Skip Navigation

Cardiovascular Research 2003 57(1):101-108; doi:10.1016/S0008-6363(02)00650-8
© 2003 by European Society of Cardiology
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Pitt, A. D
Right arrow Articles by Whalley, D. W
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Pitt, A. D
Right arrow Articles by Whalley, D. W
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

Copyright © 2003, European Society of Cardiology

Chronic amiodarone-induced inhibition of the Na+–K+ pump in rabbit cardiac myocytes is thyroid-dependent: comparison with dronedarone

Andrew D Pitta, Clyne Fernandesa,c, Nerida L Bewicka, Paul D Hemswortha, Kerrie A Buhagiara,c, Peter S Hansena,c, Helge H Rasmussena,c, Leigh Delbridgeb and David W Whalleya,c,*

aDepartment of Cardiology, Royal North Shore Hospital, Pacific Highway, St. Leonards, Sydney, NSW 2065, Australia
bDepartment of Surgery, Royal North Shore Hospital, Pacific Highway, St. Leonards, Sydney, NSW 2065, Australia
cUniversity of Sydney, Sydney, NSW 2006, Australia

* Corresponding author. Tel.: +61-2-9926-8686; fax: +61-2-9906-7807. dwhalley{at}bigpond.net.au

Received 7 June 2002; accepted 13 August 2002


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Objective: To examine the thyroid-dependence of the effect of amiodarone on the sarcolemmal Na+–K+ pump, and the effect on the pump of dronedarone, a deiodinated amiodarone congener without influence on thyroid status. Methods: New Zealand white rabbits underwent total thyroidectomy, sham thyroidectomy or thyroidectomy and concomitant oral amiodarone therapy. After 5 weeks, Na+–K+ pump current was measured using the whole-cell patch-clamp technique in isolated ventricular myocytes. Pump current was also measured in myocytes isolated from a separate group of rabbits not subjected to thyroidectomy but treated with dronedarone, or placebo for 3 weeks. Results: Treatment of thyroidectomised rabbits with amiodarone caused a significant prolongation of the corrected QT interval (QTc) and sinus cycle length. Na+–K+ pump current measured in myocytes isolated from thyroidectomised rabbits was significantly lower than pump current in myocytes from sham-operated controls. However, treatment of thyroidectomised rabbits with amiodarone did not cause any additional decrease in pump current. Treatment with dronedarone caused prolongation of QTc. However, it had no effect on Na+–K+ pump current. Conclusions: The inhibitory effect of amiodarone on Na+–K+ pump current is thyroid-dependent, whereas the effects on heart rate and QTc are at least partially mediated by thyroid-independent mechanisms. In contrast to its parent compound, dronedarone has no significant effects on the activity of the Na+–K+ pump.

KEYWORDS Antiarrhythmic agents; Hormones; Membrane currents; Na/K-pump


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
Amiodarone, an iodinated, lipophilic benzofuran derivative, is widely used in the treatment of ventricular tachyarrhythmias and atrial fibrillation [1,2]. The drug's anti-arrhythmic efficacy is usually attributed to effects on membrane ion channels. However, treatment with amiodarone also induces inhibition of the sarcolemmal Na+–K+ pump [3–5]. This is expected to alter transmembrane electrochemical ion gradients and may make an important contribution to the electrophysiological properties of the drug. The mechanism for the inhibitory effect of amiodarone on the pump has not been established.

It has been suggested that amiodarone produces many of its chronic electrophysiological effects in cardiac tissue, at least in part, by inducing a state of ‘cellular hypothyroidism’ [6,7]. This suggestion is supported by several lines of evidence. Firstly, the effects of the drug on action potential duration (APD), atrial and ventricular refractoriness, corrected QT interval (QTc), heart rate and systolic time intervals all mimic those observed with hypothyroidism [8]. Furthermore, prolongation of APD and QT interval may be prevented by concomitant administration of thyroid hormone [6,7,9]. Finally, amiodarone inhibits peripheral conversion of thyroxine (T4) to the biologically active thyroid hormone (T3) [10] and competes with T3 for binding to its nuclear receptor, as does desethylamiodarone (DEA), the active metabolite of amiodarone [11,12].

Since thyroid hormone is an important regulator of Na+–K+ pump synthesis [13] amiodarone-induced Na+–K+ pump inhibition might be thyroid-dependent. However, pump inhibition might also be secondary to direct effects on the sarcolemma because amiodarone, is highly lipophilic and alters membrane fluidity with chronic treatment [14]. Changes in membrane fluidity, in turn, may alter pump function [15].

