Abstract
Bradycardia is one of the inevitable and undesirable responses when the muscle weakness induced by nondepolarizing muscle relaxants is reversed by AChE inhibitors. The current study was designed to compare the bradycardiac effects of the two AChE inhibitors used widely in clinical anesthesia, neostigmine and edrophonium. Isolated, spontaneously beating guinea pig right atrial preparations were used as the experimental model, and in some cases, electrical field stimulation was utilized to stimulate parasympathetic nerve terminals within the atria. Neostigmine decreased the spontaneously beating rate in a concentration-dependent manner at concentrations up to 10 μM. At higher concentrations, the agent restored the beating rate to the predrug control level. Atropine abolished the biphasic response of the atrium to neostigmine. In contrast, edrophonium had no effect on the spontaneous beating rate. However, edrophonium (3 μM) potentiated the field stimulation-induced negative chronotropic effect. Tetrodotoxin did not inhibit the chronotropic effect of neostigmine. Both neostigmine and edrophonium at higher concentrations inhibited the negative chronotropic effect of carbachol. In conclusion, neostigmine possesses potential dual effects on cardiac muscarinic ACh receptors. Low concentrations of neostigmine may stimulate the receptors directly, and at higher concentrations neostigmine may act as an antimuscarinic agent. On the other hand, edrophonium may inhibit the cardiac muscarinic ACh receptors exclusively without stimulating the receptors. These results could at least partially explain the difference between the bradycardiac effects of the agents observed clinically.
The AChE inhibitor neostigmine has been widely used to reverse the muscle weakness induced by nondepolarizing muscle relaxants, although another inhibitor, edrophonium, has recently been accepted as an effective substitute for neostigmine (Bevan, 1994) because of its rapid onset of action with less bradycardiac side effect (Cronnelly et al., 1982). It is generally accepted that facilitation of cholinergic neurotransmission by an AChE inhibitor in both neuromuscular junction and postganglionic parasympathetic synapse is due to increases in the number and lifetime of ACh molecules released from nerve terminals and to the subsequent increase in concentration of ACh in the synaptic clefts (Bevan, 1994). However, the reason why neostigmine produces more serious bradycardia is still unclear. Several reports have suggested that neostigmine acts directly at sites where ACh acts and produces additional effects that are not related to inhibition of AChE. For example, neostigmine depolarizes autonomic ganglion cells by both a hexamethonium-sensitive mechanism (Mason, 1962) and an atropine-sensitive mechanism (Takeshige and Volle, 1963). It also inhibits single-channel currents activated by ACh in BC3H1 mouse tumor cells (Wachtel, 1990). Radioligand binding studies reveal the direct interaction of neostigmine with cardiac muscarinic ACh receptors (Dunlap and Brown, 1983) and nicotinic ACh receptors (Sherby et al., 1985). Therefore, it is conceivable that additional effects of the AChE inhibitors—effects that are not related to AChE inhibition—are responsible for the difference in bradycardiac effects of the agents.
The aim of the present study was to clarify the difference between the mechanisms underlying the bradycardiac effects of neostigmine and edrophonium. For this purpose, we used the spontaneously beating right atrium with intact SA node cells and postganglionic cholinergic nerve terminals isolated from guinea pig heart. We show that neostigmine directly decreases the beating rate of the right atrium, whereas edrophonium does not. However, the negative chronotropic effect of edrophonium is manifest when endogenous ACh is released by electrical FS from parasympathetic nerve terminals embedded in the preparation.
Materials and Methods
All animal experiments were done in conformity with the “Guiding Principles for Research Involving Animals and Human Beings” of Yokohama City University School of Medicine.
Hartley guinea pigs (Japan SLC Co., Shizuoka, Japan) of either sex weighing 250 to 350 g were killed by cervical dislocation under light anesthesia with diethyl ether. After midline thoracotomy, the heart was rapidly excised and placed in a dissection dish filled with oxygenated Krebs-Henseleit solution of the following composition (mM): NaCl, 119.0; CaCl2, 2.5; KCl, 4.8; MgSO4, 1.2; KH2PO4, 1.2; NaHCO3, 24.9; glucose, 10.0.
