HTML

ePub

PDF

Introduction

The quotient between the fourth (T4) and first (T1) muscular tensions generated by train-of-four (TOF) stimulus (2.00 Hz during 2.00 s) applied on motor nerve is TOFratio (T4/T1). TOFratio reductions are referred to as TOFfade and indicate the degree of patients' curarization (Ali, Utting, & Gray, 1970; Murphy, 2006). The patients’ curarization may also be monitored by assessing the ability of skeletal muscle to maintain a sustained muscular contraction (tetanic contraction) when high electrical frequencies of stimulation (tetanizing stimulus) are applied on the motor nerve. Tetanic fade (Fade) or Wedensky inhibition occurs when the tetanic

contraction is characterized by a poorly sustained muscular contraction that follows a fast muscular contraction of high amplitude. Fade is assessed by measuring the ratio (R) between the amplitude of muscular contraction obtained at the end (B) and that (A) recorded at the beginning of tetanizing stimulus (R = B/A). Although better reflecting the motor nerve-firing rate under physiological conditions, high frequency (e.g. 50 Hz) nerve-evoked muscle contractions produce patient discomfort in clinical practice. Thereby, TOF

stimulus is clinically employed with more frequency. However, it has been shown that the values of TOFfade and Fade obtained in the presence of antinicotinic agents may express different conditions of neuromuscular transmission, as different patterns of stimulation applied on motor nerve in the absence or in the presence of drugs, may modify the autoregulatory cholinergic (facilitatory-nicotinic receptors expressing α3β2 subunits, facilitatory-M1 and inhibitory-M2 subtypes of muscarinic receptors) and the modulatory (facilitatory-A2A and inhibitory-A1 adenosine receptors) mechanisms on the terminal (Bornia, Bando, Machinsky, Pereira, & Prado, 2009; Bornia, Sá, & Prado, 2011; Pereira, Bornia, Sá, & Prado, 2011). Therefore, the Fade caused by antinicotinic (hexamethonium) agent may be severe (~ 89%) when the concentration of such agent is causing 25% TOFfade (Pereira, Sá, & Prado, 2012). In contrast, the difference between the percentages of Fade (~ 17%) and TOFfade (~ 25%) caused by an antinicotinic agent may not be so drastic if the antinicotinic (cisatracurium) agent also inhibits the acetylcholinesterase activity (Ki ~ 0.32 µmol L-1) (Pereira et al., 2012).

In clinical practice, atropine, a nonselective muscarinic receptor blocker, is employed to prevent the parasympathomimetic effects induced by neostigmine when the anticholinesterase agent is clinically used to reverse the blockage of neuromuscular transmission caused by non-depolarizing muscular relaxants (Barrio, Miguel, García, & Pelegrín, 2007). Nevertheless, it has been shown that atropine alone already improves the recovery of Fade induced by antinicotinic (hexamethonium, D-tubocurarine) agents in the anterior tibial muscle nerve preparation of cats (Prado, Corrado, & Prado, 1987). This effect of atropine might be determined by the ability of the drug to reduce the action of acetylcholine on the presynaptic inhibitory-M2 muscarinic receptors when the motor nerve is being stimulated at tetanizing frequencies (Prado et al., 1987). On the other hand, it has been shown that cisatracurium, an antinicotinic muscular relaxant agent that exhibits anticholinesterase activity in its molecules at clinical concentration (Bornia et al., 2009), also causes Fade in the phrenic nerve diaphragm muscle preparations of rats, but the Fade caused by the agent is not mediated by presynaptic inhibitory-M2 receptors. In fact, Fade caused by cisatracurium seems to depend more on the activation of facilitatory-M1 and facilitatory-A2A adenosine receptors than on activation of presynaptic inhibitory-M2 receptors on

motor nerve (Bornia et al., 2009; Bornia et al., 2011; Pereira et al., 2012). Contrastingly, it has been reported that the cisatracurium-induced TOFfade occurs mainly by the activation of M2 and A2A receptors caused by acetylcholine released from motor nerve and from the adenosine respectively released from neuronal and muscular sources (Pereira et al., 2012).