We have previously reported that in euthyroid rabbits chronic treatment with amiodarone reduces Na+–K+ pump current (Ip) by 33% [5]. The aim of the present study was to examine the thyroid-dependence of the amiodarone-induced reduction in Ip. We have measured electrogenic Na+–K+ pump current in cardiac myocytes isolated from thyroidectomised rabbits. There was no effect on Ip when thyroidectomised hypothyroid rabbits were treated with amiodarone. This suggests amiodarone-induced pump inhibition is thyroid-dependent rather than due to a direct effect of amiodarone on membrane properties. To gain independent support for this we also examined the effect on Ip of dronedarone, a deiodinated amiodarone congener with minimal effects on thyroid hormone metabolism. Treatment of euthyroid rabbits with dronedarone had no effect on Ip. Taken together the findings suggest effects of amiodarone on the Na+–K+ pump are thyroid-dependent.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
2.1 Treatment protocols
Male New Zealand white rabbits of weight 2.5–3.2 kg were assigned to one of five groups. Three experimental groups were used to examine the effect of amiodarone: thyroidectomised rabbits (TX-CON), thyroidectomised rabbits treated with amiodarone (TX-AM), and sham-operated rabbits (SHAM). Two groups of euthyroid rabbits were used to examine the effect of dronedarone. One group was treated with the active drug while the other group was given placebo capsules. All animal procedures conformed with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No 85-23, revised 1996).

2.2 Amiodarone treatment protocol (SHAM, TX-CON and TX-AM groups)
Blood samples were taken from the marginal ear vein to measure baseline levels of serum Ca2+ and free-T3 1 day before surgery in all rabbits. Rabbits were anaesthetised with 2.5% halothane. A longitudinal incision was made ventral to the trachea. The pretracheal fascia was retracted to access the right and left lobes of the thyroid gland and minimise the risk of damage to the parathyroid glands. Once isolated from the surrounding soft tissue, the thyroid was removed from rabbits assigned to the TX-CON and TX-AM groups. In the SHAM group the procedure was identical except that the thyroid gland was left intact. Rabbits were monitored closely during recovery to determine the need for post-operative analgesia. The number of experimental groups and the number of rabbits in each of these was limited to the minimum necessary to achieve the objectives of the study. Serum Ca2+ was measured 1 day and 1 week post-operatively to monitor for disruption of parathyroid gland function. T3 assays were carried out on blood from all animals 1 week post-surgery using an ACS180 automated immunoanalyser (Ciba Corning Diagnostics Corp., Medfield, MA) and chemiluminescence to confirm the development of hypothyroidism in thyroidectomised animals, and maintenance of euthyroid state in the SHAM group.

Treatment with amiodarone was commenced 7–9 days post-thyroidectomy. Amiodarone was provided as a hydrochloride powder by Sanofi-Synthelabo (France), and was administered orally in gelatin capsules. Serum amiodarone levels were measured after 2 weeks of treatment, and again 1 day before sacrifice using the HPLC method of Law et al. [16]. In preliminary experiments on four thyroidectomised rabbits we used the same dose of amiodarone (80 mg/kg/day) as that used in a previous study demonstrating the inhibitory effect of amiodarone on Ip in rabbits who had not undergone thyroidectomy [5]. In the thyroidectomised rabbits, this regimen produced toxic plasma amiodarone levels <4.5 µM and caused pneumonitis, hepatic failure and marked bradycardia. We therefore used 40 mg/kg/day in all subsequent experiments as this dose was well tolerated and was found to produce serum amiodarone levels and effects on QTc and heart rate (HR) similar to those reported previously using 80 mg/kg/day in euthyroid rabbits [5].

All experimental groups were kept under identical conditions for the same period of time (5 weeks) with free access to water and chow and their weight was monitored weekly. Serum free-T3 and calcium levels were determined pre-sacrifice. A surface electrocardiogram was obtained at baseline and at completion of the treatment protocol as described previously [5]. We measured the heart rate (HR) from the electrocardiogram and calculated QTc using Bazett's formula (QTc=QT/{surd}RR) [17]. Intervals were recorded and measured at 50 mm/s sweep speed and were averaged over ten cardiac cycles.

2.3 Dronedarone treatment
In a separate series of experiments dronedarone, provided by Sanofi-Synthelabo, was administered in gelatin capsules for 3 weeks at a dose of 100 mg/kg/day. This regimen has been previously reported to produce therapeutic plasma levels, antiarrhythmic effects and prolongation of action potential duration and QTc [18,19].