Chronotropic response of the right atrium to the AChE inhibitors.
The spontaneously beating right atrium was carefully dissected and mounted vertically in a 10-ml double-walled glass chamber filled with Krebs-Henseleit solution, gassed continuously with 95% O2 and 5% CO2 (pH 7.4) and maintained at 36°C. The lower end of the right atrium was fixed on a hook, and the upper end was connected by a silk thread to an isometric force-displacement transducer (model UL-10GR; Minebea, Nagano, Japan); changes in tension were recorded with a pen recorder (model Recti-Horiz-8K; Sanei-Sokki, Tokyo, Japan) through a preamplifier (model AS2103; Sanei-Sokki). Diastolic tension was adjusted to 0.5 g. Spontaneously beating rate was counted on a chart recording of its developed tension. The preparation was equilibrated for at least 60 min before the experiments began.
The concentration-response curves for the chronotropic effects of neostigmine and edrophonium were determined in a cumulative manner by increasing their concentrations in steps of 0.5 log units. When examining influences of several kinds of inhibitors on the chronotropic effect of neostigmine, we added the drugs 30 min before obtaining the concentration-response curve.
Chronotropic response of the right atrium to transmural parasympathetic nerve stimulation.
Parasympathetic nerve endings were stimulated transmurally. Field stimuli were delivered at a frequency of 25 Hz by a pair of spiral platinum electrodes and consisted of 50 consecutive square-wave pulses 3 msec in duration and 50 V in intensity generated by a stimulator (model SEN-3301, Nihon Kohden, Tokyo, Japan). Propranolol (1 μM) was added to the incubation medium throughout the experiment to prevent the activation ofbeta adrenoceptor by norepinephrine released from sympathetic nerve terminals. Each preparation was stimulated four times. The second response was taken as control. Either of the AChE inhibitors was added to the incubation solution 30 min before the third FS. After the preparation was washed three times, once every 15 min, in fresh Krebs-Henseleit solution, the forth field stimulation was added to the preparation.
Influences of the AChE inhibitors on the negative chronotropic effects of CCh and adenosine.
The concentration-response curves for the negative chronotropic effects of CCh and adenosine were determined in a cumulative manner by increasing their concentrations in steps of 0.5 log units. At the completion of the concentration-response curve, the preparations were washed thoroughly. After beating rate returned to the predrug level, one concentration of each AChE inhibitor was applied. The incubation time for the drugs was 30 min. The concentration-response curves for CCh and adenosine were then obtained in the presence of the AChE inhibitors. We confirmed that repetitive application of CCh and adenosine to the preparations did not affect the concentration-response curves for the negative chronotropic effects of CCh and adenosine, respectively. The concentration of CCh causing half-maximal response in the absence and presence of various concentrations of the tested drugs was estimated from log-probit plots of the individual response vs. concentrations. Affinity measurement for edrophonium was estimated by Schild analysis (Arunlakshana and Schild, 1959).
Drugs.
The following drugs were used: neostigmine bromide, edrophonium chloride, carbamylcholine chloride, atropine sulfate, pirenzepine dihydrochloride, adenosine, dl-propranolol hydrochloride, (±)nicotine, hexamethonium bromide (Sigma Chemical Co., St. Louis, MO) and tetrodotoxin (Wako Pure Chemical Co., Osaka, Japan). Nicotine was dissolved in ethanol, and further dilution was made with distilled water. All other chemicals were dissolved in distilled water.
Statistics.
