole, lansoprazole and pantoprazole was greater than apical-to-basal transport in Caco-2 and L-MDRI cells. Addition of PSC-833 (1 microM) showed a clear effect only for lansoprazole, suggesting that other transporters contribute to omeprazole and pantoprazole cellular translocation. Furthermore, all of the tested compounds inhibited digoxin transport with IC50 values of 17.7, 17.9 and 62.8 microM for omeprazole, pantoprazole and lansoprazole, respectively. In summary, our data provide evidence that proton pump inhibitors are substrates and inhibitors of P-glycoprotein. These findings might explain some of the drug interactions with proton pump inhibitors observed in vivo.

Experimental urethane anaesthesia prevents digoxin intoxication: electrocardiographic and histological study in rabbit

An electrocardiographic and histological study was performed in rabbit to detect the effects of urethane (ethyl carbamate) intraperitoneal (i.p.) anaesthesia in digoxin intoxication, since it has been previously shown that this anaesthetic and digitalis glycosides exert specific peripheral effects on the cardiovascular system involving central structures of the autonomic nervous system. We observed that i.p. urethane anaesthesia prevented the onset of the electrocardiographic signs of digitalis intoxication, as well as inhibiting the appearance of histological myocardial alterations after treatment with toxic digoxin doses. On the other hand, lethal arrhythmias and severe myocardial damage were observed in animals that had not undergone preliminary urethane anaesthesia. These results indicate that the effect exerted by urethane in preventing the toxic action of digoxin is probably due to a decrease of sympathetic activity in anaesthetized animals by centrally mediated sympathetic inhibition.

Ketanserin inhibits digoxin-induced arrhythmias in the anaesthetized guinea-pig

Digoxin inhibits the membrane-bound ATPase enzyme, resulting in a rise in intracellular sodium and activated outward potassium current, predisposing to arrhythmias. In this study, the effect of ketanserin, thought to block outward potassium currents, was investigated on digoxin-induced arrhythmias. Twenty-four guinea-pigs were studied in four groups (control, ketanserin 0.5 mg/kg, ketanserin 1 mg kg, ketanserin 2 mg/kg). Under pentobarbital anaesthesia (40 mg/kg), 15 min after injection of saline or ketanserin, digoxin (0.6 mg/kg) was administered through the jugular vein. Carotid artery blood pressure and electrocardiogram (ECG) were recorded. The time for the onset of the first arrhythmia and incidence of ventricular tachycardia (VT), ventricular fibrillation (VF), and premature ventricular contraction (PVC) were determined. Arrhythmias were scored according to the MacLeod scale. Ketanserin produced minor haemodynamic effects and lacked, by itself, arrhythmogenic effects at the doses studied. However, it increased the time for the onset of the first digoxin-induced arrhythmia and decreased the incidence of VT, VF and PVC. We conclude that ketanserin inhibits digoxin-induced arrhythmias in guinea-pigs.

A model for the prediction of digoxin-drug interactions at the renal tubular cell level

Digoxin-drug interactions are relatively common causes of digitalis toxicity. Recently, the clinical importance of the renal tubular secretion of digoxin has been proven by documenting drug interactions at this level. The authors describe a model using cultured renal tubular cell monolayers that can be used to predict drug interactions with the cardiac glycoside. This model accurately documents known clinical digoxin interactions such as those with verapamil and propafenone. The common feature of these interactions is that they involve P-glycoprotein substrates (e.g., digoxin, vincristine, vinblastine) or inhibitors (e.g., quinidine, cyclosporine). In the case of the newly described interaction of digoxin with itraconazole, the model preceded the emergence of clinical cases.

Absence of a sertraline-mediated effect on digoxin pharmacokinetics and electrocardiographic findings

The effects of oral administration of sertraline on the plasma concentration profile and renal clearance of digoxin were assessed in 20 healthy male subjects in a double-blind, randomized study.

METHOD

All subjects first received digoxin 0.5 mg twice daily on Day 1, 0.25 mg twice daily on Day 2, and 0.25 mg daily thereafter. On Day 11, 10 subjects began concomitant sertraline administration with an initial dose of 50 mg/day that was titrated upward over 7 days to 200 mg/day, which was given over the remainder of the study period. The other 10 subjects received concomitant digoxin and placebo for 17 days beginning on Day 11. Trough plasma concentrations of digoxin were monitored daily beginning on Day 7. Blood samples and 24-hour urine collections were used to determine steady-state digoxin concentration and renal clearance before, during, and after sertraline coadministration.

RESULTS

Sertraline had no effect on digoxin pharmacokinetics, except for a decrease in the time to reach the maximum plasma digoxin concentration (Tmax) compared with placebo (p = .0046), a finding thought to be of limited clinical significance. Side effects of mild-to-moderate severity were reported by 5 of 10 sertraline-treated subjects and by 6 of 10 placebo-treated subjects.

CONCLUSION

The results of this study suggest that dosing adjustments of digoxin may not be necessary in patients receiving concomitant sertraline administration.