Weight and ECGs were recorded at weekly intervals during the treatment period. Venous samples for measurement of T3 were taken the day prior to commencement of treatment and weekly thereafter until sacrifice. Whole blood dronedarone levels were measured at baseline and then weekly up to the time of sacrifice to assess the adequacy of the dosing regimen. The assays were conducted using HPLC at the Sanofi Laboratories in Montpelier, France. The mean trough blood level of dronedarone after 3 weeks of therapy was 60.0±4.0 µg/dl which approximates the therapeutic range.

2.4 Measurement of Ip
Rabbits were anaesthetised with xylazine (0.2 ml/kg) and ketamine (0.5 ml/kg), and then heparinised. The heart was removed and ventricular myocytes were isolated and used for experimentation within 8 h. For measurement of Ip myocytes were superfused with modified Tyrode solution containing (in mM): 140 NaCl, 5.6 KCl, 2.16 CaCl2, 1 MgCl2, 0.44 NaH2PO4, 10 glucose and 10 HEPES, titrated to pH 7.4 with NaOH at 35 °C. They were voltage-clamped with wide-tipped pipettes (0.8–1.2 M{Omega}) constructed as described by Whalley et al. [20]. They were filled with solution containing (in mM) 70 potassium-glutamate, 1 KH2PO4, 5 HEPES, 5 EGTA, 2 MgATP, and either 10 or 80 sodium glutamate with 80 or 10 TMA-Cl, respectively, to maintain intracellular osmotic balance. A pipette Na+ concentration of 10 mM was used because it is near physiological levels for intracellular Na+ while 80 mM was chosen because it is at a level expected to nearly saturate intracellular Na+ binding sites on the pump. The pipette solution was titrated to pH 7.05±0.01 at 35 °C with KOH.

After the whole-cell configuration had been established cells were voltage-clamped at –40 mV to inactivate the voltage-dependent Na+ current. The superfusate was then switched from normal Tyrode solution to Ca2+-free Tyrode solution which contained 2 mM BaCl2 and 0.2 mM CdCl2 to block voltage-dependent K+ and Ca2+ currents. Ip was measured as the change in holding current induced by superfusion with 100 µM ouabain, a concentration which produces complete block of Ip in the presence of amiodarone [5]. Ip was normalized for membrane capacitance and hence cell size. Membrane currents were recorded with an Axoclamp 2A amplifier using pClamp or Axotape (Axon Instruments, Foster City, CA).

2.5 Statistics
For experiments examining the effects of amiodarone, paired and unpaired Student's t-tests were used to analyse HR, QTc, and thyroid hormone levels. Three-way analysis of variance (ANOVA) or Dunnett's test were used for comparison of Ip. For experiments using dronedarone we used multivariate repeated measures ANOVA for analysis of QTc, HR and thyroid hormone levels and Student's t-test for Ip. Results are expressed as mean±standard error of the mean (S.E.M.). Differences were regarded as significant at P<0.05.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
3.1 Effect of thyroidectomy and treatment with amiodarone on thyroid function and ECG characteristics
The mean pre-sacrifice serum amiodarone level was 1.9±0.4 µM. This is similar to the therapeutic plasma levels found in patients on chronic therapy with amiodarone [21,22] and levels in our previous study in rabbits [5].

Table 1 summarises the effects of thyroidectomy and amiodarone therapy on thyroid function. There was no significant difference in pre-surgery baseline thyroid status between the sham-operated and thyroidectomised animals. The pre-sacrifice serum T3 levels were significantly reduced, relative to their baseline values, in both the TX-CON and TX-AM groups. This confirms that a state of hypothyroidism existed in rabbits subjected to thyroidectomy. Rabbits in the sham group remained euthyroid. The decrease in T3 concentration observed in the TX-AM group (~80±15%) was significantly greater than that seen in the group undergoing thyroidectomy alone (~55±12%).


View this table:
[in this window]
[in a new window]

  		Table 1 Effect of thyroidectomy and thyroidectomy plus 4 weeks amiodarone treatment on serum free T3 levels

 
The effects of thyroidectomy with and without concomitant amiodarone treatment on QTc and heart rate are summarized in Fig. 1. QTc did not change significantly during the treatment period in either the SHAM or TX-CON groups (Fig. 1A). However, in the group of hypothyroid rabbits undergoing amiodarone therapy a significant prolongation (approximately 17%) of the QTc was noted over the 5-week treatment period. This prolongation is similar in magnitude to that reported previously with amiodarone treatment under euthyroid conditions [5,22].


