Department of Anaesthesiology and Critical Care Medicine, Hokkaido University Graduate School of Medicine, N-15, W-7, Kita-ku, Sapporo 060-8638, Japan E-mail: mazuma@med.hokudai.ac.jp
Accepted for publication: August 30, 2002
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Abstract |
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Methods. Electrophysiological studies of the effects of bupivacaine, epinephrine, amrinone and milrinone in normal and high K+ Tyrodes solution were measured with guinea-pig papillary muscles. Specifically, epinephrine, amrinone and milrinone reversal of bupivacaine-induced depression was measured.
Results. Bupivacaine reduced the action potential duration (APD), the maximum rate of rise of the AP (V·max) and contractile force. Although epinephrine increased the contractile force similarly to amrinone and milrinone, it shortened the APD at 50% repolarization (APD50) and 90% repolarization (APD90). A high concentration of amrinone shortened APD, while milrinone did not affect APD except for a prolongation of APD20. In high K+ Tyrodes solution (25 mM), epinephrine, amrinone and milrinone increased the APD and the contractile force. Epinephrine reversed bupivacaine depression of APD and contractile force to control levels. Amrinone and milrinone restored not only the contractile force but also APD. There was an incomplete recovery of APD50 for amrinone and the prolongation of APD20 for milrinone.
Conclusions. Our results suggest that bupivacaine decreases the Ca+ current (ICa) and Na+ current (INa). Epinephrine and amrinone may increase ICa and the delayed outward current (Ik), whereas milrinone may increase ICa. The profound cardiovascular depression caused by bupivacaine was effectively reversed by amrinone and milrinone in a manner similar to epinephrine.
Br J Anaesth 2003; 90: 6671
Keywords: anaesthetics local, bupivacaine; enzymes, phosphodiesterase, inhibition; nerve, transmission; sympathetic nervous system, epinephrine
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Introduction |
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Inhibition of phosphodiesterase (PDE) III leads to increased intracellular concentrations of cyclic adenosine monophosphate (cAMP)5 and activation of protein kinases,6 which in turn explain the positive inotropic effects of the drug. It has been shown that amrinone (PDE III inhibitor) increases cardiac output (CO) because of an increase in myocardial contractility7 and effectively reverses profound bupivacaine-induced cardiovascular depression in pigs through intracellular Ca2+-release mechanisms.8 Milrinone (another PDE III inhibitor), in a manner similar to amrinone, enhances cardiac contractility, increases CO,4 acts on Ca2+ channels in the sarcolemma, and increases cAMP, which activates Ca2+ influx and Ca2+ release/uptake from the sarcoplasmic reticulum.9 10 In this study, we evaluated whether amrinone and milrinone would reverse bupivacaine-induced cardiodepression in a manner similar to epinephrine.
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Methods |
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One end of the papillary muscle was hooked to an extension of the lever arm of a force transducer (Nihon Kohden, JZ-802J, Tokyo, Japan), and the other end was pinned to the bottom of the tissue chamber. The transducer was mounted on a micromanipulator and resting tension was progressively increased to 200 mg. These preparations were stimulated at 0.5 Hz through platinum field electrodes with rectangular pulses of 1 ms duration at twice the threshold voltage, delivered by an electronic stimulator (Nihon Kohden, SEN 3301) through an isolation unit (Nihon Kohden, SS-102J). Transmembrane potentials were recorded using glass microelectrodes filled with KCl 3 M (resistance of 1030 M). The microelectrodes were coupled through an Ag/AgCl junction to a high-impedance capacitance-neutralizing amplifier (Nihon Kohden, MZE2801). An agar bridge containing KCl 3 M was used as a reference electrode. An electronic differentiator with linear output 501000 V s1 was used for measurement of the maximum rate of rise of the fast action potential (AP). This rate is denoted by V·max. These amplified signals were displayed on a digital oscilloscope (Hewlett Packard 54501A, Palo Alto, CA, USA) and recorded on a chart recorder (Hewlett Packard, 2225AJ). The preparations were stabilized for
60 min before commencement of the experiments. After a 30 min stabilization period for the AP, control recordings were made and the preparations were exposed to solutions containing various concentrations of bupivacaine, epinephrine, amrinone and milrinone. The concentration was increased in a stepwise fashion at intervals of 30 min and recordings were made when the changes in the AP and the developed tension reached a steady state. In the experiments to test the recovery of bupivacaine-induced depression, the preparations were exposed for 30 min to bupivacaine and then to the solutions in the absence of bupivacaine, containing various concentrations of epinephrine, amrinone and milrinone.