Since the fades (TOFfade and Fade) caused by non-depolarizing agents exhibiting anticholinesterase activity in their molecules may be determined by mechanisms different from those involved with the fades causing muscular relaxants without anticholinesterase activity (Pereira et al., 2012), and taking into account that atropine is the muscarinic antagonist used in anesthesiology practice (Barrio et al., 2007), current study comparatively tested the protective effects of atropine and ZM241385, the latter being a blocker of A2A receptors (Pereira et al., 2012) against the cisatracurium inducing drastic levels of fades (83% TOFfade, 100% Fade) in the phrenic nerve diaphragm muscle of rats.

Material and methods

procedures used in current assay were approved by the Committee for Ethics in Experimental Studies of the State University of Maringá, Maringá, Paraná State, Brazil. Male Wistar rats (250 g) were anaesthetized with an intramuscular injection of ketamine (40 mg kg-1) and xylazine (8 mg kg-1). Phrenic nerve diaphragm–muscle preparations were isolated and assembled according to methods by Bülbring (1946). Each preparation was immersed in a 20 mL chamber with Krebs’ buffer (composition in mmol L-1: NaCl 110.0; KCl 4.70; CaCl2 3.0; MgCl2 1.3; NaHCO3 25.0; KH2PO4 1.0; glucose 11.1) at 37°C, continuously gassed with a mixture of oxygen (95%) and carbon dioxide (5%). Hemidiaphragms were connected to an isometric force transducer (FT10; Grass Instruments Division, West Warwick, RI, USA). Muscle contraction responses were recorded continuously at a resting tension of 50 mN with a PowerLab data-acquisition system (Chart Software; ADInstruments, Castle Hill, NSW, Australia). The phrenic nerve was stimulated with a bipolar platinum electrode (supramaximal rectangular pulses, 0.05 ms). The preparations were stimulated indirectly at 0.2 Hz for 15 min (equilibration protocol). Afterward, the phrenic nerve was supramaximally stimulated with TOF stimuli delivered at 2 Hz, which was repeated once every 15 s over a period of 7.3 min (TOF protocol). The above sequence was repeated three times. The muscular tension produced at the beginning of each train (T1) was compared with the muscular tension obtained at the end the train (T4). TOFratio, the quotient of these two values (T4/T1), was taken as a measure of drug-induced neuromuscular transmission failure (TOFfade). TOF

stimulus obtained at instants 15 and 30 min after the cisatracurium administration (Figure 1a) was systematically used for comparison. Preparations were stimulated indirectly with 10 s-duration high-frequency (50 Hz) tetanic trains applied at 15 min intervals so that the fading of tetanic contractions could be investigated (Figure 1c). This interval was selected to avoid the possible influence of previous tetanic stimulation on fade induced by subsequent stimuli. Initial tetanic tension at the beginning of the tetanic stimulus (A) and the tension at the end of the tetanic stimulus (B) were recorded and the B/A ratio (Faderatio) was calculated (Figure 1d). A, B and Faderatio values were obtained at the same instants used to assess the TOFfade, i.e., in the absence of drugs (control, one record of Fade), after atropine (0.2 µM) or 4-(2-[7-amino-2-(2-furyl)[1,2,4] triazol [2,3-a][1,3,5] triazin-5-ylamino] ethyl) phenol] (ZM 241385; 10 nmol L-1), separately (two records of Fade) and in combined administration of drugs (atropine or ZM 241385 followed by administration of cisatracurium). The cisatracurium concentration (2.2 µM) used was that previously tested capable of causing 25% TOFfade after 3 min from its administration in Krebs buffer (Pereira et al., 2012). Muscarinic and adenosine receptor antagonists were applied to the bath solution at a constant volume of 10 µL 1 min before the second sequence of TOF or tetanic stimulation, with the antinicotinic muscle relaxing agents, then applied 1 min before the third sequence of TOF or tetanic stimulation, respectively (Figure 1a and c).

Statistical analysis

Data are expressed as mean ± SEM of n individual experiments. The statistical significance of experimental results was analyzed by two-sided one-way analysis of variance (ANOVA) followed by Dunnett’s post-hoc test (p < 0.05, Prism 3.02; GraphPad, San Diego, CA, USA).