Figure 1
View larger version (18K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 1 Effect of sham thyroidectomy (SHAM), thyroidectomy alone (TX-CON) and thyroidectomy with chronic amiodarone therapy (TX-AM) on (A) QTc and (B) heart rate. In both panels solid bars indicate values at baseline and open bars indicate values after 5 weeks (presacrifice). In panel (B) * denotes P<0.01 for difference of presacrifice heart rate in TX-CON vs. SHAM, ** denotes P=0.02 for difference of presacrifice heart rate in TX-AM vs. TX-CON.

 
The effects of the three protocols on heart rate are summarised in Fig. 1B. The heart rate decreased significantly from baseline to pre-sacrifice in the TX-CON and TX-AM groups. There was no significant change in the SHAM group. The reduction in heart rate was significantly greater in the TX-AM group (33.1±2.5%) than the TX-CON group (16.5±1.9%). Fig. 2 illustrates the combined effects of thyroidectomy and amiodarone therapy on QTc and heart rate in an animal from the TX-AM group.


Figure 2
View larger version (40K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 2 Electrocardiographs recorded from a rabbit prior to thyroidectomy (A) and after thyroidectomy and 5 weeks amiodarone therapy (B) demonstrating prolongation of QTc and reduction of heart rate.

 
3.2 Effect of thyroidectomy and amiodarone treatment on Na+–K+ pump current
Fig. 3 shows recordings of Ip from one cell from each of the SHAM, TX-CON and TX-AM groups. To allow comparison, three cells of approximately equal capacitance, and hence cell size, were selected. Fig. 4 summarises Ip from all treatment groups using pipette Na+ concentrations ([Na]pip) of 10 or 80 mM. At 10 mM [Na]pip, Ip in both the thyroidectomised control group and the amiodarone treated group were significantly decreased compared to the sham group (P<0.05, ANOVA and Dunnett's test). There was no significant difference between Ip of the TX-CON and TX-AM groups. At 80 mM [Na]pip the mean Ip for both thyroidectomised groups was significantly smaller than in the SHAM group but not significantly different from each other. Thus, after thyroidectomy amiodarone did not produce any apparent additional inhibitory effect on Ip beyond that observed with hypothyroidism.


Figure 3
View larger version (13K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 3 Holding currents recorded in cells isolated from rabbits undergoing sham thyroidectomy (A), thyroidectomy alone (B), and thyroidectomy with subsequent amiodarone therapy (C). [Na]pip was 80 mM for all traces. The shift in holding current induced by exposure to ouabain represents the Na+–K+ pump current (Ip). The capacitances of the three cells were similar (130–140 pF) to allow comparison of Ip. Hypothyroidism decreases Ip (trace A vs. B), however, amiodarone treatment produces no additional inhibitory effect (trace B vs. C).

 

Figure 4
View larger version (17K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 4 The influence of amiodarone on Ip is thyroid-dependent. Effects of thyroidectomy and thyroidectomy with 5 weeks amiodarone therapy on Ip at near-physiological intracellular sodium concentration (10 mM [Na]pip), and concentrations producing near-maximal Na+–K+ pump stimulation (80 mM [Na]pip). Numbers in brackets indicate the number of experiments performed.

 
3.3 The effect of chronic dronedarone treatment on thyroid function, and ECG characteristics
If the effect of amiodarone is dependent on thyroid hormone and independent of a direct effect on membrane properties, treatment with dronedarone should have no effect on on Ip. To examine this we gave five rabbits dronedarone while a separate control group of five rabbits were given placebo capsules for the same treatment period.

The results of thyroid function tests are summarised in Table 2. At the time of sacrifice there were no significant differences in mean T3 levels between the two groups of animals confirming that the animals were not rendered hypothyroid by dronedarone therapy.


View this table:
[in this window]
[in a new window]

  		Table 2 Effect of 3 weeks dronedarone or placebo treatment on T3 levels

 
QTc and heart rate at baseline and pre-sacrifice in the dronedarone treated and placebo groups are summarised in Fig. 5. Dronedarone treatment increased the QTc by approximately 15%. This is similar to the increase observed with chronic amiodarone therapy in euthyroid rabbits [5,6,9,23] (Fig. 5A). The QTc was unchanged in the placebo group over the treatment period. As has been reported recently in rat heart, dronedarone therapy resulted in small (~10%) reduction in mean HR (Fig. 5B) but this failed to reach statistical significance


Figure 5
View larger version (17K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 5 Effect of chronic dronedarone therapy on (A) QTc and (B) heart rate. Measurements at baseline and after 3 weeks are indicated by solid bars and open bars, respectively.

