Skip Navigation

Cardiovascular Research 1998 40(3):492-501; doi:10.1016/S0008-6363(98)00200-4
© 1998 by European Society of Cardiology
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow E-letters: Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when E-letters are posted
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
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Wolk, R.
Right arrow Articles by Hicks, M. N
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Wolk, R.
Right arrow Articles by Hicks, M. N
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

Copyright © 1998, European Society of Cardiology

Regional electrophysiological effects of hypokalaemia, hypomagnesaemia and hyponatraemia in isolated rabbit hearts in normal and ischaemic conditions

Robert Wolkb, Kathleen A Kaneb, Stuart M Cobbea and Martin N Hicksa,*

aDepartment of Medical Cardiology, Royal Infirmary, 10 Alexandra Parade, Glasgow G31 2ER, UK
bDepartment of Physiology and Pharmacology, University of Strathclyde, Royal College, 204 George Street, Glasgow G1 1XW, UK

* Corresponding author. Tel.: +44-(141)-211-0461; fax: +44-(141)-552-4683.

Received 26 January 1998; accepted 21 April 1998


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 5 Conclusions
 References
 
Objective: The aims of this study were to establish an isolated working heart model for electrophysiological recordings from the epicardium and endocardium and to examine regional effects of changes in ion concentrations in normal and ischaemic conditions. Methods: Monophasic action potential duration (MAPD90), effective refractory period (ERP) and conduction delay were measured simultaneously in the epicardium and endocardium of rabbit hearts paced at 3.3 Hz, subjected to 30 min of regional ischaemia and 15 min of reperfusion. The hearts were exposed before and throughout ischaemia and reperfusion to hypokalaemia (K+=2 mM), hypomagnesaemia (Mg2+=0.5 mM) or hyponatraemia (Na+=110 mM). Results: In the control hearts, no regional electrophysiological differences were seen before ischaemia, but ischaemia-induced MAPD90 shortening and postrepolarisation refractoriness were greater in the epicardium than in the endocardium and conduction delay increased only in the epicardium. Hypokalaemia shortened ERP in the epicardium (but not endocardium) and increased conduction delay in all areas before ischaemia, but it had no effects during ischaemia. During reperfusion hypokalaemia increased the incidence of recurrent tachyarrhythmias. Hypomagnesaemia had no effect before ischaemia, increased epicardial (but not endocardial) MAPD90 shortening during ischaemia, although it had no pro-arrhythmic action. Hyponatraemia increased conduction delay in all areas before ischaemia and produced asystole or severe bradycardia in all hearts. During ischaemia, hyponatraemia decreased ERP shortening and inducibility of arrhythmias in the epicardium (but not endocardium). Conclusions: We conclude that the more pronounced effect of ischaemia upon the epicardium than the endocardium can be explained by the contact of the endocardium with intracavitary perfusate. We also conclude that changes in ion concentrations may have differential regional electrical effects in normal or ischaemic conditions.

KEYWORDS Hypokalaemia; Hypomagnesaemia; Hyponatraemia; Epicardium; Endocardium; Ischaemia; Rabbit


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 5 Conclusions
 References
 
Disturbances in ion homeostasis have long been recognised as important and often life-threatening conditions, mainly because of their cardiac electrophysiological effects [1, 2]. Hypokalaemia, in particular, has been studied very extensively and is associated with an increased risk of complex ventricular arrhythmias in a variety of clinical situations, including myocardial infarction [3–7]. Similarly, hypomagnesaemia has been implicated in the pathogenesis of cardiac arrhythmias, especially in the presence of hypokalaemia, although the role of isolated hypomagnesaemia as a cause of arrhythmias is rather uncertain [8, 9]. Hyponatraemia, which is a common finding in patients with decompensated heart failure [10, 11], is also likely to be associated with disturbances in cardiac electrophysiology. An association of dilutional hyponatraemia with cardiac arrest has been reported during transurethral prostatectomy [12], but its potential pathophysiological and prognostic significance in heart failure is not known.

Regional electrical heterogeneity has been proposed to be an important determinant of susceptibility to cardiac arrhythmias [13]. Differential electrical responses of isolated epicardial and endocardial preparations to hypokalaemia and hyponatraemia in normoxic conditions have also been reported [13, 14]. However, is has not been established whether these differential effects are present in the whole heart at physiological heart rates and whether they are related to the occurrence of cardiac arrhythmias. Also, it is not known whether changes in ion concentration may have different regional effects during myocardial ischaemia. Consequently, the aims of this study were twofold. Firstly, to establish a working isolated whole heart model, in which ischaemia-induced electrical changes could be recorded simultaneously from the epicardium and the endocardium and could be correlated with the occurrence of ischaemic arrhythmias. Secondly, to test the hypothesis that regional electrical heterogeneity might underlie the pathophysiological effects of hypokalaemia, hypomagnesaemia and hyponatraemia in normal and regionally ischaemic hearts.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 5 Conclusions
 References
 
The investigation conforms 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 1985) and with the provisions of the Animals (scientific procedures) Act 1986.