Values are given as means ± S.E.M. Comparisons of values obtained from the concentration-response curves were made by ANOVA followed by Scheffé’s t test or Bonferroni’s t test. Comparison of more than two groups was performed by ANOVA followed by Scheffé’s t test. The slope of Schild plot analyzed with respect to difference from unity and 95% confidence limits of the slope were calculated according to Brown and Hollander (1977). A value of P < .05 was considered statistically significant.
Results
Chronotropic effects of neostigmine and edrophonium.
Figure1 shows the chronotropic effects of neostigmine and edrophonium on the spontaneously beating right atria. Neostigmine at concentrations up to 10 μM produced the negative chronotropic effect in a concentration-dependent manner. The basal level of the spontaneous beating rate was 186.2 ± 5.2 beats/min, and neostigmine at a concentration of 10 μM decreased it by 46.9 ± 4.5% (n = 7). However, subsequent application of neostigmine increased the beating rate in a concentration-dependent manner. The beating rate of 173.8 ± 7.4 beats/min in the presence of 1 mM neostigmine was not different from the basal value. Pretreatment of the atria with atropine at a concentration of 1 μM abolished completely the chronotropic effect of neostigmine. Atropine itself did not affect the spontaneous beating rate (185.3 ± 4.0 beats/min before the treatment with atropine and 183.3 ± 1.2 beats/min after the treatment; n = 6), which suggests that endogenous ACh did not affect the basal rate.
In contrast to neostigmine, edrophonium did not decrease the beating rate but rather increased it slightly, though insignificantly, at higher concentrations (n = 6).
We also examined the chronotropic effect of nicotine because the intracardiac ganglion neurons are located in the posterior aspect of the atria within the subepicardial connective tissue, in the interatrial septum and in the entrance of the venae cavae (Löffelholz and Pappano, 1985). In the presence of 1 μM propranolol, nicotine at a concentration of 300 μM slightly but significantly decreased the spontaneously beating rate by 15.1 ± 1.0% (n = 4, data not shown). However, neither hexamethonium at a concentration of 300 μM nor TTX at a concentration of 1 μM inhibited the negative chronotropic effect of nicotine, which indicates that in the present experimental model, the soma of the postganglionic parasympathetic neuron that innervated the SA node cells was cut out of the tissue.
Effects of edrophonium and neostigmine on the spontaneous beating rate that is modulated by endogenous ACh released by electrical FS.
As shown in figure 2A, the electrical FS in the presence of 1 μM propranolol evoked a transient negative chronotropic effect on the spontaneously beating right atrium. Atropine at a concentration of 1 μM abolished the response (fig. 2B), which indicates that the FS released ACh from the cholinergic nerve terminals. Pretreatment with 1 μM TTX also abolished the FS-evoked transient decrease in the beating rate (fig. 2C), a result that further indicates the involvement of endogenous ACh in the FS-evoked bradycardiac effect. The inhibitory effect of TTX was confirmed in three other preparations. Seifert and Eldefrawi (1974) reported that an inhibition constant (K i) of edrophonium on AChE was 1.5 μM. Thus we examined the influence of edrophonium at a concentration of 3 μM on the FS-evoked bradycardiac effect. As shown in figure 2D, 3 μM edrophonium potentiated the negative chronotropic response of the right atrium to the FS. The negative chronotropic effect of the FS in the presence of edrophonium was also abolished by pretreatment with atropine (1 μM) (fig. 2E). These results suggest that edrophonium at a concentration of 3 μM attenuates the hydrolysis of ACh in the synaptic cleft and potentiates the negative chronotropic effect of the FS. The potentiating effect of edrophonium is summarized in figure 3A. The time elapsing until the appearance of the first contraction of the atria after the FS was 1.82 ± 0.19 sec (n = 4). Edrophonium at a concentration of 3 μM significantly increased the time elapsing to 4.23 ± 0.38 sec (P < .01). After washing of the preparations with fresh Krebs-Henseleit solution three times, once every 15 min, the negative chronotropic effect of the FS returned to the predrug control level (1.82 ± 0.14, P > .05). The basal beating rate of the atrium did not change significantly throughout the experiment (179.6 ± 5.4 before the control FS, 172.1 ± 5.0 before the FS in the presence of edrophonium and 170.2 ± 5.0 before the FS after washing). Atropine abolished the time lag between the end of the FS and the appearance of the first contraction after the FS in both edrophonium-untreated and treated preparations (n = 4).