Problems in determining levels of free digoxin in patients treated with digoxin immune FAb

Determination of free digoxin levels in patients treated with digoxin immune FAb has long been a problematic area in clinical laboratory testing. The older radioimmunoassays resulted in inaccurate and variable results due to the competition of the administered drug with the radioactively labelled forms. The 1995 Physicians' Desk Reference continues to state that digoxin immune FAb will interfere with digitalis immunoassay measurements. This statement, however, is based primarily on the RIA methods. We evaluated the Stratus digoxin assay and the TDX digoxin II assay. Increasing amounts of immune FAb were added in a stepwise fashion to 12 patient samples containing high normal to elevated digoxin levels. Results showed a progressive decrease in digoxin levels when assayed with the Stratus kit. However, five patient samples tested with the TDX kit resulted in constant digoxin values despite the presence of increasing digibind levels. These results suggest that the Stratus method measures free digoxin, whereas the TDX method measures the total digoxin. Measurement of digoxin by the Stratus method is simple and quick. The Stratus digoxin assay may be an accurate and objective way of measuring free digoxin levels in patients on digoxin immune FAb.

Magnesium sulfate in the treatment of ventricular arrhythmias due to digoxin toxicity

Although digoxin antibodies are the definitive treatment of cardiac arrhythmias due to digoxin toxicity, magnesium can also be effective especially with low serum magnesium levels. The case report describes a patient with digoxin toxicity, ventricular tachycardia and a slightly elevated serum magnesium. Two 10 mmol doses of intravenous magnesium sulfate were associated with a more stable junctional rhythm with bigeminy. Magnesium is known to suppress early after depolarizations, and in supraphysiological doses, may act as an indirect antagonist of digoxin at the sarcolemma Na(+)-K(+)-ATPase pump. Intravenous magnesium may be used to treat cardiac arrhythmias due to digoxin poisoning where there is likely to be a delay in the availability of digoxin antibodies, even in the presence of elevated serum magnesium.

Effect of R56865 on cardiac sarcoplasmic reticulum function and its role as an antagonist of digoxin at the sarcoplasmic reticulum calcium release channel

1. The effect of R56865 (N-[1-[4-(4-fluorophenoxy)-butyl]-4-piperidinyl]-N-methyl-2- benzothiazolamine) on cardiac sarcoplasmic reticulum (SR) Ca(2+)-release channel function was investigated. The effect of R56865 on [3H]-ryanodine and [3H]-digoxin binding to SR vesicles and its effect on the ATP-stimulated 45Ca2+ uptake into SR vesicles was also studied. 2. R56865 (0.5-50 microM) had no effect on single-channel open probability (Po) when added to native cardiac SR Ca(2+)-release channels, incorporated into planar phospholipid bilayers, that had previously been activated by 10 microM Ca2+. The single-channel conductance (93 pS) and the Ca2+/Tris+ permeability ratio (12.5) were also unaffected by R56865. 3. R56865 failed to affect the rapid Ca(2+)-induced efflux of 45Ca2+ from cardiac SR vesicles. The initial efflux rate at an extravesicular [Ca2+] of 0.1 microM was 176 +/- 33 nmol 45Ca2+ mg-1 protein s-1 (n = 5). Addition of 0.5-50 microM R56865 to the efflux solution did not affect the initial efflux rate or the total amount of 45Ca2+ released from the vesicles. 4. The specific binding of [3H]-ryanodine to SR vesicles can be viewed as a marker for SR Ca(2+)-release channel activation. R56865 (0.05-50 microM) did not change the amount of specific [3H]-ryanodine bound at 10 microM activating Ca2+. Taken together these data (points 2, 3 and 4) suggest that R56865 does not affect the Ca2+ activation of the cardiac SR Ca(2+)-release channel. 5. R56865 (0.5-50 microM) decreased the ATP-stimulated uptake of 45Ca2+ into cardiac SR vesicles. The total amount of 45Ca2+ taken up into the vesicles was decreased by 26-37% and the initial rate of uptake was also reduced.6. R56865 decreased the binding of [3H]-digoxin to cardiac SR vesicles. It inhibited binding of[3H]-digoxin to both sites on SR membranes. However, it acted as a mixed type of non-competitive antagonist, as it increased the Kd and reduced the Bmax for [3H]-digoxin. Additionally, when SRCa2+-release channels incorporated into bilayers were activated by 1 nM digoxin at 10 micro M Ca2+, 5 microMR56865 decreased single-channel Po to that seen prior to activation by digoxin, confirming that R56865is an antagonist at the cardiac glycoside receptor site on the cardiac SR Ca2+-release channel.7. The results presented here show that R56865 decreases 45Ca2+ uptake into SR vesicles and also non-competitively decreases the binding of cardiac glycosides to their activation sites on the SRCa2+-release channel. Since this compound fails to affect other aspects of the functioning of the SR, we conclude that inhibition of Ca2+-uptake into the SR and/or antagonism of digoxin binding to its high affinity sites on the cardiac SR Ca2+-release channel may contribute to the protection against glycoside induced toxicity seen with R56865. Indeed the reduction of Ca2+ uptake into the SR during Ca2+overload may contribute to the protective action of R56865 against arrhythmias induced by veratridine or by ischaemia. Notwithstanding this, it is possible that the multitude of other actions on specific ion translocation processes, as well as the possible non-specific membrane effects of R56865, may be as responsible for the protection against arrhythmias as the effects on SR function we have documented here. However, the simultaneous interference with more than one of these processes by R56865 may be required for its anti-arrhythmic action.