2.1 Whole heart preparation
Male New Zealand White rabbits (weight 2.5–3.6 kg) were heparinised (2000 I.U. i.v.) and anaesthetised with sodium pentobarbitone (100 mg kg–1 i.v.). The hearts were excised, the aorta was cannulated rapidly and the hearts were perfused retrogradely (Langendorff preparation) with a modified Tyrode solution (composition in mM: Na+ 142.0, K+ 4.0, Ca2+ 1.8, Mg2+ 1.0, Cl– 121.0, HCO3– 28.0, H2PO4– 0.4, glucose 11.0), equilibrated with O2–CO2 (95:5, v/v) to provide a pH of 7.4. The heart was enclosed in a temperature controlled chamber and the epicardial surface temperature was maintained at 35±0.1°C. The left atrium was cannulated to commence perfusion in the working heart mode. The perfusion pressures were set at a preload of 10 cm H2O and an afterload of 75 cm H2O. The right atrium was paced at the cycle length of 300 ms. A loose loop of atraumatic silk was placed through a snare around the obtuse marginal branch of the left coronary artery, which could be occluded to produce regional ischaemia.

In the experiments performed in the Langendorff mode, the heart was perfused retrogradely with a modified Tyrode solution in a constant perfusion pressure mode (75 cm H2O). The left atrium was cannulated only to enable insertion of the endocardial electrode (see Section 2.2).

2.2 Electrophysiological measurements
A diagram of the experimental heart preparation is shown in Fig. 1. Epicardial monophasic action potentials (MAPs) and effective refractory periods (ERPs) were recorded using custom-made suction electrodes and bipolar stimulating electrodes, respectively, located in the apical part of the left ventricle in an area that was subsequently to be made ischaemic by coronary occlusion (epicardium) and in a non-ischaemic area above the site of occlusion (normal zone). For the endocardial recordings and local pacing, a contact electrode catheter (EP Technologies, UK) was placed in the apical part of the left ventricular endocardium (opposite the respective epicardial suction electrode) in the area that was subsequently made ischaemic (endocardium).


Figure 1
View larger version (28K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 1 Diagram of the experimental working heart preparation. Monophasic action potentials (MAPs) were measured simultaneously in the normal zone (area 1), in the epicardium (area 2) and in the endocardium (area 3). Effective refractory periods (ERPs) were measured first in area 1, followed immediately by ERP in area 2 and then in area 3. See the text for details. The shaded region indicates the area at risk. Abbreviations: RA, right atrium; LA, left atrium; RV, right ventricle; LV, left ventricle; LAD, left anterior descending coronary artery; OMB, obtuse marginal branch of the left circumflex artery.

 
ERPs were determined during local ventricular pacing by the extrastimulus technique, using square wave impulses of 2-ms duration at twice diastolic threshold, generated by a constant voltage stimulator (model DS2, Digitimer, UK). Following a train of eight regular stimuli (S1) with a cycle length of 300 ms, an early ‘ineffective’ extrastimulus (S2) was introduced. S2 was subsequently introduced progressively later in 5-ms steps until it triggered an action potential. The ERP was defined as the longest S1S2 interval at which S2 failed to produce a propagated ventricular response. Ventricular fibrillation (VF) threshold was determined using a constant current stimulator (model DS7, Digitimer). It was measured by local application of a train of 10 consecutive stimuli (all with a duration of 2 ms and at 10-ms intervals) during the vulnerable period while the heart was atrially paced. The local stimulus strength was set at 5 mA and then it was increased at 5 mA steps until VF was produced or until 100 mA was achieved.

2.3 Other measurements
An inline flow meter (model T106, Transonic, Ithaca, NY, USA) was used for continuous recording of mean aortic forward flow and coronary flow in the working hearts and in the Langendorff perfused hearts, respectively. In the working hearts, left ventricular peak systolic and end diastolic pressures were measured with a 20-gauge venflon catheter inserted into the left ventricle and connected to a pressure transducer (model: P23XL, Gould, UK).

The epicardial and endocardial temperatures were monitored simultaneously during the duration of the experiment using temperature probes connected to a temperature monitor (model ET 250, Libra Medical, UK).

2.4 Experimental protocol
Before ischaemia, two series of measurements (15 min apart) were performed at baseline, and then 15 and 30 min after the solution change (the new solution being either normal Tyrode solution or Tyrode solution with lowered ion concentrations). MAPs were recorded simultaneously at all three sites, whereas ERPs and diastolic stimulation thresholds were measured first in the normal zone, followed immediately by measurements in the epicardium and then in the endocardium. Subsequently, a period of 30 min of ischaemia and 15 min of reperfusion was induced. During this period, local MAPs were measured continuously in all three areas and ERPs were measured 15 min into ischaemia and 15 min into reperfusion, in the same order as previously. When any sustained arrhythmia appeared spontaneously or was induced by the stimulation protocol, defibrillation was used after 30 s to restore normal rhythm. If no arrhythmia was induced during ERP measurements, VF threshold was measured.