 
Chronic dronedarone treatment and Ip. The effect of chronic dronedarone therapy on Ip measured using [Na]pip of 10 and 80 mM is shown in Fig. 6. In contrast to the inhibitory effects of chronic amiodarone administration demonstrated previously in euthyroid animals [5], Ip was not significantly affected by dronedarone treatment at either [Na]pip. Thus, at concentrations sufficient to exert effects on QTc similar to those observed with amiodarone therapy, dronedarone does not alter the activity of the Na+–K+ pump at a near-physiological concentration of intracellular Na+, or at a concentration which should produce near-maximal pump activity. These data provide further support for the notion that the effect of chronic amiodarone therapy on the activity of the Na+–K+ pump is dependent on alteration of thyroid metabolism.


Figure 6
View larger version (13K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 6 Effect of chronic dronedarone therapy on Ip. Ip was determined at 10 mM [Na]pip and 80 mM [Na]pip. Numbers in brackets indicate the number of experiments performed.

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
The major findings of this study are that induction of a state of systemic hypothyroidism abolishes the inhibitory effect of chronic amiodarone therapy on the Na+–K+ pump but does not eliminate prolongation of the QTc and sinus cycle length induced by the drug. Treatment of thyroidectomised rabbits with amiodarone in this study induced serum amiodarone levels and increases in QTc and sinus cycle lengths similar to those we have measured previously in euthyroid rabbits [5]. However, in contrast to the 33% reduction in Ip observed in euthyroid rabbits treated chronically with amiodarone [5], in the present study there was no effect of treatment on Ip recorded in myocytes from hypothyroid animals. When taken together with our previous study these findings indicate that amiodarone-induced inhibition of the sarcolemmal Na+–K+ pump is largely thyroid dependent. It also suggests that it is unlikely that the amiodarone-induced decrease in Ip reported previously [3–5] is mediated by an effect of the drug on membrane fluidity. This conclusion is also supported by the absence of any effect of the amiodarone congener dronedarone on Ip. Persistence of the effects of amiodarone on QTc and sinus cycle length in the hypothyroid state suggests that these are mediated to a significant extent by thyroid-independent mechanisms and presumably direct effects on sarcolemmal ion channels.

Thyroidectomy reduced, but did not eliminate, detectable levels of T3. This suggests that there are extrathyroidal sites of production of thyroid hormone in rabbits as previously described in rats [24]. Treatment with amiodarone induced an additional decrease in levels of T3 in thyroidectomised rabbits (Table 1), presumably due to the known inhibitory effect of the drug on peripheral deiodination of T4 [10]. Despite a level of T3 significantly lower than in rabbits not treated with amiodarone there was no additional inhibitory effect of the drug on Ip. This suggests that there is a threshold level for a decrease in T3 below which no further decrease in Na+–K+ pump function occurs.

There are several possible mechanisms by which amiodarone may mediate a thyroid-dependent reduction of Ip. Firstly, it may decrease Ip by producing a state of systemic hypothyroidism, a condition which may occur in up to 6–13% of patients on long term therapy [25]. Hypothyroidism decreases synthesis and incorporation of functional pump subunits into the sarcolemma, thereby reducing maximal Ip [26,27]. In support of this hypothesis, Hensley et al. [28] demonstrated a significant decrease in expression of the {alpha}1, {alpha}2, and β1 subunits of Na+–K+ ATPase in rat heart after 3 weeks of oral amiodarone therapy. These changes were associated with a significant decrease in T3 and T4 indicating a state of systemic hypothyroidism.

Notwithstanding the effects of amiodarone on systemic thyroid metabolism, it is possible that the drug may inhibit the Na+–K+ pump by induction of a state of ‘cellular hypothyroidism’ since amiodarone and its active metabolite desethylamiodarone competitively inhibit binding of T3 to its nuclear receptor [11,12,29]. This hypothesis is supported by a study in rabbits demonstrating that chronic amiodarone therapy may reduce maximal Ip despite maintenance of a euthyroid state [5].