Contribution of a single heavy chain residue to specificity of an anti-digoxin monoclonal antibody

Two distinct spontaneous variants of the murine anti-digoxin hybridoma 26-10 were isolated by fluorescence-activated cell sorting for reduced affinity of surface antibody for antigen. Nucleotide and partial amino acid sequencing of the variant antibody variable regions revealed that 1 variant had a single amino acid substitution: Lys for Asn at heavy chain position 35. The second variant antibody had 2 heavy chain substitutions: Tyr for Asn at position 35, and Met for Arg at position 38. Mutagenesis experiments confirmed that the position 35 substitutions were solely responsible for the markedly reduced affinity of both variant antibodies. Several mutants with more conservative position 35 substitutions were engineered to ascertain the contribution of Asn 35 to the binding of digoxin to antibody 26-10. Replacement of Asn with Gln reduced affinity for digoxin 10-fold relative to the wild-type antibody, but maintained wild-type fine specificity for cardiac glycoside analogues. All other substitutions (Val, Thr, Leu, Ala, and Asp) reduced affinity by at least 90-fold and caused distinct shifts in fine specificity. The Ala mutant demonstrated greatly increased relative affinities for 16-acetylated haptens and haptens with a saturated lactone. The X-ray crystal structure of the 26-10 Fab in complex with digoxin (Jeffrey PD et al., 1993, Proc Natl Acad Sci USA 90:10310-10314) reveals that the position 35 Asn contacts hapten and forms hydrogen bonds with 2 other contact residues. The reductions in affinity of the position 35 mutants for digoxin are greater than expected based upon the small hapten contact area provided by the wild-type Asn. We therefore performed molecular modeling experiments which suggested that substitution of Gln or Asp can maintain these hydrogen bonds whereas the other substituted side chains cannot. The altered binding of the Asp mutant may be due to the introduction of a negative charge. The similarities in binding of the wild-type and Gln-mutant antibodies, however, suggest that these hydrogen bonds are important for maintaining the architecture of the binding site and therefore the affinity and specificity of this antibody. The Ala mutant eliminates the wild-type hydrogen bonding, and molecular modeling suggests that the reduced side-chain volume also provides space that can accommodate a congener with a 16-acetyl group or saturated lactone, accounting for the altered fine specificity of this antibody.

BRL 38227--a potassium channel opener, antagonizes digoxin-induced convulsions

Experimental evidence suggests that potassium channel openers play an important role in convulsions. In this study, the anticonvulsant activity of BRL 38227, a new potassium channel opener against digoxin-induced convulsions, is reported. Intraventricular administration of digoxin (7.5 micrograms), included "popcorn-type" convulsions in rats. BRL 38227, injected centrally increased the onset time of convulsions and decreased the mortality rate in a dose-dependent manner. Pretreatment with 4-aminopyridine, a potassium channel blocker antagonized the protective effect of BRL 38227. These findings show the involvement of potassium channels in digoxin-induced convulsions. Further these results indicate that in the future potassium channels might be a target for new anticonvulsant drugs.

Cyclosporin and quinidine inhibition of renal digoxin excretion: evidence for luminal secretion of digoxin

We studied the in vivo luminal and contraluminal uptake of [3H]digoxin in dog kidney using the single-pass multiple indicator dilution method. A bolus tracer of 125I-albumin (plasma reference), creatinine, or L-[14C]glucose [extracellular reference (ecf)] and [3H]digoxin (or [3H]ouabain) was injected into the left renal artery, and timed serial samples were collected from the left renal vein (basolateral uptake) and left and right ureters (luminal uptake). [3H]ouabain was excreted solely by filtration and exhibited saturable and irreversible binding at the basolateral surface. Uptake of [3H]digoxin across the basolateral membrane was large and nonsaturable. Despite urine flow-dependent reabsorption and approximately 20% protein binding, the urine recovery ratio for [3H]-digoxin/glomerular (ecf) marker was 0.97 +/- 0.04 (n = 29), indicating net digoxin secretion. After intravenous infusions of cyclosporin in Cremophor EL (0.5-3.5 microM), the urine recovery ratio decreased in a dose-dependent manner from control values of 1.13 +/- 0.06 (n = 12) to 0.62 +/- 0.03 (n = 14). There was no change in the relative renal vein recovery. Left renal artery infusion of quinidine (37.5 micrograms.min-1.kg-1) decreased the relative urine recovery of [3H]digoxin by 46% (n = 6) but had no effect on postglomerular extraction. Cyclosporin and quinidine are known inhibitors of P-glycoprotein. But digoxin did not compete with [3H]azidopine for binding in rat brush-border membranes or membranes prepared from the multidrug-resistant cell line CHRC5. The exact mechanism for renal digoxin secretion remains to be determined, but our results point to a luminal localization of this secretory system.

