Presynaptic effect of muscle relaxants on the release of 3H-norepinephrine controlled by endogenous acetylcholine in guinea pig atrium

Hypothermia down-regulates the LPS-induced norepinephrine (NE) release in ischaemic human heart cells

Hypothermia has been widely acknowledged as the fundamental component of myocardial protection during cardiac operations. In this work, we studied in human atrial tissue the effect of the common hypothermic protection used in cardiac surgery, and we assessed this effect by comparing catecholamine release among normoxic, ischaemic, and inflammatory conditions. Our results provide the first evidence that lipopolysaccharide treatment results in an extremely dramatic and significant increase in the resting norepinephrine release under ischaemic conditions that can be normalised by hypothermia. These findings demonstrate that inflammatory conditions increase the temperature sensitivity of the norepinephrine transporter in human cardiac tissue. When the possible pharmacological interventions are taken into consideration, the results presented here provide new insight into the protection against ischaemia/reperfusion injury during cardiac surgery.

Investigation of the relaxant effects of pancuronium, rocuronium, vecuronium and mivacurium on rat thoracic aorta

BACKGROUND AND OBJECTIVE

Pancuronium, vecuronium, mivacurium and rocuronium are nondepolarizing neuromuscular blocking agents, which are competitive antagonists against acetylcholine at nicotinic receptors, and considered to have no direct actions on vascular smooth muscle. We aimed to investigate the relaxant effects and possible underlying mechanisms of these agents on isolated rat thoracic aorta.

METHODS

The preparations were precontracted with prostaglandin F2alpha (10(-7) mol l(-1)) and pancuronium (10(-7)-10(-4) mol l(-1)), rocuronium (10(-7)-10(-4) mol l(-1)), vecuronium (10(-7)-10(-4) mol l(-1)) and mivacurium (10(-7)-10(-4) mol l(-1)) added at cumulative concentrations in the presence or absence of a prostaglandin synthesis inhibitor, indomethacin (10(-6) M), and a nitric oxide synthesis inhibitor, N(omega)-nitro-L-arginine methylester (3 x 10(-5)). The same protocol was applied to both endothelia (+) and endothelia (-) aortic rings. The preparations precontracted with prostaglandin F2alpha (10(-7) mol l(-1)) were stimulated with electrical field stimulation at a frequency of 10 Hz as square-wave pulses of 50 V (0.2 ms) in the presence of a noradrenaline reuptake inhibitor desipramine (10(-7) mol l(-1)) and a nonselective beta-blocker propranolol (10(-6) mol l(-1)). Drugs were added at ineffective concentration of 10(-7) mol l(-1). Tetrodotoxin (10(-7) mol l(-1)) was added to test whether the changes were dependent on the neuronal response.

RESULTS

Pancuronium and rocuronium relaxed aortic rings precontracted by prostaglandin F2alpha in a dose-dependent manner, but vecuronium and mivacurium did not. The relaxation effect of pancuronium and rocuronium was endothelium independent because there was not a significant response difference from the endothelium-denuded group.

CONCLUSION

In conclusion, their relaxation effect may be due to an increase in prostaglandin synthesis. The increased relaxation effect of these agents at electrical field stimulation may be by the decreasing effect of noradrenaline reuptake from nerve endings because a noradrenaline reuptake inhibitor desipramine did not change this effect. Also, these neuromuscular agents may affect beta-receptors, because a nonselective beta-blocker agent, propranolol, decreased their electrical field stimulation-induced relaxations.

A new short-acting non-depolarizing muscle relaxant (SZ1677) without cardiovascular side-effects

BACKGROUND

In order to facilitate rapid tracheal intubation, the development of a rapid onset, short duration, non-depolarizing muscle relaxant without cardiovascular side-effects would be a significant accomplishment in the field of anesthesiology. The aim of the present study was to test the action of a new non-depolarizing muscle relaxant (SZ1677) on neuromuscular transmission, muscarinic (M2, M3) receptors and cardiovascular reactions and to compare it with clinically used muscle relaxants.

METHODS

Neuromuscular transmission was studied by recording muscle contractions elicited by indirect electrical stimulation, using (i). in vitro isolated phrenic nerve-hemidiaphragm preparation of mice, rats and guinea pigs and (ii). in vivo sciatic nerve-anterior tibial muscle preparation of anesthetized rats, guinea pigs and cats. Cardiovascular effects of muscle relaxants were evaluated on the grounds of their effects on changes of blood pressure and heart rate induced by electrical stimulation of the right vagal nerve in anesthetized cats. To study postsynaptic antimuscarinic affinity of muscle relaxants on M3 receptors, oxotremorine-induced contractions of longitudinal muscle strip of guinea pig ileum were registered in their presence and absence.