At the end of the reperfusion period the obtuse marginal branch of the left coronary artery was re-occluded at the same site as during ischaemia and Evans blue dye was injected into the perfusate to distinguish the area at risk (calculated as a percentage of the left ventricular mass).

2.5 Study groups
The hearts were randomly assigned to one of five experimental groups: Langendorff control (n=6), working heart control (n=6), hypokalaemia (K+=2 mM, n=7), hypomagnesaemia (Mg2+=0.5 mM, n=6) and hyponatraemia (choline substitution) (Na+=110 mM, n=7). The hearts in the latter three groups were perfused in the working heart mode and the results were compared with the working heart control group.

2.6 Data analysis
The signals were recorded on a videotape recorder (Matsui VHS HQ, VX 2000Y) via an A/D VCR Adapter (Model PCM 4/8, Medical Systems, Greenvale, NY, USA). Subsequently, the signals were analysed off-line using an appropriate computer software package (WCP, John Dempster, University of Strathclyde). The action potential duration was measured at 90% repolarisation (MAPD90) from an averaged signal of 15 consecutive MAPs; the conduction delay was taken as the time from the atrial pacing trigger to the onset of the MAP. Transepicardial MAPD90 dispersion was calculated as: MAPD90 in the normal zone–MAPD90 in the epicardium. Transepicardial ERP dispersion was calculated in a similar manner. Postrepolarisation refractoriness was calculated as local ERP–MAPD90.

For statistical analysis paired t-test was used to assess changes within each group. Analysis of variance (ANOVA) followed by t-tests with Bonferroni's correction were employed for comparisons between the groups and between different areas in the same group. p value of less than 0.05 was considered statistically significant. All data are expressed as means±S.E.M.


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 5 Conclusions
 References
 
3.1 Haemodynamic measurements
In the Langendorff perfused hearts, coronary flow decreased after coronary artery occlusion from 37±2 to 28±1 ml/min (p<0.05) and during reperfusion coronary flow returned to its pre-ischaemic values. In the control working heart group, coronary artery occlusion resulted in a fall in both mean forward flow and intraventricular peak systolic pressure (from 124±8 to 77±6 ml/min, p<0.05, and from 92±2 to 74±4 mm Hg, p<0.05, respectively). During reperfusion neither forward flow nor systolic pressure returned to pre-ischaemic values. There were no significant differences in end diastolic pressure between pre-ischaemic, ischaemic and reperfusion values (1.4±0.5 vs. 2.4±0.6 vs. 1.3±1 mm Hg, respectively). These haemodynamic parameters were not affected by any change in ion concentration.

3.2 Changes in temperature in the epicardium and the endocardium during ischaemia and reperfusion
As shown in Table 1, the temperature in the endocardium was higher than in the epicardium both in the working and in the Langendorff perfused hearts. In both preparations the decrease in temperature during ischaemia was similar in the epicardium and in the endocardium, so that the transmural temperature gradient was not affected by myocardial ischaemia (Table 1).


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

  		Table 1 Changes in epicardial and endocardial temperature (°C) in the working and Langendorff perfused hearts

 
3.3 Ischaemia- and reperfusion-induced arrhythmias
No spontaneous or inducible arrhythmias occurred before ischaemia in either the Langendorff perfused or in the control working hearts. The incidence of arrhythmias during ischaemia and reperfusion is shown in Table 2. During ischaemia spontaneous arrhythmias did not occur, but sustained VF (≥30 s) was induced in both groups by application of an extrastimulus in the area at risk (either in the epicardium or in the endocardium). There were no differences between these groups except that reperfusion-induced idioventricular rhythm occurred more often in the working than in the Langendorff perfused hearts (83% vs. 0%, respectively, p<0.05).


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

  		Table 2 Incidence of arrhythmias (spontaneous VF, induced VF, idioventricular rhythm) during ischaemia and reperfusion

 
Before ischaemia, incessant spontaneous VT/VF occurred in 1 heart after lowering of K+ and on one occasion VF was induced during hypomagnesaemia. Hyponatraemia extinguished spontaneous electrical activity in 5/7 hearts (p<0.05) and markedly slowed it in the remaining 2/7 hearts (cycle length>1000 ms), although all hearts exposed to hyponatraemia remained responsive to electrical stimulation. During ischaemia no spontaneous arrhythmias occurred in any group (Table 2). Inducibility of VF was not affected by either hypokalaemia or hypomagnesaemia, but hyponatraemia decreased inducibility of VF in the epicardium (from 100% to 28%, p<0.05) (although not in the endocardium) (Table 2). Neither hypomagnesaemia nor hyponatraemia had any effect on reperfusion-induced arrhythmias. However, in the hypokalaemic hearts reperfusion was always followed by incessant VT/VF cycles (preceded by more and more extrasystoles or idioventricular rhythm) (Table 2). VT was not liable to defibrillation. Successful defibrillations of VF were immediately followed by another cycle of VT/VF. Only 1 heart in this group survived until the 15th min of reperfusion so that ERPs could not be measured.