Morphine and the Endogenous Opioid Dynorphin in the Brain Attenuate Digoxin-Induced Arrhythmias in Guinea Pigs

The effects of the opioid receptor agonists morphine and dynorphin on digoxin-induced arrhythmias were examined in guinea pigs that had received intravenous digoxin (50 mu/kg bolus plus 500 mu/kg/hr intravenously). Animals received either morphine (50 or 100 micrograms/kg) or dynorphin A(1-13) (50 or 100 micrograms/kg) or saline (the diluent) into the lateral cerebral ventricle (intracerebroventricularly) prior to digoxin. Morphine and dynorphin produced significant (P < 0.05) dose-dependent increases in the threshold of digoxin-induced arrhythmias. The mean digoxin dose at the development of fatal arrhythmias was 775 +/- 42 micrograms/kg in the control group but was significantly higher namely 958 +/- 45 micrograms/kg after 100 micrograms/kg of morphine ICV, and 984 +/- 47 micrograms/kg after 100 micrograms/kg of dynorphin A (1-13) intracerebroventricularly. In the absence of digoxin, the highest doses of each of these opioids did not produce arrhythmias. Changes in blood pressure and heart rate were unlikely explanations for the observed actions of these opioids as morphine accentuated the increase in blood pressure that accompanied digoxin while dynorphin was associated with a lower blood pressure with digoxin, despite similar effects on arrhythmias. In the control group, fatal digoxin-induced arrhythmias were ventricular tachyarrhythmias in two-thirds of cases and complete heart block in the rest. Morphine and dynorphin reduced the development of ventricular tachyarrhythmias. The role of the cholinergic system was explored, with morphine, utilizing atropine sulfate which crosses the blood brain barrier and atropine methylnitrate which does not enter the CNS. Atropine sulfate but not atropine methylnitrate reversed the effects of morphine on digoxin-induced arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cyclooxygenase and lipoxygenase inhibitors on digoxin-induced arrhythmias and haemodynamics in guinea-pigs

Effects of cyclooxygenase and lipoxygenase inhibitors of digoxin-induced arrhythmias and haemodynamics were studied in guinea-pigs. ECG, mean arterial blood pressure heart rate, pressure rate index and arrhythmias were recorded, starting 15 min before digoxin administration and continuing for 30 min afterwards. The cyclooxygenase inhibitor aspirin (50 mg kg-1) and the dual cyclooxygenase/lipoxygenase inhibitor BW 755C (0.25-10.0 mg kg-1) were found to produce a significant protection against the arrhythmias, whereas aspirin (100 mg kg-1) and CGS 8515 were found to be ineffective. SK&F 104 353, a potent and selective peptidoleukotriene receptor antagonist significantly attenuated the arrhythmias and mortality in a dose-dependent manner. It is concluded that production of both cyclooxygenase and lipoxygenase metabolites could favour the occurrence and/or the maintenance of digoxin-induced cardiac toxicity.

Inhibition by basic drugs of digoxin secretion into human bile

Bile samples from previous interaction studies in man were re-analysed by a combined HPLC/radioimmunoassay method. Quinidine, quinine and verapamil but not probenecid or spironolactone were found to inhibit the biliary secretion of unchanged digoxin.

Digoxin-specific Fab fragments in the treatment of oleander toxicity in a canine model

STUDY OBJECTIVE

To examine the efficacy of digoxin-specific Fab fragments (dsFab) in the treatment of experimentally induced Nerium oleander cardiac glycoside toxicity in a dog model.

DESIGN

A nonblined, placebo-controlled experiment.

SUBJECTS

Ten adult greyhound dogs of either sex divided into treatment and control groups of five dogs each.

INTERVENTIONS

A tincture of oleander was prepared and administered intravenously to each animal. After the onset of cardiotoxicity, the treatment group received 60 mg/kg dsFab IV.

MEASUREMENTS AND MAIN RESULTS

All dogs exhibited dysrhythmias meeting our criteria for cardiac glycoside cardiotoxicity within 27 minutes of beginning the infusion. Three of five control dogs had lethal dysrhythmias during the three-hour observation period. The remaining two control dogs exhibited dysrhythmias throughout the three-hour experiment. All five of the dsFab-treated dogs survived and converted to normal sinus rhythm within eight minutes of dsFab infusion. Three treatment animals reverted back to nonlethal and hemodynamically stable dysrhythmias after a mean of 107 minutes.

CONCLUSION

Large doses of dsFab are efficacious in the treatment of dysrhythmias in this canine model of N oleander cardiac glycoside poisoning.

Effect of aminophylline preperfusion on digoxin-induced cardiac arrest in isolated frog heart

Digoxin (DGN) and aminophylline (theophylline ethylenediamine, APH) being frequently prescribed cardioactive drugs, the present study investigated the effect of APH (10(-4) M) preperfusion on DGN-cardiotoxicity employing the isolated frog heart preparation. The mean DGN perfusion time (sec) and mean DGN exposure (microgram/10 mg heart wt.) for cardiac arrest were the parameters studied. APH preperfusion caused a significant elevation in both the parameters, signifying that it afforded protection against DGN-cardiotoxicity. This protective effect was not observed with the preperfusion of ethylenediamine (EDA) instead of APH, which led to the inference that the protective effect of APH was solely due to its theophylline component. The present finding that APH-pretreatment might modulate DGN-cardiotoxicity, of considerable pharmaco-toxicological interest.