We also examined the influence of neostigmine on the negative chronotropic effect of the FS (fig. 3B). The inhibition constant (K i) of neostigmine on AChE is reported to be 20 nM (Seifert and Eldefrawi, 1974). Neostigmine at a concentration of 30 nM increased the time elapsing until the appearance of the first contraction after the FS from 1.76 ± 0.31 sec to 6.17 ± 0.54 sec (n = 4, P < .01). After washing of the preparation with fresh Krebs-Henseleit solution three times, once every 15 min, the time elapsing until appearance of the first contraction returned to the predrug control value (2.22 ± 0.27, P > .05). The basal beating rate of the atrium did not change significantly throughout the experiment (177.3 ± 5.6 before the control FS, 168.4 ± 6.8 before the FS in the presence of neostigmine and 170.6 ± 9.6 before the FS after washing).
Influences of TTX, pirenzepine and propranolol on the chronotropic response of the atria to neostigmine.
Figure4 demonstrates the influence of TTX at a concentration of 1 μM on the chronotropic effect of neostigmine. Pretreatment with TTX did not inhibit the neostigmine-evoked decrease in the beating rate. Neostigmine at a concentration of 10 μM decreased the beating rate in the absence and presence of TTX to 53.1 ± 4.5% (n = 7) and 34.2 ± 8.2% (n = 4), respectively. There was no statistically significant difference between the two concentration-response curves.
Because higher concentrations of neostigmine restored the beating rate that was decreased by lower concentrations of the agent (fig. 1), we examined whether excitatory M1 muscarinic ACh receptors (Costa and Majewski, 1991) and beta adrenoceptors were involved in the increasing effect of neostigmine. Neither pirenzepine at a concentration of 0.1 μM nor propranolol at a concentration of 1 μM affected the restoration of the beating rate by neostigmine at a concentration of 1 mM (data not shown). In addition, pirenzepine at a concentration of 0.1 μM did not affect the bradycardiac effect of neostigmine.
Influences of neostigmine and edrophonium on the negative chronotropic effect of CCh and adenosine.
The muscarinic ACh receptor agonist CCh decreased the spontaneous beating rate of the right atria in a concentration-dependent manner (fig.5). Neostigmine at a concentration of 1 mM shifted rightward the concentration-response curve for the negative chronotropic effect of CCh. EC50 values of CCh in the absence and presence of neostigmine were 0.34 ± 0.04 μM and 5.97 ± 1.11 μM, respectively (n = 5). The pKB value of neostigmine at a concentration of 1 mM was 4.20 ± 0.07. On the other hand, the negative chronotropic effect of adenosine was not affected by neostigmine (fig. 5; n= 4).
Edrophonium also shifted the concentration-response curve for the negative chronotropic effect of CCh to the right in a parallel manner (fig. 6). The EC50 values of CCh in the absence of edrophonium and in its presence at concentrations of 30 μM, 100 μM and 300 μM were 0.31 ± 0.03 μM (n = 20), 0.82 ± 0.14 μM (n = 6), 1.84 ± 0.19 μM (n = 8) and 3.46 ± 0.59 μM (n = 6), respectively. The slope of the regression line (1.03) obtained from the Schild plot was not significantly different from unity, which indicates that edrophonium interacted competitively with cardiac muscarinic ACh receptors. The pA2 value of edrophonium was 4.64.
Recovery of the beating rate after exposure to neostigmine by washing the preparation.