3.4 Electrophysiological changes in epi- and endocardium in the working and Langendorff perfused hearts during coronary artery occlusion
Occlusion of the coronary artery resulted in a significant shortening of MAPD90 in the area at risk, without any apparent changes in the normal zone (Fig. 2). In the working hearts, the magnitude of ischaemia-induced MAPD90 shortening was greater in the epicardium than in the endocardium (65±4 vs. 30±3 ms, p<0.05) (Fig. 2). The original signals illustrating this regional differential sensitivity to ischaemia are shown in Fig. 3. In contrast to the differential effect of ischaemia on epicardial and endocardial MAPD90 shortening, the degree of ERP shortening was similar in both areas (Fig. 4). Consequently, during ischaemia, postrepolarisation refractoriness in the working hearts decreased in the endocardium (from 15±3 to –7±4 ms, p<0.05), but not in the epicardium (from 23±3 to 32±2 ms, p<0.08). Diastolic stimulation threshold did not change during ischaemia in either area. Ischaemia also resulted in a significant increase in conduction delay in the epicardium (from 103±2 to 122±6 ms, p<0.05), but not in the endocardium (90±3 vs. 92±2 ms, ns) in the working heart preparations.


Figure 2
View larger version (19K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 2 Monophasic action potential duration (MAPD90) in the normal zone ({blacktriangleup}), in the epicardium ({blacksquare}) and in the endocardium (bullet) in working and in Langendorff perfused isolated rabbit hearts subjected to regional ischaemia and reperfusion. * and {diamondsuit} indicate a statistically significant difference from pre-ischaemic values and from the epicardium, respectively (p<0.05).

 

Figure 3
View larger version (10K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 3 Original epicardial and endocardial monophasic action potential (MAP) recordings before (a) and during (b) ischaemia.

 

Figure 4
View larger version (18K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 4 Effective refractory period (ERP) in the normal zone ({blacktriangleup}), in the epicardium ({blacksquare}) and in the endocardium (bullet) in working and in Langendorff perfused isolated rabbit hearts subjected to regional ischaemia and reperfusion. * and {diamondsuit} indicate a statistically significant difference from pre-ischaemic values and from the epicardium, respectively (p<0.05).

 
In contrast to the working hearts, in the Langendorff perfused hearts MAPD90 shortening was similar in both areas (Fig. 2). Although baseline ERP was shorter in the endocardium than in the epicardium (125±4 vs. 143±6 ms, p<0.05), in both areas it shortened to a similar value during ischaemia (Fig. 4). Postrepolarisation refractoriness increased in both areas during ischaemia (from 6±4 to 34±8 ms, p<0.05, and from –7±3 to 34±14 ms, p<0.05, in the epicardium and in the endocardium, respectively). Diastolic stimulation threshold did not change during ischaemia in the epicardium, but it increased in the endocardium (from 0.7±0.1 to 1.5±0.3 V, p<0.05). Ischaemia caused an increase in conduction delay within the ischaemic area both in the epicardium (from 105±3 to 127±6 ms, p<0.05) and in the endocardium (from 92±3 to 98±3 ms, p<0.05).

3.5 Electrophysiological effects of changes in ion concentrations in working hearts
3.5.1 Hypokalaemia
MAPD90 was not affected by hypokalaemia either before or during ischaemia. However, after lowering [K+]o in the perfusate, epicardial ERP was shorter in the hypokalaemic than in the control hearts (113±8 vs. 137±2 ms, respectively, p<0.05) (Fig. 5) and, therefore, postrepolarisation refractoriness decreased from 7±4 to –10±9 ms (p<0.05). Neither ERP (Fig. 5) nor postrepolarisation refractoriness (–4±8 vs. –6±9 ms before and during hypokalaemia, respectively) in the endocardium were affected. Hypokalaemia had no effect on ischaemic ERP shortening (Fig. 5) or postrepolarisation refractoriness in either area. ERP values were not obtainable during reperfusion in the hypokalaemic group due to incessant VT/VF (see Section 3.3).


Figure 5
View larger version (19K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 5 Epicardial and endocardial effective refractory period (ERP) in the hypokalaemic ({blacktriangleup}) and in the control ({blacksquare}) groups in isolated working rabbit hearts subjected to regional ischaemia and reperfusion. * and {diamondsuit} indicate a statistically significant difference from pre-ischaemic values and from the control group, respectively (p<0.05).

 
Conduction delay was significantly increased by hypokalaemia before ischaemia, but these changes were similar in the epicardium and in the endocardium (from 96±4 to 116±6 ms, p<0.05, and from 84±4 to 101±6 ms, p<0.05, respectively). In addition, hypokalaemia increased epicardial stimulation threshold before ischaemia from 0.95±0.11 to 1.26±0.17 V (p<0.05), but the change in the endocardial stimulation threshold was not significant (from 1.12±0.23 to 1.37±0.19 V, p<0.09). The magnitude of ischaemia-induced changes in conduction delay or stimulation threshold were not affected by hypokalaemia.