GABA/BZ-and NMDA-receptor interaction in digoxin-induced convulsions in rats

Digoxin (7.5 micrograms icv) induced 'pop-corn' type of convulsions and 100% mortality. The GABA-ergic agents produced varying degree of protection against digoxin-induced neurotoxicity. Diazepam (4 mg/kg) offered significant protection whereas pentobarbital (5 mg/kg) and baclofen (5 mg/kg) markedly reduced per cent mortality, but ethanol (2 g/kg), progabide (50 mg/kg) and muscimol (0.5 mg/kg) as well as GABA (50 mg/kg) could not offer significant protection in doses used. GABA-ergic agonists; GABA, baclofen, diazepam and pentobarbital when administered along with MK-801 (0.5 mg/kg) a non-competitive NMDA antagonist, a potentiation of anticonvulsant action of MK-801 was observed. MK-801 showed potent anticonvulsant profile in dose range (0.25-1 mg/kg) studied. A synergistic influence of Mg2+ and K+ ions on NMDA receptor antagonism was also observed. A role of GABA-ergic facilitation and NMDA antagonism as a potential anticonvulsant approach in digoxin-induced convulsions in rats has been suggested.

Cardiac arrhythmias are ameliorated by local inhibition of angiotensin formation and bradykinin degradation with the converting-enzyme inhibitor ramipril

We investigated the influence of the angiotensin-converting enzyme (ACE) inhibitor ramipril on cardiac arrhythmias in guinea pigs and rats. Ramiprilat, the active moiety of ramipril, did not influence action potentials of isolated guinea-pig papillary muscle or rabbit sinus node, thereby excluding cellular electrophysiological evidence of anti-arrhythmic properties. Ramipril protected against cardiac arrhythmias induced by digoxin infusion in guinea pigs. This effect was comparable with that of lidocaine. In isolated perfused ischemic working rat hearts, angiotensin (ANG) I (3 x 10(-9) M/l) and ANG II (1 x 10(-9) M/l) aggravated reperfusion arrhythmias, accompanied by deterioration of cardiodynamic and metabolic events. Bradykinin (BK) (1 x 10(-10)-1 x 10(-8) M/l), in contrast, protected against reperfusion arrhythmias, which corresponded to an increase in energy-rich phosphates and glycogen stores and a decrease in lactate levels in myocardial tissue. Identical changes were seen in hearts from rats pretreated with ramipril (1 mg/kg PO) or perfused with ramiprilat (2.58 x 10(-7)-2.58 x 10(-5) M/l). Local ACE inhibition in these ischemic hearts antagonized ANG I but not ANG II effects and enhanced BK effects. The BK antagonist D-Arg-(Hyp2, Thi5,8, D-Phe7)BK abolished the beneficial effects of BK, ramipril, and ramiprilat. Increased concentrations of BK or ramiprilat were able to reverse the antagonism. The antiarrhythmic agent nicainoprol, a fast-sodium-channel blocking drug (class Ib), also protected isolated rat hearts against reperfusion arrhythmias, but was without beneficial effects on cardiac hemodynamics and biochemical parameters, in contrast to the ACE inhibitor. These results suggest that the beneficial effects of the ACE inhibitor ramipril on digoxin and reperfusion arrhythmias are not mediated by their direct actions on ionic channels in the cell membrane. It seems that other factors are responsible for its beneficial effects on reperfusion arrhythmias, cardiac function, and metabolism, which are associated with a reduction in ANG II generation and BK degradation by local ACE inhibition in the heart.

Modification of the positive inotropic effects of catecholamines, cardiac glycosides and Ca2+ by the orally active male contraceptive, gossypol, in isolated guinea-pig heart

Gossypol is an orally active male contraceptive with cardio-depressant side effects. To understand the mechanism of its cardiac actions, the interaction of gossypol with positive inotropic drugs was examined in isolated atrial muscle preparations obtained from guinea-pig heart. Gossypol delayed the onset of arrhythmias caused by digoxin. In the presence of gossypol, the positive inotropic effect of isoproterenol declined rapidly, and the effect of isoproterenol to increase tissue cyclic AMP concentrations was smaller. Pretreatment of atrial muscle with the combination of gossypol and isoproterenol markedly reduced effects of isoproterenol on developed tension and cyclic AMP concentrations when these effects were tested after the washout of the first dose of isoproterenol. These effects, however, were not specific to isoproterenol. The gossypol-isoproterenol pretreatment reduced the positive inotropic effect of ouabain or extracellular Ca2+. These results indicate that gossypol has pharmacodynamic interactions with several positive inotropic agents that are known to enhance developed tension by increasing intracellular Ca2+ transients.

Reevaluation of digoxin-encainide interactions using an animal model

The effects of intravenous encainide on digoxin-induced atrial ectopic tachycardia (AET) were investigated in the rat using 3-channel simultaneous limb-lead electrocardiography. Pentobarbital-anesthetized (35 mg/kg, intraperitoneal) adult male rats were given digoxin subcutaneously, 30 mg/kg. After onset of AET, rats received either saline (0.5 ml/kg) or encainide; 0.25, 0.5, 1.0, or 2.0 mg/kg intravenously in repeated doses at 15-min intervals. At all doses, encainide converted digoxin-induced AET to ventricular arrhythmias, prolonged recovery time, and increased mortality in comparison to saline-treated animals. An additional group of anesthetized rats was not given digoxin. These animals received encainide (2.0 mg/kg, intravenously) in repeated doses at 15-min interval and developed dose-related increase in the P-R interval only. Blood samples were obtained by cardiac puncture from 12 additional anesthetized, digoxin-treated rats 5 min after the fourth intravenous dose of saline (0.5 ml/kg, n = 6) or encainide (1.0 mg/kg, n = 6). Serum was prepared and analyzed by affinity column-mediated immunoassay. Digoxin levels were the same in both groups. These results suggest that encainide may exacerbate digoxin-induced arrhythmias (proarrhythmic effect) in this species. In view of our findings of digoxin-encainide interactions in the rat, we recommend caution if these drugs are coadministered in humans.