Results and discussion

Cisatracurium (2.2 µM) causes 25% TOFfade (2 Hz) 3 min after its administration in the Krebs buffer (Pereira et al., 2012). In these conditions, cisatracurium determines 17% of Fade (50 Hz) (Pereira et al., 2012). Current study shows that the fades (TOFfade and Fade) caused by 2.2 µM cisatracurium are progressively worsened (TOFfade = 52.73 ± 7.2%, n = 4, at T = 15 min and 82.74 ± 6.35%, n = 4, at T = 30 min; Fade = 100 ± 0.0%, n = 4, at T = 15 or 30 min), when the parameters (T4/T1 and R = B/A) used to verify levels of fades in neuromuscular transmission were analyzed 15 min after the instant of administration of neuromuscular relaxant (Figure 2a and d). Atropine (0.2 µM) and the antagonist of A2A receptors ZM241385 (10 nM) separately, or in combination, reduced the TOFfade caused by 2.2 µM cisatracurium (from 82.74% to ~ 48%) at T = 30 min (Figure 2a). However, there was no significant difference (p > 0.05) between these treatments (Figure 2a). Atropine (0.2 µM) alone, but not (p < 0.05) ZM241385 (10 nM) or the combined administration of atropine with ZM241385, expressed a significant (p < 0.01) reduction (from 100 ± 0.0%, n = 4 to 81.83 ± 3.64%, n = 4) in the cisatracurium-induced 100% Fade at T = 15 min (Figure 2d). This effect caused by atropine (0.2 µM) was determined by significant (p < 0.05) improvements in A (from 77.8 ± 2.78%, n = 4 to 59.27 ± 4.63, n = 4) and B (from 100 ± 0.0%, n = 4 to 85.21 ± 2.78%, n = 4) values (Figure 2e and f).

The blockage of presynaptic A2A receptors by ZM241385, or the blockages of M1 and M2 muscarinic receptors on the motor nerve by atropine, reduces (~ 48%) TOFfade caused by cisatracurium (T = 30 min). However, no significant (p > 0.05) difference has been detected between the effect caused by the blockade of presynaptic A2A and blockade of presynaptic M1 and M2 receptors by ZM241385 and atropine, respectively, in TOFfade (2 Hz) caused by

cisatracurium. Neither the combined administration of ZM241385 with atropine produces (p > 0.05) any reduction in the cisatracurium-induced TOFfade (2 Hz) better than that caused by ZM241385 or atropine. In contrast, it has been verified that reductions of M1 and M2 presynaptic muscarinic receptors activity by atropine decreased (~ 18%) the cisatracurium-induced Fade (50 Hz, T = 15 min). However, Fade caused by such non depolarizing neuromuscular relaxant was not affected by treatment of preparations with ZM241385, or by combined administration of ZM241385 and atropine.

It has previously been shown that the anticholinesterase activity of cisatracurium (Bornia et al., 2011; Pereira et al., 2012) plays a key role in TOFfade and Fade caused by this agent, since the mechanisms that regulate acetylcholine release via presynaptic muscarinic and adenosine receptors are differently affected by TOF and tetanic stimulations conditions (Bornia et al., 2011; Pereira et al., 2012). In fact, the activations of inhibitory-M2 and facilitatory-A2A receptors by acetylcholine and adenosine, respectively, are involved with TOFfade-caused cisatracurium when this agent causes 25% TOFfade, as the blockades of M2 or A2A receptors by methoctramine or ZM241385, respectively, improved 25% TOFfade caused by cisatracurium (Pereira et al., 2012). Nevertheless, the blockages of M1 or M2 muscarinic receptors by pirenzepine or methoctramine, respectively, worsen Fade (50 Hz) induced by cisatracurium when this neuromuscular relaxant causes 17% Fade (50 Hz) (Pereira et al., 2012). Current study shows that the mechanisms involved with 25% TOFfade (Pereira et al., 2012) and drastic (82.74%) TOFfade caused by cisatracurium seem to be similar, i.e., they involve presynaptic inhibitory-M2 and facilitatory-A2A receptors activations by acetylcholine and adenosine, respectively. However, the mechanisms involved with cisatracurium-induced 17% Fade (Pereira et al., 2012) seem to be different from those involved with cisatracurium which induces 100% Fade, In fact, in this last condition, only atropine, but not ZM241385, was able to cause a small (~ 18%), but significant (p < 0.05) improvement in the cisatracurium-induced 100% Fade (50 Hz).