3.5.2 Hypomagnesaemia
Pre-ischaemic MAPD90 was not affected by hypomagnesaemia in any area. During ischaemia, the magnitude of MAPD90 shortening was increased by hypomagnesaemia in the epicardium (86±4 vs. 65±4 ms in the hypomagnesaemic and in the vehicle-treated group, respectively, p<0.05), although not in the endocardium (40±7 vs. 30±3 ms, respectively, ns) (Fig. 6). Consequently, the magnitude of ischaemia-induced increase in transepicardial MAPD90 dispersion was increased by hypomagneasemia from 74±4 to 86±3 ms (p<0.05).


Figure 6
View larger version (21K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 6 Epicardial and endocardial monophasic action potential duration (MAPD90) in the hypomagnesaemic ({blacktriangleup}) and in the control ({blacksquare}) groups in isolated working rabbit hearts subjected to regional ischaemia and reperfusion. * and {diamondsuit} indicate a statistically significant difference from pre-ischaemic values and from the control group, respectively (p<0.05).

 
Similarly to MAPD90, pre-ischaemic ERP or postrepolarisation refractoriness were not affected by hypomagnesaemia in any area. However, during ischaemia, whilst hypomagnesaemia had no effect on ERP shortening or on postrepolarisation refractoriness in the epicardium, the magnitude of ischaemia-induced ERP shortening in the endocardium was smaller in the hypomagnesaemic than in the control group (22±9 vs. 52±1 ms, respectively, p<0.05) (although the difference between endocardial ERP during ischaemia in the control and in the hypomagnesaemic hearts did not reach statistical significance). Also, hypomagnesaemia prevented the ischaemia-induced decrease in postrepolarisation refractoriness in the endocardium observed in the control hearts, the respective values before and during ischaemia being 20±9 vs. 38±11 ms (ns) and 15±3 vs. –7±4 ms (p<0.05) in the hypomagnesaemic and in the control group.

3.5.3 Hyponatraemia
Pre-ischaemic or ischaemic values of MAPD90 were not affected by hyponatraemia in any area. Similarly, hyponatraemia had no effect on pre-ischaemic ERP or postrepolarisation refractoriness. However, during ischaemia hyponatraemia significantly decreased the magnitude of ERP shortening in the epicardium (23±8 vs. 56±4 ms, p<0.05) (Fig. 7), and epicardial postrepolarisation refractoriness increased more in the hyponatraemic than in the control hearts (by 48±10 and 9±4 ms, respectively, p<0.05). As a result of these effects, an ischaemia-induced increase in transepicardial ERP dispersion was decreased from 62±6 to 35±8 ms (p<0.05). In contrast to the epicardium, the magnitude of ischaemia induced ERP shortening in the endocardium was not affected by hyponatraemia (Fig. 7). Hyponatraemia prevented the ischaemia- induced decrease in postrepolarisation refractoriness in the endocardium observed in the control hearts, the respective values before and during ischaemia being 12±9 vs. 24±7 ms and 15±3 vs. –7±4 ms (p<0.05) in the hyponatraemic and in the control group.


Figure 7
View larger version (19K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 7 Epicardial and endocardial effective refractory period (ERP) in the hyponatraemic ({blacktriangleup}) and in the control ({blacksquare}) groups in isolated working rabbit hearts subjected to regional ischaemia and reperfusion. * and {diamondsuit} indicate a statistically significant difference from pre-ischaemic values and from the control group, respectively (p<0.05).

 
Also, before ischaemia, hyponatraemia markedly increased conduction delay in all areas, with the magnitude of these changes being similar in the epicardium (from 99±5 to 154±10 ms, p<0.05) and in the endocardium (from 86±5 to 141±10 ms, p<0.05). However, the magnitude of ischaemia-induced changes in conduction delay was not affected by hyponatraemia.

3.5.4 Area at risk
The area at risk was similar in all groups, the respective values being 25±2, 24±1, 27±1, 27±1 and 26±1% in the Langendorff perfused, working heart control, hypokalaemic, hypomagnesaemic and hyponatraemic groups, respectively.


   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
5 Conclusions
 References
 
We have described an isolated working rabbit heart model, in which various electrical parameters were recorded simultaneously in the epicardium and the endocardium and could be correlated with inducibility of arrhythmias during local ventricular stimulation. We have found that the sensitivity of the endocardium to the electrophysiological effects of myocardial ischaemia was greater in the Langendorff perfused hearts compared with the working hearts, as reflected in the larger magnitude of MAPD90 shortening, development of postrepolarisation refractoriness and conduction delay. The mechanisms underlying these differences between the working and the Langendorff perfused preparations are unclear, but, as we have demonstrated, are unlikely to result from differences in the area at risk or transmural temperature gradients. It seems that the most likely explanation for the observed differences is the contact of the endocardium with the cavity Tyrode solution in the working, but not in the Langendorff perfused, hearts. This conclusion is in agreement with previous studies suggesting the role for the cavity blood/solution in nourishing and oxygenating the endocardium (through either diffusion or perfusion via the thebesian circulation), although the extent of such luminal flow to the endocardium is uncertain (for references and discussion see [15]).