Digoxin-induced cardiac toxicity in the anaesthetized guinea-pigs and effect of heparin infusion

We have examined the effects of heparin infusion on the arrhythmias induced by digoxin. Digoxin treatment consisted of 0.6 mg kg-1 given i.v. 15 min after the beginning of heparin infusion. Heparin infusions (1.7 IU, 3.4 IU and 6.8 IU kg-1 min-1) were begun 15 min before digoxin injection and continued for another 30 min. ECG, blood pressure, heart rate and arrhythmias were recorded starting 15 min before and continuing for 60 min after the digoxin injection. Heparin at the infusion rates of 3.4 IU and 6.8 IU kg-1.min-1 reduced significantly the arrhythmia scores. On the other hand, heparin did not significantly alter the arterial blood pressure and heart rate values affected by digoxin.

Will treatment with digoxin antibody benefit pregnant patients with toxemia and elevated digoxin like factor?

Some pregnant patients with toxemia, especially those with thrombocytopenia, liver or renal dysfunction have elevated serum DLF (digoxin like factor). Since antidigoxin serum reduces the hypertension of DOCA hypertensive rats, it is proposed that antidigoxin antibodies be tried in the treatment of these select patients with toxemia of pregnancy.

The effect of D-Ala-2-Me-Phe-4-Met-(0)-Ol enkephalin on blood pressure, heart rate and digoxin-induced arrhythmias in the guinea pig

The cardiovascular effects of D-Ala-2-Me-Phe-4-Met-(0)-Ol enkephalin were investigated after its administration into the fourth cerebroventricle of the guinea pig. This enkephalin is a synthetic Met-enkephalin analog that is more resistant to degradation and has a high affinity for opioid receptors. It produced a significant increase in blood pressure and decline in heart rate. At high concentrations, 100 micrograms/kg plus 100 micrograms/kg/hr in the fourth ventricle, it produced bradyarrhythmias that were sometimes fatal. At doses that did not alter survival or produce arrhythmia, namely 30 micrograms/kg plus 25 micrograms/kg/hr in the fourth ventricle, the response to digitalis was assessed. Significant leftward shifts in the relationships between digoxin and arrhythmia occurrence and development of fatal arrhythmias were observed. Thus, D-Ala-2-Me-Phe-4-Met-(0)-Ol enkephalin has definite cardiovascular effects that include potentiation of digoxin arrhythmias.

Functional antagonism by monoclonal antibody to digoxin in the guinea-pig atria

A monoclonal antibody with a high affinity for digoxin (Ka = 1.14 X 10(9) M) was produced by somatic cell fusion. This antibody, designated 35/6.1, displayed little or no cross reactivity with other steroids. In the guinea-pig atria, 35/6.1 antagonized functionally the positive inotropic effect of digoxin but did not alter that of isoproterenol. Our results suggest that this monoclonal antibody may represent a useful probe for studying the physiological role of the putative endogenous digoxin-like factor in various pathophysiological conditions.

Effects of ryanodine on inotropic and arrhythmogenic actions of cardiotonic steroids

In addition to its role in myocardial excitation-contraction coupling, the sarcoplasmic reticulum may be involved in arrhythmogenic actions of cardiotonic steroids. This study in isolated cardiac muscle demonstrates that ryanodine can prevent the arrhythmias and reduce the positive inotropic effect produced by cardiotonic steroids without affecting their specific binding to sites on Na,K-ATPase. These data suggest that the antagonism is mediated by ryanodine-induced alterations in Ca2+ release from the sarcoplasmic reticulum.

Effects of BAY K-8644 on inotropic and arrhythmogenic actions of digoxin

Myocardial intracellular 'Ca2+ overload' may be involved in the direct arrhythmogenic actions of cardiotonic steroids. This proposal was examined by determining if the sensitivity of guinea-pig atrial muscle to digoxin-induced arrhythmias was affected by BAY K-8644, a 1,4-dihydropyridine derivative which promotes Ca2+ influx via slow channels. BAY K-8644 significantly reduced both the time required for a given concentration of digoxin to produce arrhythmias and the amount of digoxin bound to atrial muscle at the onset of arrhythmias. In addition, BAY K-8644 increased the maximum developed tension observed in the presence of digoxin before the onset of arrhythmias. Similar results were obtained with increasing concentrations of buffer Ca2+. In contrast, A23187, a Ca2+ ionophore, enhanced the sensitivity to digoxin-induced arrhythmias without affecting maximum developed tension. These results suggest that increases in intracellular Ca2+ enhance cardiac sensitivity to digoxin-induced arrhythmias and that the arrhythmogenic action may involve Ca2+ overload at a pool other than that which activates contractile proteins.