The conclusion of the present study that amiodarone-induced pump inhibition is thyroid-dependent is at odds with a previous study. Bergman et al. [30] cultured neonatal rat myocytes for 48 h with or without supplemental T3. There was no effect of T3 on Na+–K+ pump inhibition induced by in vitro exposure of myocytes to amiodarone. However, pump function was estimated from the ouabain-sensitive component of cellular uptake of the K+ congener 86Rb+. There was no control of the intracellular Na+ concentration and amiodarone may have caused a reduction in Na+ influx and hence cytosolic Na+ levels. Amiodarone may therefore have limited the availability of a key substrate for the Na+–K+ pump rather than directly inhibiting the pump itself. In our study, cytosolic Na+ was fixed and controlled by perfusion of the intracellular compartment with patch pipette solution. Furthermore, our study examined the effects of chronic in vivo exposure of cardiac myocytes to amiodarone while Bergman et al. [30], examined acute in vitro exposure to amiodarone with observation of Na+–K+ ATPase activity over only a 2–3-h period. One would not expect to observe an effect of amiodarone mediated via alteration in synthesis and incorporation of new Na+–K+ pump subunits into the sarcolemma over this brief observation period.

Many studies have investigated the role of altered cellular thyroid metabolism in determining the effects of amiodarone on the ECG, cardiac action potential, and cardiac membrane ionic currents [6,7,9,23,31,32]. Singh and Vaughan-Williams [6] noted that prolongation of APD, QTc and ventricular refractoriness by amiodarone could be prevented by co-administration of thyroxine and suggested that the drug's beneficial effects may be due to thyroid hormone antagonism. Using a similar model these findings were confirmed and extended to include the effects of the drug on heart rate [7,9]. The limitation of the thyroxine co-treatment model to address the thyroid dependence of amiodarone's effects is that it cannot distinguish between abolition of a thyroid-dependent effect or the independent but opposing effects of amiodarone and thyroxine on the electrophysiological parameter being examined.

More recent studies using experimental hypothyroidism to further examine this interrelationship have produced divergent results with some showing persistence of the class III effect of the drug despite hypothyroidism [33] whilst others confirm the earlier studies by demonstrating prevention or marked attenuation of the influence of amiodarone on QTc and heart rate [23,31]. In our study the effects of amiodarone on QTc and heart rate were significantly greater than that of hypothyroidism alone (Fig. 1). This could be interpreted as confirming a thyroid-independent influence of the drug. However, other explanations should be considered. Firstly T3 levels were significantly lower in the amiodarone treated thyroidectomised animals compared with those undergoing thyroidectomy alone (see Table 1) and thus the inhibitory influence of more profound hypothyroidism on QTc and sinus rate may be expected to be greater. Alternatively the divergent results between our study in rabbits and those in guinea pigs could be due to species-dependent differences in expression of the various K+ currents. In support of this proposal Bosch et al. [31] reported that hypothyroidism only reduced the slow component of the delayed rectifier K+ current (IKs) whereas amiodarone predominantly reduced the rapid component of the delayed rectifier current K+ (IKr) and the inward rectifier K+ current (IK1). Since IKr rather than IKs is the dominant K+ current in rabbits [34], it is not surprising we found that QTc was not prolonged by hypothyroidism alone whereas the combination of amiodarone and hypothyroidism significantly prolonged QTc presumably via reduction of IKr.

Dronedarone was developed with the aim of maintaining the antiarrhythmic efficacy of amiodarone whilst minimising organ toxicity [19]. With chronic administration it has similar electrophysiological effects and efficacy to the parent drug in animal models of ischaemia and reperfusion-induced ventricular tachyarrhythmias [18,19,35,36]. As confirmed in our study, dronedarone has little effect on thyroid function and is therefore an appropriate negative control to independently test the thyroid dependency of amiodarone's effects on Ip, QTc and HR. Dronedarone prolonged QTc to a similar extent as amiodarone therapy (Figs. 1 and 5Go), yet it had no inhibitory effect on Ip, providing further support for the hypothesis that the influence of amiodarone on Ip may be attributed to an interaction with cellular thyroid hormone metabolism. A further inference that may be drawn from these experiments is that, unlike its parent compound, prolongation of action potential duration and refractoriness associated with dronedarone therapy is not contributed to by inhibition of the hyperpolarising Na+–K+ pump current.

Time for primary review 31 days.


   Acknowledgements
 
This work was funded by a grant from the North Shore Heart Research Foundation (Australia). Both amiodarone and dronedarone were gifts from Sanofi-Synthelabo. Gilbert Pastor of Sanofi-Synthelabo conducted the serum dronedarone assays.


