British Journal of Anaesthesia, 2003, Vol. 90, No. 4 501-503
© 2003 The Board of Management and Trustees of the British Journal of Anaesthesia
Short Communications |
Effects of halothane on action potential configuration in sub-endocardial and sub-epicardial myocytes from normotensive and hypertensive rat left ventricle
School of Biomedical Sciences and 1 Academic Unit of Anaesthesia, University of Leeds, Leeds LS2 9JT, UK
Corresponding author. E-mail: s.m.harrison@leeds.ac.uk
Accepted for publication: December 9, 2002
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Abstract |
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Background. Halothane shortens ventricular action potential duration (APD), as a consequence of its inhibitory effects on a variety of membrane currents, an effect that is greater in sub-endocardial than sub-epicardial myocytes. In hypertrophied ventricle, APD is prolonged as a consequence of electrical remodelling. In this study, we compared the effects of halothane on transmural APD in myocytes from normal and hypertrophied ventricle.
Methods. Myocytes were isolated from the sub-endocardium and sub-epicardium of the left ventricle of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. Action potentials were recorded before, during, and after a 1-min exposure to 0.6 mM halothane and APD measured from the peak of the action potential to repolarization at –50 mV (APD–50 mV). Data are presented as mean (SEM).
Results. In WKY myocytes, halothane reduced APD–50 mV from 21 (2) to 18 (2) ms (P<0.001, n=15) in sub-epicardial myocytes but abbreviated APD–50 mV to a greater extent in sub-endocardial myocytes (37 (4) to 28 (3) ms; P<0.001, n=14). In SHR myocytes, APD–50 mV values were prolonged compared with WKY and APD–50 mV was reduced by halothane from 36 (6) to 27 (4) ms (P<0.016) and from 77 (10) to 38 (4) ms (P<0.001) in sub-epicardial and sub-endocardial myocytes, respectively.
Conclusions. In the SHR, hypertrophic remodelling was not homogeneous; APD–50 mV was prolonged to a greater extent in sub-endocardial than sub-epicardial cells. Halothane reduced APD to a greater extent in sub-endocardium than sub-epicardium in both WKY and SHR but this effect was larger proportionately in SHR myocytes. The transmural gradient of repolarization was reduced in WKY and effectively abolished in SHR by halothane, which might disturb normal ventricular repolarization.
Br J Anaesth 2003; 90: 501–3
Keywords: anaesthetics volatile, halothane; nerve, transmission
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Introduction |
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During the development of ventricular hypertrophy, remodelling of the myocardium occurs,1 which leads to changes in the expression of various proteins crucial to the normal electrical and contractile function of the heart. As a consequence, the ventricular action potential is prolonged,2 which is thought to result predominantly from a decreased expression of the transient outward K+ current (Ito) rather than changes in the L-type Ca2+ current (ICa) or other K+ currents.3
Exposure to halothane leads to a reduction in ventricular action potential duration4 5 secondary to inhibition of both inward and outward membrane currents (e.g. ICa6 and Ito7). Recently, it has been reported8 that halothane abbreviates action potential duration (APD) to a greater extent in left ventricular sub-endocardial than sub-epicardial myocytes and it was proposed that the transmural gradient in expression of Ito9 contributed to this effect. As several halothane-sensitive currents are affected during hypertrophic remodelling (e.g. Ito), our aim was to investigate whether halothane affected the action potential differentially in hypertensive and normotensive ventricle.
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Methods and results |
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Left ventricular sub-epicardial and sub-endocardial myocytes were dissociated enzymatically8 from 20-week-old Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats (Harlan UK, Oxon, UK). Myocytes were superfused with the following solution (in mM): NaCl 140; KCl 5.4; MgCl2 1.2; NaH2PO4 0.4; HEPES 5; glucose 10; CaCl2 1; pH 7.4 (NaOH) at 30°C. Halothane 0.6 mM was added from a 0.5 M stock solution made up in dimethyl sulphoxide. Action potentials were recorded using the perforated patch clamp technique in current clamp mode (Axoclamp 200A, Axon Instruments, Inc., Foster City, CA, USA).8 APD was measured from the peak of the action potential to repolarization at –50 mV (APD–50 mV). Statistical comparisons were carried out with Student’s t-tests (paired or unpaired as appropriate) or ANOVA followed by corrected t-tests (Tukey) for multiple comparisons. Data are presented as mean (SEM).
SHR myocytes were significantly longer (145 (4) µm, n=27) than WKY myocytes (128 (4) µm, n=28; P=0.005, t-test); however, the degree of hypertrophy was regional; myocyte length was greater in sub-endocardial (SHR, 148 (4) µm vs WKY, 123 (6) µm; P<0.05) than sub-epicardial myocytes (SHR, 141 (7) µm vs WKY, 134 (4) µm; P>0.05).
In WKY, APD–50 mV was shorter (P=0.003) in sub-epicardial myocytes (21 (2) ms, n=15) than sub-endocardial myocytes (37 (4) ms, n=14; Fig. 1). Halothane reduced APD–50 mV to 18 (2) ms (P<0.001) in sub-epicardial myocytes and to 28 (3) ms (P<0.001) in sub-endocardial myocytes. In SHR myocytes, APD–50 mV values were prolonged compared with WKY and APD–50 mV was reduced from 36 (6) to 27 (4) ms (P<0.016), and from 77 (10) to 38 (4) ms (P<0.001) by halothane in sub-epicardial and sub-endocardial myocytes, respectively. Halothane did not affect the resting membrane potential in either WKY or SHR myocytes although action potential amplitude was depressed significantly by halothane in all cells (P<0.05).
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Comments |
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These data are the first to describe the effects of halothane on action potential configuration in hypertrophied ventricle. Two major points arise from this study. The first is that hypertrophy was not homogeneous across the ventricular wall; hypertrophy (measured as an increase in cell length) was greater in cells from the sub-endocardium than sub-epicardium. Wall stress is known to be greater at the sub-endocardial than sub-epicardial surface of the heart.10 This gradient may be the reason for the selective hypertrophy of sub-endocardial myocytes and this may be causally related to the greater prolongation of APD noted in SHR sub-endocardial cells (Fig. 1).2
The second main point of this study is that exposure to a clinically relevant concentration of halothane (approximately twice the minimum alveolar concentration) had a greater proportionate inhibitory effect on APD in hypertrophied than in normotensive myocytes, especially in sub-endocardial cells (Fig. 1). As such, the transmural gradient of repolarization in SHR was reduced to 27% of its control value by halothane, and APD–50 mV was no longer significantly different between the sub-endocardium and sub-epicardium in the presence of halothane. This effect was greater than observed in WKY myocytes where the transmural gradient of APD was reduced to 63% of its control value by halothane.
In summary, the data show that halothane reduces APD, which could potentially reduce the incidence of re-entrant arrhythmias in the hypertrophied ventricle by reducing the increased transmural dispersion of repolarization/refractoriness. However, as sub-endocardial APD was reduced dramatically by halothane, the transmural gradient in APD–50 mV was essentially abolished in the SHR and this may also impact on repolarization in hypertrophied ventricle.
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References |
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1 Swynghedauw B. Molecular mechanisms of myocardial remodelling. Physiol Rev 1999; 79: 215–62
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8 Rithalia A, Gibson CN, Hopkins PM, Harrison SM. Halothane inhibits contraction and action potential duration to a greater extent in subendocardial than subepicardial myocytes from the rat left ventricle. Anesthesiology 2001; 95: 1213–9[CrossRef][Web of Science][Medline]
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