Modification of the inotropic effect of digoxin by diazepam in rat left atria

There is a pharmacokinetic interaction between digoxin and diazepam that increases the elimination half-life of digoxin. It may be due to a reduction of digoxin tissue concentrations and to an enhanced effect of diazepam on digoxin binding to plasma albumin. Diazepam (10(-5) M) also induces a positive inotropic effect in guinea-pig isolated atria. In a study of a possible pharmacodynamic interaction between both drugs, the inotropic response to digoxin has been examined in rat isolated atria in the presence of diazepam. The atria were kept in Tyrode at 37 degrees C, bubbled with 95% O2 and 5% CO2 and electrically stimulated at twice the threshold voltage. The results indicate that diazepam induces an inotropic effect at 10(-5) M (P less than 0.05) and reduces (P less than 0.05) at 10(-9), 10(-7) and 10(-5) M the inotropic response to digoxin (10(-5) M).

Prazosin alters free and total plasma digoxin levels in dogs

Digoxin has been associated with a variety of drug interactions. Studies in our laboratory indicate that single doses of prazosin may alter plasma steady-state digoxin levels. Adult mongrel dogs were given digoxin tablets orally (0.008 mg/kg, twice daily) until steady-state levels of digoxin were reached. Dogs were then tested in a cross-over study with prazosin and saline. Plasma samples were assayed for both free and total plasma digoxin via radioimmunoassay. Free drug was separated from protein bound drug utilizing a micropartition system. Plasma binding of digoxin as well as nonspecific tissue binding was reduced by prazosin and resulted in an increased fraction of free digoxin in plasma. The increase in free digoxin leads to an increase in the pool of digoxin available for pharmacologic activity. Acute experiments demonstrated such an increased pharmacologic effect of digoxin in the presence of prazosin. Prazosin pretreatment prior to a continuous digoxin infusion significantly increased the positive inotropic effect of digoxin compared to dogs receiving only digoxin.

Inhibition of platelet aggregation by oral digoxin and reversal with nifedipine

To study the effect of cardiac glycosides on platelet function, we obtained serial blood samples from 18 normal male volunteers before and 3, 5, and 7 weeks after beginning digoxin therapy (0.375 mg daily). The combined effect of digoxin and nifedipine (mean dose, 57 mg/day) was assessed during the 5th week. Spontaneous platelet aggregation and platelet response to adenosine diphosphate (ADP) were measured in whole blood by electronic particle sizing. Digoxin (mean serum concentration, 0.9 ng/ml) caused significant reduction in total volume and mean size of platelet aggregates formed in response to ADP. However, with addition of nifedipine, the volume and mean size of aggregates returned to baseline measurements. In vitro administration of digoxin to whole blood failed to inhibit ADP-induced platelet aggregation. The volume of spontaneously induced aggregates decreased with digoxin; however, the decrease was not statistically significant. These data indicate that digoxin given in vivo for several weeks inhibits platelet response to ADP; this effect is reversed by the addition of nifedipine.

Influence of verapamil on the digoxin-induced inotropic effect in guinea-pig isolated atria

The nature of the digoxin-verapamil interaction was explored "in vitro" by determining the effect of verapamil on digoxin-induced inotropism in isolated guinea-pig atria. Verapamil, at concentrations (2.10(-8)-3.10(-8)M) too low to have any significant effect on the amplitude or frequency of the basal contractile activity, reduced the inotropic action of digoxin in all experimental circumstances, i.e. in spontaneously beating atria from reserpine-treated or untreated animals, in electrically driven (at 1 Hz frequency stimulation) left atria. The results obtained suggest that a competitive antagonism exists between verapamil and digoxin. In spontaneously beating atria an apparently competitive antagonism was also observed between verapamil and ouabain. The interaction observed between cardiac glycosides and the Ca++ channel blocker suggests that Ca++ movements across the sarcolemma may influence the inotropic effect of digitalis or its toxicity.

Digoxin-specific antibody fragments and a calcium antagonist for reversal of digoxin-induced mesenteric vasoconstriction

The effect of digoxin-specific antibody fragments on glycoside-induced mesenteric vasoconstriction were investigated. Digoxin caused a sustained contraction of strips of isolated feline mesenteric artery lasting for several hours, while in anaesthetized cats it produced a significant decrease in blood flow and increase in resistance in the mesenteric artery. In-vitro, digoxin's contractile effect was inhibited by 'prophylactic' addition of antibody to the organ bath, but the clinical use for prophylaxis is not a practical proposition. When the antibodies were added with the contraction of the arterial strip in response to digoxin already established, the tone of the preparation decreased significantly over 3 h, but the effect of the glycoside was not fully reversible. In-vivo, control animals not treated with antibodies developed arrhythmias, mesenteric blood flow fell by more than 50% and resistance increased by more than 80% relative to the initial values. These animals died of ventricular fibrillation before the end of the experiment. Animals treated with digoxin-specific antibody fragments after receiving digoxin injections showed no further decrease in mesenteric blood flow and 90 min after the last dose of digoxin, the flow was recovering and mesenteric resistance decreasing. Furthermore, all the animals that had received antibodies remained in sinus rhythm to the end of the experiment. In view of the latent period to onset of action of the antibodies, valuable time may be lost in impaired mesenteric blood flow. To bridge the gap or, indeed, as primary treatment, calcium antagonists merit consideration; in our experiments mesenteric vasoconstriction was abolished within a few minutes by application of the dihydropyridine calcium antagonist 4-(2,1,3-benzo-oxadiazol-4-yl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylic aid, diethyl ester (PY 108-068).