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

  1. Nattel S, Talajic M, Fermini B, et al. Amiodarone: pharmacology, clinical actions, and relationships between them. J Cardiovasc Pharmacol (1992) 3:266–280.
  2. Roy D, Talajic M, Dorian P, et al. Amiodarone to prevent recurrence of atrial fibrillation. Canadian Trial of Atrial Fibrillation Investigators. New Engl J Med (2000) 342:913–920.[Abstract/Free Full Text]
  3. Broekhuysen J, Clinet M, Delisee C. Action of amiodarone on guinea pig heart sodium and potassium activated adenosine triphosphatase: Comparison with ouabain. Biochem Pharmacol (1972) 21:2951–2960.[CrossRef][ISI][Medline]
  4. Dzimiri N, Almotrefi A.A. Amiodarone: Biochemical evidence for its interaction with myocardial Na+–K+ ATPase in guinea pig microsomal preparations. Biochem Pharmacol (1991) 41:470–472.[CrossRef][ISI][Medline]
  5. Gray D.F, Mihailidou A.S, Hansen P.S, et al. Amiodarone inhibits the Na+–K+ pump in rabbit cardiac myocytes after acute and chronic treatment. J Pharmacol Exp Ther (1998) 284:75–82.[Abstract/Free Full Text]
  6. Singh B.N, Vaughan Williams E.M. The effect of amiodarone, a new antianginal drug, on cardiac muscle. Br J Pharmacol (1970) 39:657–667.[ISI][Medline]
  7. Lindenmeyer M, Sporri S, Staubli M, et al. Does amiodarone affect heart rate by inhibiting the intracellular generation of triiodothyronine from thyroxine? Br J Pharmacol (1984) 82:275–280.[ISI][Medline]
  8. Freedberg A.S, Papp G.J, Vaughan-Williams E.M. The effects of altered thyroid state on atrial intracellular potentials. J Physiol (1970) 207:357–370.[Abstract/Free Full Text]
  9. Patterson E, Walden K.M, Khazaeli M.B, et al. Cardiac electrophysiologic effects of acute and chronic amiodarone administration in the isolated perfused rabbit heart: altered thyroid hormone metabolism. J Pharmacol Exp Ther (1986) 239:179–184.[Abstract/Free Full Text]
  10. Kannan R, Ookhtens M, Chopra I, et al. Effects of chronic administration of amiodarone on kinetics of metabolism of iodothyronines. Endocrinology (1984) 115:1710–1716.[Abstract]
  11. Latham K.R, Sellitti D.F, Goldstein R.E. Interaction of amiodarone and desethylamiodarone with solubilized nuclear thyroid hormone receptors. J Am Coll Cardiol (1987) 9:872–876.[Abstract]
  12. Paradis P, Lambert C, Rouleau J. Amiodarone antagonises the effect of T3 at the receptor level: an additional mechanism for its in vivo hypothyroid-like effects. Can J Physiol Pharmacol (1991) 69:865–870.[ISI][Medline]
  13. Ewart H.S, Klip A. Hormonal regulation of the Na+–K+ ATPase: mechanisms underlying rapid and sustained changes in pump activity. Am J Physiol (Cell Physiol) (1995) 269:C295–C311.[Abstract/Free Full Text]
  14. Chatelain P, Laruel R, Gillard M. Effect of amiodarone on membrane fluidity and Na+/K+ ATPase activity in rat-brain synaptosomes. Biochem Biophys Res Commun (1985) 129:148–154.[CrossRef][ISI][Medline]
  15. Cornelius F. Functional reconstitution of the sodium pump: kinetics of exchange reactions performed in reconstituted Na+/K+ ATPase. Biochim Biophys Acta (1991) 1071:19–66.[Medline]
  16. Law B, Gill R, Moffat A.C. High-performance liquid chromatography retention data for 84 basic drugs of forensic interest on a silica column using an aqueous methanol eluent. J Chromatogr (1984) 301:165–172.[ISI][Medline]
  17. Bazett H.C. An analysis of the time-relations of electrocardiograms. Heart (1920) 7:353–370.[ISI]
  18. Aimond F, Beck L, Gautier P, et al. Cellular and in vivo electrophysiological effects of dronedarone in normal and postmyocardial infarcted rats. J Pharmacol Exp Ther (2000) 292:415–424.[Abstract/Free Full Text]
  19. Manning A, Thisse V, Hodeige D, et al. SR33589, a new amiodarone-like antiarrhythmic agent: electrophysiological effects in anesthetized dogs. J Cardiovasc Pharmacol (1995) 25:252–261.[ISI][Medline]
  20. Whalley D.W, Hool L.C, Ten Eick R.E, et al. Effect of osmotic swelling and shrinkage on Na+–K+ pump activity in mammalian cardiac myocytes. Am J Physiol (1993) 265:C1201–1210.[ISI][Medline]