4.1 Hypokalaemia
Hypokalaemia did not have any regional differential effect on MAPD90 in normoxic conditions. This is not consistent with the results obtained in isolated canine epicardial and endocardial preparations, in which lowering [K+]o to 2 mM prolonged APD, with a greater effect in the epicardium [13]. This effect was ascribed to the presence of a prominent Ito in the epicardium, but not in the endocardium [13, 16]. The discrepancy between these results can be explained by the fact that, although differential distribution of Ito has also been described in the rabbit heart [17], it may not be demonstrable at faster stimulation rates (300 ms in our study vs. 1000 ms in the canine preparations) due to slow reactivation kinetics of Ito [16]. In contrast to its effects on MAPD90, hypokalaemia had a differential effect on ERP, shortening it in the epicardium with no effect in the endocardium. Although the mechanism underlying this differential effect is not known, one possibility is that it is related to different characteristics of potassium channels in the epicardium and in the endocardium [18]. It has been recognised that recovery of excitability cannot be ascribed solely to the availability of Na+ channels, but is also influenced by a repolarising action of IK and IK1 [19]. These currents have been proposed to constitute background conductance (even during diastole), which opposes depolarising currents and, therefore, determines the magnitude of a threshold depolarising current and excitability [19]. Since the potassium conductance is sensitive to [K+]o [20], it is possible that the differential effect of hypokalaemia on the potassium conductance in the epicardium and in the endocardium is responsible for its differential effect on ERP and, consequently, on postrepolarisation refractoriness.

Another effect of hypokalaemia observed in all areas was an increase in conduction delay. This is consistent with previous reports and ascribed to an increase in the voltage change and, therefore, the time required to bring the tissue to threshold for a propagated response (as a result of an increase in the difference between the resting and threshold potentials) [1, 3]. This mechanism can also be implicated to explain a hypokalaemia induced increase in stimulation threshold seen in our experiments [21, 22].

Hypokalaemia did not have any effects on arrhythmias before or during ischaemia, but it was extremely pro-arrhythmic during reperfusion, facilitating the occurrence of spontaneous VT/VF cycles. A non-reentrant mechanism for VT (which subsequently degenerated into VF) is suggested by our inability to terminate it by defibrillation (in contrast to VF which was always susceptible to defibrillation), its temporal association with an increase in idioventricular rate as well as by the fact that it sometimes started as single extrasystoles of ever increasing frequency which gradually transformed into VT. Such frequent transitions of VT into VF during reperfusion have been previously described in other species and VT has been proposed to be due to enhanced automaticity [23, 24]. Indeed, hypokalaemia has long been known to enhance automaticity by increasing the slope of diastolic depolarisation [3, 25, 26]. Another possible mechanism is facilitation of triggered activity [27].

4.2 Hypomagnesaemia
Hypomagnesaemia did not have any electrophysiological effects before ischaemia, but it increased epicardial MAPD90 shortening during ischaemia. Since magnesium has been proposed to block KATP channels [28]and lowering extracellular magnesium concentration has been shown to decrease intracellular magnesium and deplete high-energy phosphates [29], activation of KATP channels is a possible explanation for increased ischaemia induced MAPD90 shortening in the hypomagnesaemic epicardium. The lack of such effect in the ischaemic endocardium may be due to a lesser degree of metabolic deprivation as a result of intracavitary perfusion in this preparation.

The absence of any pro-arrhythmic effects of hypomagnesaemia in either ischaemic or non- ischaemic conditions is consistent with the opinion that isolated hypomagnesaemia is an unlikely cause of cardiac arrhythmias [8].

4.3 Hyponatraemia
The only effect of hyponatraemia before ischaemia was a marked increase in conduction delay, which was similar in all areas. This observation is in agreement with other studies which suggest that a decrease in extracellular sodium concentration decreases the magnitude of the Na+ inward current, and, consequently, Vmax and conduction velocity [30, 31]. During ischaemia, hyponatraemia increased postrepolarisation refractoriness and decreased ERP shortening in the epicardium, but not in the endocardium. Our explanation is a further decrease in the inward Na+ current superimposed on a previous partial INa inactivation due to ischaemia induced extracellular K+ accumulation and membrane depolarisation. The absence of this effect in the endocardium is likely to be due to a lesser degree of K+ accumulation and membrane depolarisation (suggested by the lack of postrepolarisation refractoriness or conduction delay in the control endocardium subjected to ischaemia). The decrease in epicardial ERP shortening was associated with a decrease in inducibility of VF in the epicardium (but not in the endocardium). This is in agreement with our previous observations in the rabbit heart that changes in the inducibility of VF during ischaemia are associated with differences in the magnitude of regional ERP shortening, but not the magnitude of MAPD90 shortening [32].