Mechanism of additive effects of digoxin and quinidine on contractility in isolated cardiac muscle

To evaluate the mechanism of the effect of the interaction of digoxin and quinidine on myocardial contractility, ferret right ventricular papillary muscles were isolated and the effects of digoxin, 4 x 10(-7) M, quinidine, 1 x 10(5) M and atropine, 1.5 x 10(-6) M, on peak developed force, peak rate of development of force (dF/dt) and time to peak tension were determined. The addition of quinidine to muscles treated with digoxin increased developed force 18 percent (p = 0.006) and dF/dt 35 percent (p = 0.001) without significantly changing time to peak tension. This effect was abolished by pretreatment with atropine. Quinidine alone increased developed force 35 percent (p less than 0.001) and dF/dt 70 percent (p less than 0.001) and decreased time to peak tension 22 percent (p less than 0.001) from pretreatment control values. Atropine alone increased developed force 17 percent (p = 0.02) and dF/dt 32 percent (p = 0.001) and decreased time to peak tension 13 percent (p = 0.003) from pretreatment control values. The addition of quinidine to muscles treated with atropine or of atropine to muscles treated with quinidine did not significantly change developed force, dF/dt or time to peak tension from values with either drug alone. It is concluded that digoxin and quinidine in these doses have additive effects of myocardial contractility, and that this interaction is at least partially mediated through antagonism of cholinergic influences by quinidine.

Canrenoate reversal of inhibitory effects of digoxin on basal and furosemide-stimulated renin secretion

Changes in plasma renin activity (PRA) were monitored in six mildly hypertensive men after intravenous doses, in seven separate experiments, of placebo, digoxin, potassium canrenoate, potassium canrenoate with digoxin, furosemide, furosemide with digoxin, and potassium canrenoate with furosemide and digoxin. Potassium canrenoate has been used as a rapid source of canrenone, which has been recently shown to be a competitive antagonist of ouabain at its Na-K-ATPase receptor site. Potassium canrenoate infusion reversed the hyporeninemic effect of digoxin. This result has been taken as evidence that: (1) antialdosteronic drugs can also reverse digoxin effects at extracardiac level and (2) the Na-K-ATPase system is involved in the renin secretory mechanism. A seemingly identical reversal of the hyporeninemic effect of digoxin was induced by furosemide, which, when given alone, stimulated renin secretion. The simultaneous administration of potassium canrenoate, digoxin, and furosemide induced an increase in PRA on the same order as that after furosemide alone. This result indicates that furosemide stimulates renin release by affecting a biochemical system other than that affected by digoxin.

Involvement of endogenous digitalis-like substance in genesis of deoxycorticosterone-salt hypertension

In an attempt to evaluate the role of endogenous digitalis-like substance in the genesis of deoxycorticosterone (DOCA) -salt hypertension, the effects of intravenous anti-digoxin antibody on the blood pressure response were observed in male Wistar rats that underwent heminephrectomy followed by treatment with DOCA and saline. Administration of anti-digoxin antibody caused a marked decrease in the blood pressure which continued for about an hour. Such a change in the blood pressure was not observed in pertinent control animals. Thus, it seems that endogenous digitalis-like substance plays an important role in the genesis of DOCA-salt hypertension.

Cardiac drug overdose

Toxicity from cardiac drugs is a particular management challenge since the manifestations of an acute overdose and the initial indications for the drug are often similar. Plasma drug levels are essential but must be interpreted in light of the clinical picture. Hypotension, due to either vasodilatation or decreased myocardial contractility, and arrhythmias are the principal cardiac manifestations, but noncardiac effects are sometimes more troublesome. An antiarrhythmic agent of the same class should not be used in treating an arrhythmia resulting from an overdose.

Amiloride-induced changes in digoxin dynamics and kinetics: abolition of digoxin-induced inotropism with amiloride

Digoxin dynamics and kinetics were studied in six healthy subjects with and without amiloride. Amiloride increased mean renal digoxin clearance from 1.3 to 2.4 ml . kg-1 . min-1 (p less than 0.001) due to increased tubular secretion of digoxin, while the glomerular filtration rate was unchanged. This might be caused by an increase in intracellular potassium concentration in the tubular cells provoked by amiloride. In contrast, the extrarenal clearance of digoxin was almost blocked by amiloride; it fell from a mean of 2.1 to 0.2 ml . kg-1 . min-1 (p less than 0.025). Total body clearance tended to fall, but the decrease was not statistically significant. EValuation of myocardial contractility by systolic time intervals revealed a concentration-response relationship between digoxin and changes in preejection period index when digoxin was given alone (rs = 0.750, p less than 0.001). Pretreatment with amiloride abolished this relationship (rs = 0.307, p = NS). Blood pressure and echocardiographically determined left ventricular end-diastolic diameter measurements indicated no changes in the left ventricular post- and preload. It is concluded that amiloride suppressed digoxin-induced inotropism.

Reduction of digoxin-induced inotropism during quinidine administration

A kinetic and dynamic study of digoxin was performed in 6 healthy subjects, and repeated in the same subjects after administration of quinidine for 1 wk. Myocardial performance evaluated by systolic time intervals increased in parallel with plasma digoxin concentration, whereas left ventricular end-diastolic diameter on echocardiography and arterial blood pressure remained constant. The positive inotropic effect of digoxin was abolished during concomitant treatment with quinidine. Quinidine has been reported to increase the risk of digitoxicity, and therefore the treatment with digoxin and quinidine in combination should be reconsidered.