The drastic (82.74%) TOFfade caused by cisatracurium seems to exhibit an amount of adenosine in the synaptic cleft which might be lower than that extant during 100% Fade, as at 50 Hz there is a higher activation of presynaptic M1 receptors by acetylcholine. Thereby, the adenosine outflow through the equilibrative transport system is increased (Oliveira, Timóteo, & Sá, 2009). Previous findings of our research group show that adenosine A2A receptors play a pivotal role in motor nerves, favoring muscarinic M2 inhibition by breaking M1 receptor counteractions caused by downstream signaling involving activation of protein kinase A (PKA) and Ca++ level in the motor nerve (Oliveira & Sá, 2005). In fact, current study shows that atropine improved the Fade caused by cisatracurium by mitigating more the reduction in A than in B values caused by muscular relaxant in R (B/A) parameter. These data indicate that a higher activation of M1 receptors on motor nerve by acetylcholine might be occurring in the cisatracurium-induced 100% Fade. It has already been demonstrated that A-value is influenced by presynaptic M1 receptors activity when the motor nerve is stimulated at 50.00 Hz trains (Oliveira, Timóteo & Sá, 2002). Since an improvement in the cisatracurium-induced Fade would be the expected effect when the preparations were previously treated with ZM241385 (Oliveira et al., 2002; Pereira et al., 2012), it was supposed that the anticholinesterase activity of cisatracurium (Bornia et al., 2009; Bornia et al., 2011; Pereira et al., 2011) in the experimental condition causing 100% Fade reduced the expression of facilitatory effect induced by the blockade of A2A receptors by producing a level of M1/M2 activation by acetylcholine higher than that determined by the separate treatment of preparations with cisatracurium. Indeed, the previous treatment of preparations with ZM241385 caused a reduction in the A-value induced by cisatracurium which was not significantly (p > 0.05) different from that obtained after the administration of cisatracurium. However, such apparent reduction caused by ZM241385 might be indicative of a previous increment in the acetylcholine release, thereby inducing a higher activation of presynaptic inhibitory-M2 receptors. Moreover, it has been proposed that failure to activate A2A receptors may be exaggerated by blocking A2A receptors with ZM241385, mainly when the level of extracellular adenosine is reduced. This situation is expected when the postsynaptic action of cisatracurium on muscular nicotinic receptors reduces the amount of adenosine from postsynaptic source in 100% Fade condition (Matos et al., 2011).

As a whole, data show that the blockage of presynaptic A2A adenosine receptors by ZM241385 contributes more to recover the blockage of neuromuscular transmission produced by a non-depolarizing muscular relaxants exhibiting inhibitory acetylcholinesterase activity in their molecules when the level of acetylcholine in the synaptic cleft is not yet causing intense M1/M2 muscarinic receptors activity on motor nerve terminal, i.e. when the motor nerve is electrically stimulated with moderate (TOF) rate frequencies (Figure 3). In contrast, for the first time it is being shown that the simultaneous blockages of M1 and M2 presynaptic receptors by atropine seem to be more efficient than blockage of A2A receptors to reduce the Fade caused by cisatracurium. In fact, a higher M1/M2 muscarinic receptors activity on motor nerve terminal seems to exist in this condition (Figure 3). Since an anticholinesterase agent is used in anesthesiology practice to induce recovery of patients from neuromuscular blockade and, since blockers of A2A receptors are efficient to reduce the fades caused by cisatracurium when this agent is not causing drastic level of fades (Pereira et al., 2012), it is supposed that atropine is useful for initial recovery of patients from neuromuscular blockade caused by cisatracurium. However, the blockage of A2A receptors might contribute towards a more secure recovery of neuromuscular transmission, mainly when the muscular nicotinic receptors blockade caused by cisatracurium decreases.