Although in our model hyponatraemia did not promote any tachyarrhythmias, it produced either asystole or severe bradycardia in all hearts. This finding is of potential clinical significance. It has been reported that in patients with severe heart failure the mechanism of unexpected cardiac arrest is severe bradycardia or electromechanical dissociation in 62% and ventricular tachyarrhythmia in only 38% [33]. Hyponatraemia is a common finding in patients with decompensated heart failure [10, 11]and it is plausible that it has a pathophysiological role in sudden cardiac death in these patients. This association could be of prognostic and therapeutic significance and requires further studies.


   5 Conclusions
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
 5 Conclusions
References
 
We have demonstrated that, in our working isolated whole heart model, ischaemia-induced electrophysiological changes can be recorded simultaneously from the epicardium and the endocardium and can be correlated with the occurrence of ischaemic arrhythmias. A lesser sensitivity of the endocardium to the electrophysiological consequences of ischaemia may be related to the contact of the endocardium with the intraventricular perfusate. Although changes in ion concentrations may have regional differential electrophysiological effects in either normal (hypokalaemia) or ischaemic (hypomagnesaemia, hyponatraemia) conditions, these effects are not pro-arrhythmic in the present model (in fact, hyponatraemia was anti-arrhythmic during ischaemia by decreasing the magnitude of ERP shortening in the epicardium). An increase in the occurrence of reperfusion induced tachyarrhythmias caused by hypokalaemia and in that of non-ischaemic bradyarrhythmias produced by hyponatraemia may be of clinical significance, but isolated hypomagnesaemia is an unlikely cause of cardiac arrhythmias in either normal or ischaemic conditions.

This model provides a useful tool for investigators to search for other risk factors in cardiac pathology. It may also be used for screening drugs that may have a therapeutic potential for cardiac arrhythmias, especially under ischaemic conditions.

Time for primary review 29 days.


   Acknowledgements
 
Robert Wolk holds a University of Strathclyde postgraduate scholarship and an ORS award.


   References
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
 5 Conclusions
 References
 