Figure 7shows the consecutive changes in the beating rate that followed the addition and removal of neostigmine. Neostigmine at a concentration of 10 μM decreased the basal level of 181.9 ± 3.8 beats/min (n = 5) to 53.7 ± 27.3 beats/min. Subsequent application of neostigmine at a concentration of 1 mM restored the beating rate to the predrug control level (176.9 ± 5.4 beats/min). Washing the preparations with fresh Krebs-Henseleit solution three times, once every 15 min, slowed the beating rate again to 39.9 ± 27.8 beats/min. This value was not different from the value obtained in the presence of 10 μM neostigmine. Application of atropine at a concentration of 1 μM after washing the preparations restored the beating rate to the predrug control level (185.3 ± 1.9 beats/min).
Discussion
In the present study we showed that neostigmine possessed dual chronotropic effects on the spontaneously beating right atrium in the guinea pig. Neostigmine decreased the beating rate at lower concentrations in a concentration-dependent manner. Subsequent application of higher concentrations of the agent returned the beating rate to the predrug control level. Both responses were abolished by atropine. In contrast, edrophonium did not affect the basal beating rate. Although edrophonium lacked the direct negative chronotropic effect, it potentiated the negative chronotropic effect of endogenous ACh released from the cholinergic nerve terminals by the electrical FS, which indicated that edrophonium could evoke bradycardia only by inhibiting the hydrolysis of ACh molecules released from the cholinergic nerve terminals. The bradycardiac effect of the FS was also potentiated by neostigmine at a concentration lower than concentrations that decreased the beating rate directly. TTX did not inhibit the negative chronotropic effect of neostigmine. Both neostigmine and edrophonium at higher concentrations inhibited the negative chronotropic effect of CCh. These results may suggest that neostigmine, but not edrophonium, stimulates postsynaptic cardiac muscarinic ACh receptors directly at lower concentrations, whereas both agents may inhibit those receptors at higher concentrations.
Activation of muscarinic ACh receptors is thought to be causally related to the negative chronotropic effect of neostigmine, because atropine abolished the bradycardiac effect of the agent (fig. 1). There are at least three possible underlying mechanisms: 1) inhibition of AChE in the synaptic cleft, resulting in the increases in the amount and half-life of ACh discharged spontaneously from the nerve terminals and activation of postsynaptic muscarinic ACh receptors in the SA node cells; 2) release of ACh from the postganglionic parasympathetic nerve terminals, resulting in the activation of postsynaptic muscarinic ACh receptors in the SA node cells and 3) direct activation of postsynaptic muscarinic ACh receptors in the SA node cells.
Brown et al. (1982) reported that the AChE inhibitors physostigmine and neostigmine produced a marked attenuation of isoproterenol-stimulated cAMP accumulation in the mouse atria. Although the attenuation was inhibited by atropine, the concentration of the agents required to attenuate isoproterenol-stimulated cAMP accumulation was lower than the concentration required to compete for [3H]QNB binding to the cardiac muscarinic ACh receptors. Thus the authors concluded that the AChE inhibitors could activate the cardiac muscarinic ACh receptors by inhibiting hydrolysis of ACh discharged spontaneously from the cholinergic nerve terminals. However, the present study does not support this conclusion, because another AChE inhibitor, edrophonium, failed to decrease the spontaneously beating rate (fig. 1A). Inasmuch as edrophonium possesses an antimuscarinic effect (fig. 6), it may be that the bradycardiac effect of edrophonium was masked by the antimuscarinic effect of the agent. However, edrophonium at a concentration of 3 μM potentiated the negative chronotropic effect of endogenous ACh released from the nerve terminals by the FS (figs. 2; 3A). This concentration of edrophonium is 2 times higher than its inhibition constant (K i) on AChE (1.5 μM; Seifert and Eldefrawi, 1974) and 8 times lower than its dissociation constant on the cardiac muscarinic ACh receptors (pA2 = 4.62, fig. 6).