Conclusion

Blockage of presynaptic A2A adenosine receptors by ZM241385 contributes more to recover the blockage of neuromuscular transmission produced by a non-depolarizing muscular relaxants exhibiting inhibitory acetylcholinesterase activity in their molecules when the level of acetylcholine in the synaptic cleft is not yet causing intense M1/M2 muscarinic receptors activity on motor nerve terminal. However, the simultaneous blockages of M1 and M2 presynaptic receptors by atropine seem to be more efficient than blockage of A2A receptors to reduce the drastic level of fade caused by cisatracurium at 50 Hz.

References

Ali, H. H., Utting, J. E., & Gray, C. (1970). Stimulus frequency in the detection of neuromuscular block in humans. British Journal of Anesthesiology, 42(11), 967-978.

Barrio, J., Miguel, G. S., García, V., & Pelegrín, F. (2007). Influence of neostigmine on the course of neuromuscular blockade with rocuronium or cisatracurium: a randomized, double-blind trial. Revista Española de Anestesiología y Reanimación, 54(7), 399-404.

Bornia, E. C. S., Bando, E., Machinsky, M. J., Pereira, M. W., & Prado, W. A. (2009). Presynaptic M1, M2 and A1 receptors play roles in tetanic fade induced by pancuronium or cisatracurium. Journal of Anesthesiology, 23(4), 513-519.

Bornia, E. C. S., Sá, P. C., & Prado, W. A. (2011). Presynaptic facilitatory adenosine A2A receptors mediate fade induced by neuromuscular relaxants that exhibit anticholinesterase activity. Clinical and Experimental Pharmacology and Physiology, 38(3), 164-169.

Bülbring, E. (1946). Observations on the isolated phrenic nerve diaphragm preparation of the rat. British Journal of Pharmacology, 1(1), 38-61.

Matos, J. B. N., Morais, T., Trigo, D., Timóteo, M. A., Cardoso, M. T. M., Oliveira, L., & Sá, P. C. (2011). Tetanic failure due to decrease endogenous adenosine A2A tonus operating neuronal Cav1 (L-type) influx in Myastenia gravis. Journal of Neurochemistry, 117(5), 797-811.

Murphy, G. S. (2006). Residual neuromuscular blockade: Incidence, assessment, and relevance in the postoperative period. Minerva Anesthesiology, 72(3), 97-109.

Oliveira, L., & Sá, P. C. (2005). Protein kinase A and Cav1 (L-type) channels are common targets to facilitatory adenosine A2A and muscarinic M1 receptors on rat motoneurons. Neurosignals, 14(5), 262-272.

Oliveira, L., Timóteo, M. A., & Sá, P. C. (2002). Modulation by adenosyne of both muscarinic M1-facilitation and M2-inhibition of [3H]-acetylcholine release from rat motor nerve terminals. European Journal of Neuroscience, 15(11), 1728-1736.

Oliveira, L., Timóteo, M. A., & Sá, P. C. (2009). Negative crosstalk between M1 and M2 muscarinic autoreceptors involves endogenous adenosine activating A1 receptors at the rat motor endplate. Neuroscience Letters, 459(3), 127-131.

Pereira, M. W., Bornia, E., Sá, P. C., & Prado, W. A. (2011). Presynaptic muscarinic and adenosine receptors involved in 2Hz-induced train-of-four fade caused by antinicotinic neuromuscular relaxants in the rat. Clinical and Experimental Pharmacology and Physiology, 38(11), 764-770.

Pereira, M. W., Sá, P. C., & Prado, W. A. (2012). Adenosine A2A receptor antagonists are broad facilitators of antinicotinic neuromuscular blockade monitored either with 2Hz train-of-four or 50Hz tetanic stimuli. Clinical and Experimental Pharmacology and Physiology, 39(10), 869-877.

Prado, W. A., Corrado, A. P., & Prado, W. A. (1987). Reversal by atropine of tetanic fade induced in cats by antinicotinic and anticholinesterase agents. Anesthesia and Analgesia, 66(6), 492-496.

Author notes

waprado@uem.br

IR