  21. Ikeda N, Nademanee K, Kannan R, et al. Electrophysiologic effects of amiodarone: Experimental and clinical observation relative to serum and tissue drug concentrations. Am Heart J (1984) 108:890–898.[CrossRef][ISI][Medline]
  22. Debbas N.M.G, du Cailar C, Bexton R.S, et al. The QT interval: a predictor of the plasma and myocardial concentrations of amiodarone. Br Heart J (1984) 51:316–320.[Abstract/Free Full Text]
  23. Talajic M, Nattel S, Davies M, et al. Attenuation of class 3 and sinus node effects of amiodarone by experimental hypothyroidism. J Cardiovasc Pharmacol (1989) 13:447–450.[ISI][Medline]
  24. Yin Y.-L, Perret G.Y, Nicholas P, et al. In vivo effects of amiodarone on cardiac β-adrenoceptor density and heart rate require thyroid hormones. J Cardiovasc Pharmacol (1992) 19:541–545.[ISI][Medline]
  25. Newman C.M, Price A, Davies D.W, et al. Amiodarone and the thyroid: a practical guide to the management of thyroid dysfunction induced by amiodarone therapy. Heart (1998) 79:121–127.[Free Full Text]
  26. Horowitz B, Hensley C.B, Quitero M, et al. Differential regulation of Na,K-ATPase {alpha}1, {alpha}2 and β subunit mRNA and protein levels by thyroid hormone. J Biol Chem (1990) 265(24):14308–14314.[Abstract/Free Full Text]
  27. Kamitani T, Ikeda U, Muto S, et al. Regulation of Na,K ATPase gene expression by thyroid hormone in rat cardiomyocytes. Circ Res (1992) 71:1457–1464.[Abstract/Free Full Text]
  28. Hensley B.C, Bershon M.M, Sarma J.S.M, et al. Amiodarone decreases Na,K-ATPase {alpha}2 and β2 expression specifically in cardiac ventricle. J Mol Cell Cardiol (1994) 26:417–424.[CrossRef][ISI][Medline]
  29. Drvota V, Carlsson B, Haggblad J, et al. Amiodarone is a dose dependent competitive and non-competitive inhibitor of T3 binding to human thyroid hormone receptor β1 whereas disopyramide, lignocaine, propafenone, metoprolol, sotalol and verapamil have no effect [Abstract]. Circulation (1994) 90:I38.
  30. Bergman M, Cohen F, Schlesinger H, et al. Effect of amiodarone on beating rate and Na-K-ATPase activity in cultured neonatal rat heart myocytes. Gen Pharmacol (1995) 26(2):285–290.[CrossRef][ISI][Medline]
  31. Bosch R.F, Li G.-R, Gaspo R, Nattel S. Electrophysiologic effects of chronic amiodarone therapy and hypothyroidism, alone and in combination, on guinea pig ventricular myocytes. J Pharmacol Exp Ther (1999) 289:156–165.[Abstract/Free Full Text]
  32. Polikar R, Goy J.J, Sclapfer J, et al. Effect of oral triiodothyronine during amiodarone treatment for ventricular premature complexes. Am J Cardiol (1986) 58:987–991.[CrossRef][ISI][Medline]
  33. Lambert C, Cardinal R, Vermeulen M, et al. Lack of relation between the ventricular refractory period prolongation by amiodarone and the thyroid state in rats. J Pharmacol Exp Ther (1987) 242:320–325.[Abstract/Free Full Text]
  34. Colatsky T.J, Follmer C.H, Starmer C.F. Channel specificity in antiarrhythmic action: Mechanism of potassium channel block and its role in supressing and aggravating cardiac arrhythmias. Circulation (1990) 82:2235–2242.[Abstract/Free Full Text]
  35. Finance O, Manning A, Chatelain P. Effects of a new amiodarone-like agent, SR 33589, in comparison to amiodarone, D,L-sotalol, and lignocaine, on ischemia-induced ventricular arrhythmias in anesthetized pigs. J Cardiovasc Pharmacol (1995) 26:570–576.[ISI][Medline]
  36. Sun W, Sarma J.S.M, Singh B.N. Electrophysiological effects of dronedarone (SR33589), a noniodinated benzofuran derivative, in the rabbit heart: comparison with amiodarone. Circulation (1999) 100:2276–2281.[Abstract/Free Full Text]

Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?



This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Pitt, A. D
Right arrow Articles by Whalley, D. W
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Pitt, A. D
Right arrow Articles by Whalley, D. W
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?