This observation led us to two important conclusions. First, it is unlikely that the antimuscarinic effect of edrophonium masked its bradycardiac effect produced by inhibiting the activity of AChE. Second, the amount of ACh molecules liberated spontaneously from the parasympathetic nerve terminals was not sufficient for edrophonium to evoke the negative chronotropic effect, and activation of the parasympathetic nerve was prerequisite to the appearance of its negative chronotropic effect. Seifert and Eldefrawi (1974) also reported that the inhibition constant (K i) of neostigmine on AChE was 20 nM, which is much lower than the concentrations required to decrease the spontaneously beating rate in the present study (fig. 1). Indeed, neostigmine at a concentration of 30 nM potentiated the bradycardiac effect of the FS without affecting the basal beating rate (fig. 3B). Taken together, these results suggest that it is highly unlikely that the inhibitory effect of neostigmine on AChE is causally related to the negative chronotropic effect of the agent observed in the present study.
Backman et al. (1993) reported that neostigmine evoked bradycardia by activating intracardiac ganglion cells and producing ACh release from the presynaptic nerve terminal in the cat. Carlyle (1963)also demonstrated that neostigmine contracted guinea pig isolated tracheal smooth muscle by releasing ACh from postganglionic parasympathetic nerve endings. However, TTX did not inhibit the negative chronotropic effect of neostigmine (fig. 4), which suggests that in the present study, neostigmine decreased the spontaneously beating rate by a mechanism other than the activation of parasympathetic nerve fibers. Because the soma of the postganglionic parasympathetic neurons that innervate muscarinic ACh receptors in the SA node cells was absent in the present experimental condition, we cannot rule out the possibility that neostigmine activated the intracardiac ganglion neurons, resulting in release of ACh, as advocated by Backman et al. (1993) and Carlyle (1963).
Although we do not have any decisive evidence now, a direct activation of cardiac muscarinic ACh receptors seems to be the mechanism most likely to underlie the negative chronotropic effect of neostigmine.Brown et al. (1982) argued against the direct interaction of neostigmine with the cardiac muscarinic ACh receptors on the basis of the result from the [3H]QNB binding study described above. It should be noted that there is considerable effective receptor reserve as defined by Kenakin (1986) with regard to cardiac muscarinic ACh receptors. That is, 10% and 50% receptor occupancies by CCh suffice to induce maximal inhibitions of cardiac contractile force and adenylate cyclase activity, respectively (Delhaye et al., 1984). Thus neostigmine could produce the agonistic effect by binding to a small number of the receptors, and if so, it might be difficult to detect the binding to the receptor of neostigmine working as an agonist by analyzing its displacement curve for [3H]QNB binding.
All the responses of the atrium to neostigmine were abolished by atropine (fig. 1), which suggests that neostigmine at higher concentrations might increase the beating rate by activation of muscarinic ACh receptors. In this regard, Costa and Majewski (1991)have reported the existence of facilitatory M1 ACh receptor subtype on the presynaptic sympathetic nerve terminal in the heart. However, the restoration of the beating rate by neostigmine was inhibited neither by propranolol nor by pirenzepine, a selective M1 subtype antagonist (Hammer et al., 1980). It is well known that the negative chronotropic response of SA node fades with time during continuous parasympathetic nerve stimulation or continuous application of ACh (Jalife et al., 1980; Boyettet al., 1988). Thus the restoration of the beating rate might be due to desensitization of the negative chronotropic response of the atrium to higher concentrations of neostigmine. However, the desensitization is also unlikely to explain the biphasic concentration-response curve for the chronotropic effect of neostigmine, because CCh did not exhibit the biphasic concentration-response curve, and CCh still could stop the spontaneous beating even in the presence of 1 mM neostigmine (fig. 5). An alternative possibility is that neostigmine at higher concentrations exhibits an antimuscarinic effect. The results depicted in figure 5support this hypothesis. Because the intracellular signal transduction pathways for the negative chronotropism of CCh and adenosine are thought to be common (Belardinelli and Isenberg, 1983; Kurachi et al., 1986), the results strongly suggest that neostigmine inhibits selectively the interaction of CCh with the cardiac muscarinic ACh receptors.