To induce slow AP, contributing to an indirect estimate of the Ca2+-mediated slow inward current, a 25 mM K+ Tyrodes solution for depolarizing the papillary muscles was made by isotonic substitution of NaCl with KCl. Resting membrane potential (RMP) was usually depolarized to approximately 50 mV in this solution, which is sufficient to inactivate fast Na+ channels. Slow AP was induced electrically by delivering rectangular pulses of 1011 ms duration to the field electrodes at a frequency of 0.5 Hz. After a 15 min stabilization period of the slow AP, the experimental procedure commenced. A concentration of 5x105 M bupivacaine was selected for the slow AP study because this concentration caused a 75% decrease in contractile force with normal Tyrodes solution. Epinephrine 5x107 M, amrinone 103 M and milrinone 5x105 M were used because these concentrations produced recovery of the contractile force to control values after bupivacaine-induced depression.
The following drugs were used: bupivacaine (Sigma Chemical, St Louis, MO, USA); epinephrine (Daiichi Pharmaceuticals, Tokyo, Japan); amrinone (Meiji Pharmaceuticals, Tokyo, Japan); and milrinone (Yamanouchi Pharmaceuticals, Tokyo, Japan).
All data are mean (SEM). Students t-test was used for analysis of paired and unpaired observations. P<0.05 was considered statistically significant.
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Results |
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Milrinone increased contractile force and APA, but did not influence APD except for a prolongation of APD20 (Table 3). Milrinone at 5x105 M, which, in normal Tyrodes solution, increased contractile force to 237.6 (26.2)%, also prolonged APD50 and increased contractile force in high K+ Tyrodes solution (Table 2).
As shown in Table 4, epinephrine 5x107 M administration restored APD and contractile force to control levels (Fig. 1A). Amrinone at 5x104 M and 103 M restored APD20 and APD90, but not APD50, to control levels. Contractile force was restored to control at amrinone concentration of 103 M (Fig. 1B). For milrinone, 105 M and 5x105 M caused recovery of APD50 and APD90 to control and prolonged APD20. Contractile force was also restored to control at 105 M, and significantly increased at 5x105 M. V·max was restored at both concentrations (Fig. 1C).
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Discussion |
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Epinephrine caused AP shortening in guinea-pig papillary muscles. However, it also increased contractile force in normal Tyrodes solution. For slow AP in high K+ Tyrodes solution, epinephrine prolonged APD50 and increased contractile force. This finding suggests that epinephrine increases Ca2+ influx through the myocardial Ca2+ channel. It has been reported that beta adrenoceptor agonists increase ICa and the delayed outward current (IK) in ventricular cells. Bennett and Begenisich12 suggested that an increase in IK produced by catecholamines may serve to limit the degree of calcium-current-induced AP prolongation during increased sympathetic tone and rapid heart rates. Our observations may indicate a possible mechanism to explain the AP shortening caused by epinephrine which may result from an increase in IK in the guinea-pig ventricular muscle. Epinephrine may not only enhance ICa, but also IK, both of which contribute to the APD.
Amrinone shortened APD at 5x104 M and 103 M and increased contractile force. For the slow AP, amrinone prolonged APD50 and increased contractile force. These results suggest that amrinone increases Ca2+ influx through myocardial Ca2+ channels. However, the shortening of APD at 5x104 M and 103 M in normal Tyrodes solution may be explained by an increase in IK. It is well documented that an increase in intracellular cAMP leads to an increase in the K+ current in isolated ventricular myocytes.13 We suggest that the underlying mechanism of amrinone may be to increase ICa and Ik via phosphorylation of the channel secondary to an increase in cAMP.
Conversely, milrinone did not affect APD50 or APD90, but increased contractile force and prolonged APD20. Under slow AP conditions, milrinone prolonged APD50 and increased contractile force. The positive inotropic effect of milrinone may be attributable to a greater influx of calcium during the AP and more efficient intracellular calcium handling.10 Indeed, milrinone increased ICa measured using conventional microelectrode and patch clamp techniques.14
In the reversal study, the washout of bupivacaine with drug-free Tyrodes solution did not restore APD nor contractile force to control level. Epinephrine (5x107 M) restored contractile force, APD and V·max, while amrinone restored these only at 103 M. Milrinone also produced recovery of contractile force at 105 M, and an increase at 5x105 M.
Comparisons between epinephrine, amrinone and milrinone of the concentration required to produce >200% increase in the contractile force, indicated that epinephrine was the most potent and amrinone the least potent. Epinephrine induced severe tachycardia, hypertension and cardiac arrhythmia during resuscitation from ropivacaine-induced cardiovascular toxicity in pigs, whereas milrinone did not.4 Amrinone was also superior to epinephrine for the treatment of bupivacaine-induced cardiovascular depression in sevoflurane-anaesthetized dogs according to a study in which tachycardia occurred in response to epinephrine, compared with those receiving amrinone.15
In conclusion, for reversal of the cardiodepressive effects of bupivacaine in guinea-pig papillary muscles, amrinone and milrinone were similar to epinephrine. Recovery of cardiodepression may be because of the positive inotropic effect of epinephrine, amrinone and milrinone accompanied by an increase in ICa.
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Acknowledgement |
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References |
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