  1. Surawicz B, Gettes LS. Effects of electrolyte abnormalities on the heart and circulation. In: Conn, HL Jr, Horwitz, O, editors. Cardiac and vascular diseases. Philadelphia: Lea and Febiger, 1971:539–576.
  2. Caralis P.V, Perez-Stable E. Electrolyte abnormalities and ventricular arrhythmias. Drugs (1986) 31:85–100.[CrossRef][Web of Science][Medline]
  3. Helfant R.H. Hypokalemia and arrhythmias. Am J Med (1986) 80:13–22.[Web of Science][Medline]
  4. Holland O.B. Potassium loss, ventricular irritability and the risk of sudden death in hypertensive patients. Drugs (1986) 31:78–84.[Web of Science][Medline]
  5. Solomon R.J. Ventricular arrhythmias in patients with myocardial infarction and ischemia: the role of serum potassium. Drugs (1986) 31:112–120.[Web of Science][Medline]
  6. Campbell R.W.F, Higham D, Adams P, Murray A. Potassium: its relevance for arrhythmias complicating acute myocardial infarction. J Cardiovasc Pharmacol (1987) 10:S25–S27.[CrossRef]
  7. Wong K.C, Schafer P.G, Schultz J.R. Hypokalemia and anesthetic implications. Anesth Analg (1993) 77:1238–1260.[Free Full Text]
  8. Millane T.A, Ward D.E, Camm A.J. Is hypomagnesemia arrhythmogenic? Clin Cardiol (1992) 15:103–108.[Web of Science][Medline]
  9. Arsenian M.A. Magnesium and cardiovascular disease. Prog Cardiovasc Dis (1993) 35:271–310.[CrossRef][Web of Science][Medline]
  10. Takasu T, Lasker N, Shalhoub R.J. Mechanisms of hyponatraemia in chronic congestive heart failure. Ann Intern Med (1961) 55:368–583.[Abstract/Free Full Text]
  11. Schaer G.L, Covit A.B, Laragh J.H, Cody R.J. Association of hyponatremia with increased renin activity in chronic congestive heart failure: impact of diuretic therapy. Am J Cardiol (1983) 51:1635–1638.[CrossRef][Web of Science][Medline]
  12. Charlton A.J. Cardiac arrest during transurethral prostatectomy after absorption of 1.5% glycine. Anaesthesia (1980) 35:804–806.[CrossRef][Web of Science][Medline]
  13. Antzelevitch C, Sicouri S, Litovsky S.H, et al. Heterogeneity within the ventricular wall. Electrophysiology and pharmacology of epicardial, endocardial, and M cells. Circ Res (1991) 69:1427–1449.[Free Full Text]
  14. Di Diego J.M, Antzelevitch C. High [Ca2+]o-induced electrical heterogeneity and extrasystolic activity in isolated canine ventricular epicardium. Phase 2 reentry. Circulation (1994) 89:1839–1850.[Abstract/Free Full Text]
  15. Fixler D.E, Wheeler M, Huffines D. Extent of myocardial flow from luminal collateral circulation. J Appl Physiol (1974) 37:282–285.[Free Full Text]
  16. Litovsky S.H, Antzelevitch C. Transient outward current prominent in canine ventricular epicardium but not endocardium. Circ Res (1988) 62:116–126.[Abstract/Free Full Text]
  17. Fedida D, Giles W.R. Regional variations in action potentials and transient outward current in myocytes isolated from rabbit left ventricle. J Physiol (1991) 442:191–209.[Abstract/Free Full Text]
  18. Antzelevitch C, Sicouri S, Lukas A, et al. Regional differences in the electrophysiology of ventricular cells: physiological and clinical implications. In: Zipes, DP, Jalife, J, editors. Cardiac electrophysiology: from cell to bedside. 2nd ed. Philadelphia: WB Saunders, 1994:228–245.
  19. Delmar M. Role of potassium currents on cell excitability in cardiac ventricular myocytes. J Cardiovasc Electrophysiol (1992) 3:474–486.[CrossRef][Web of Science]
  20. Carmeliet E. K+ channels in cardiac cells: mechanisms of activation, inactivation, rectification and K+o sensitivity. Pflüg Arch (1989) 414:S88–S92.[CrossRef][Medline]
  21. Dominguez G, Fozzard H.A. Influence of extracellular K+ concentration on cable properties and excitability of sheep cardiac Purkinje fibers. Circ Res (1970) 26:565–574.[Abstract/Free Full Text]
  22. Peon J, Ferrier G.R, Moe G.K. The relationship of excitability to conduction velocity in canine Purkinje tissue. Circ Res (1978) 43:125–135.[Abstract/Free Full Text]
  23. Penkoske P.A, Sobel B.E, Corr P.B. Disparate electrophysiological alterations accompanying dysrhythmias due to coronary occlusion and reperfusion in the cat. Circulation (1978) 58:1023–1035.[Abstract/Free Full Text]
  24. Kaplinsky E, Ogawa S, Michelson E.L, Dreifus L.S. Instantaneous and delayed ventricular arrhythmias after reperfusion of acutely ischemic myocardium: evidence for multiple mechanisms. Circulation (1981) 63:333–340.[Free Full Text]
  25. Gettes L.S, Surawicz B, Shieue J.C. Effect of high K, low K and quinidine on QRS duration and ventricular action potential. Am J Physiol (1962) 203:1135–1140.[Abstract/Free Full Text]
  26. Vassale M. Cardiac pacemaker potentials at different extra and intracellular K concentrations. Am J Physiol (1965) 208:770–775.[Abstract/Free Full Text]
  27. Nakata T, Hearse D.J, Curtis M.J. Are reperfusion-induced arrhythmias caused by disinhibition of an arrhythmogenic component of ischemia? J Mol Cell Cardiol (1990) 22:843–858.[CrossRef][Web of Science][Medline]
  28. Horie M, Irisawa H, Noma A. Voltage-dependent magnesium block of adenosine-triphosphate sensitive potassium channel in guinea-pig ventricular cells. J Physiol (1987) 387:251–272.[Abstract/Free Full Text]
  29. Altura B.M, Barbour R.L, Dowd T.L. et al. Low extracellular magnesium induces intracellular free Mg deficits, ischemia, depletion of high-energy phosphates and cardiac failure in intact working rat hearts: A 31P-NMR study. Biochim Biophys Acta (1993) 1182:329–332.[Medline]
  30. Walton M.K, Fozzard H.A. Experimental study of the conducted action potential in cardiac Purkinje strands. Biophys J (1983) 44:1–8.[Web of Science][Medline]
  31. Buchanan J.W Jr., Saito T, Gettes L.S. The effects of antiarrhythmic drugs, stimulation frequency, and potassium-induced resting membrane potential changes on conduction velocity and dV/dtmax in guinea pig myocardium. Circ Res (1985) 56:696–703.[Abstract/Free Full Text]
  32. Wolk R, Cobbe S.M, Hicks M.N, Kane K.A. Effects of lignocaine on dispersion of repolarisation and refractoriness in a working rabbit heart model of regional myocardial ischaemia. J Cardiovasc Pharmacol (1998) 31:253–261.[CrossRef][Web of Science][Medline]
  33. Luu M, Stevenson W.G, Stevenson L.W, Baron K, Walden J. Diverse mechanisms of unexpected cardiac arrest in advanced heart failure. Circulation (1989) 80:1675–1680.[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 has been cited by other articles:


Home page
 Am. J. Physiol. Cell Physiol.Home page
A. Michailova, W. Lorentz, and A. McCulloch
Modeling transmural heterogeneity of KATP current in rabbit ventricular myocytes
Am J Physiol Cell Physiol, August 1, 2007; 293(2): C542 - C557.
[Abstract] [Full Text] [PDF]

Home page
 Cardiovasc ResHome page
F. L Burton and S. M Cobbe
Dispersion of ventricular repolarization and refractory period
Cardiovasc Res, April 1, 2001; 50(1): 10 - 23.
[Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow E-letters: Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when E-letters are posted
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
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Wolk, R.
Right arrow Articles by Hicks, M. N
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Wolk, R.
Right arrow Articles by Hicks, M. N
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?