Edrophonium also possesses an antimuscarinic effect (fig. 6). The slope of a regression line obtained from the Schild plot was not different from unity, which indicates that edrophonium inhibits cardiac muscarinic ACh receptors in a competitive manner. It is difficult to compare the potencies of the antimuscarinic effects of neostigmine and edrophonium, because neostigmine possesses the agonistic effect simultaneously. However, the pKB value of neostigmine at a concentration of 1 mM (4.20) is similar to the pA2 value of edrophonium (4.64), which suggests that there is little difference between the antimuscarinic potencies of the two agents, compared with the about 75 times difference between their anti-AChE potencies (Seifert and Eldefrawi, 1974).
By what molecular mechanism could neostigmine activate the cardiac muscarinic ACh receptors at lower concentrations and inhibit the receptors at higher concentrations? The results shown in figure 7strongly suggest that neostigmine possesses two binding sites on the cardiac muscarinic ACh receptor. That is, the binding site with high affinity for neostigmine would mediate the negative chronotropic effect of the agent, and the binding is too tight to be dissociated by washing. On the other hand, the binding site with low affinity would mediate the antagonistic effect of neostigmine, and the binding is easily dissociated. In addition, the fact that neostigmine-produced bradycardia was hard to reversed by washing off is in contrast to the reversible inhibitory effect of neostigmine on AChE (fig. 3B), which further suggests that the negative chronotropic effect of neostigmine observed in the present study is independent of its inhibitory effect on AChE. Taken together, these results imply that edrophonium inhibits cardiac muscarinic ACh receptor in a competitive manner. On the other hand, neostigmine would interact with the agonist binding site in the cardiac muscarinic ACh receptors and produce the negative chronotropic effect. Higher concentrations of neostigmine would also interact with the different site with low affinity (allosteric site) to inhibit activation of the receptors. Electrophysiologic experiments using single atrial myocytes and radioligand binding studies will be needed to elucidate the exact mechanisms of neostigmine-produced dual effects.
After the administration of neostigmine at a dose of 0.07 mg/kg, peak serum levels may approach about 1 μM (Cronnelly et al., 1979). This concentration of neostigmine produces a significant decrease in the beating rate in the present study. Thus, in addition to the inhibition of AChE, a direct stimulation of cardiac muscarinic ACh receptor might contribute to the bradycardiac response of the heart to neostigmine. On the other hand, peak serum levels of 50 μM are expected after administration of edrophonium at a dose of 1 mg/kg (Morris et al., 1981). This concentration of edrophonium is sufficient to produce an antimuscarinic effect in the present study. Therefore, the different ways in which neostigmine and edrophonium interact with the cardiac muscarinic ACh receptors could at least partially explain the difference between the parasympathomimetic effects of the agents observed clinically.
In conclusion, neostigmine possesses possible dual effects on postsynaptic muscarinic ACh receptors in the SA node cells of guinea pig hearts. We speculate that the agent at lower concentrations stimulates the receptors, resulting in a decrease in the beating rate. However, at higher concentrations neostigmine exhibits an antimuscarinic effect. On the other hand, edrophonium, which produces less parasympathomimetic effect than neostigmine in a clinical setting, possesses only an antimuscarinic effect without stimulating the receptors.
Footnotes
-
Send reprint requests to: Masayuki Endou, M.D., Department of Anesthesiology, Yokohama City University School of Medicine, Yokohama 236, Japan.
- Abbreviations:
- CCh
- carbachol
- FS
- field stimulation
- QNB
- quinuclidinyl benzilate
- TTX
- tetrodotoxin
- Received October 7, 1996.
- Accepted May 16, 1997.
- The American Society for Pharmacology and Experimental Therapeutics