RESULTS

One of more than 120 newly synthesized non-depolarizing muscle relaxants compounds, 1-3[alpha-hydroxy-17beta-acetyloxy-2beta-(1,4-dioxa-8-azaspiro[4,5]dec-8-yl)-5alpha-androstane-16beta-il] -1-(2-propenyl)pyrrolidinium bromide (SZ1677), excelled with its advantageous pharmacological properties: relatively short duration of action, no accumulation and lack of unwanted side-effects. Pharmacodynamic studies show that SZ1677 is a non-depolarizing neuromuscular blocking agent with a relatively short duration and rapid onset of action in a variety of laboratory animal species. It is without cumulative effect, does not reduce blood pressure, and fails to produce tachycardia. Significant cardiac vagal blocking effects were not observed even at concentrations or dosages of 8 times the ED90. This compound, unlike many other muscle relaxants, does not have atropine-like effects on human atrial tissue; it does not increase the release of NA from sympathetic innervation in the heart. In all practical ways, at least from the vantage point of the preclinical study, SZ1677 compares favorably with all presently available short-acting muscle relaxants, including rapacuronium.

CONCLUSION

In experiments, SZ1677 proved to be a short-acting neuromuscular blocking compound having a large safety margin between the doses required to produce neuromuscular block and those likely to lead to cardiovascular side-effects.

Tetra-butyl ammonium attenuates evoked release of acetylcholine from mouse hemidiaphragm preparation

Tetra-butyl ammonium is a homologous member of mono-quaternary ammonium salts, and it has been reported to have a property of nondepolarizing neuromuscular relaxant. However, no neurochemical evidences exist that tetra-butyl ammonium may interfere with quantal release of acetylcholine from motor nerve terminals. In this study, using the neurochemical method, we investigated the effect of tetra-butyl ammonium on stimulation-evoked release of acetylcholine from mouse hemidiaphragm preparation. The preparation was loaded with [3H]choline (5 microCi/ml). Low concentrations of tetra-butyl ammonium (10(-5) M) had no effects. On the other hand, at concentrations of 4x10(-5) and 10(-4) M, this compound significantly reduced the [Ca2+]o-dependent release of acetylcholine from phrenic nerves. This finding indicates that tetra-butyl ammonium possesses a presynaptic inhibitory effect on acetylcholine release from the phrenic nerve terminal.

Cardiac effects of non-depolarizing neuromuscular blocking agents pancuronium,vecuronium, and rocuronium in isolated rat atria

Pancuronium, vecuronium, and rocuronium produce different cardiac effects. Using spontaneously beating right and electrically stimulated left rat atria, while measuring developed force, effective refractory period, and heart rate, we determined and compared the concentration-dependent cardiac effects of the compounds. The preparations were exposed to five progressively increasing concentrations of these compounds (10(-9), 10(-8), 10(-7), 10(-6), and 10(-5) mol/L). Pancuronium increased heart rate; vecuronium and rocuronium produced positive inotropic effects; and vecuronium shortened refractoriness. These effects may be the result of a blockade of the M2 muscarinic receptors. However, the concentrations required to produce changes were higher than those observed in patients under neuromuscular blockade.

M4 muscarinic receptor activation modulates calcium channel currents in rat intracardiac neurons

Modulation of high-voltage-activated Ca2+ channels by muscarinic receptor agonists was investigated in isolated parasympathetic neurons of neonatal rat intracardiac ganglia using the amphotericin B perforated-patch whole cell recording configuration of the patch-clamp technique. Focal application of the muscarinic agonists acetylcholine (ACh), muscarine, and oxotremorine-M to the voltage-clamped soma membrane reversibly depressed peak Ca2+ channel current amplitude. The dose-response relationship obtained for ACh-induced inhibition of Ba2+ current (IBa) exhibited a half-maximal inhibition at 6 nM. Maximal inhibition of IBa amplitude obtained with 100 microM ACh was approximately 75% compared with control at +10 mV. Muscarinic agonist-induced attenuation of Ca2+ channel currents was inhibited by the muscarinic receptor antagonists pirenzepine (</=300 nM) and m4-toxin (</=100 nM), but not by AF-DX 116 (300 nM) or m1-toxin (60 nM). The dose-response relationship obtained for antagonism of muscarine-induced inhibition of IBa by m4-toxin gave an IC50 of 11 nM. These results suggest that muscarinic agonist-induced inhibition of high-voltage-activated Ca2+ channels in rat intracardiac neurons is mediated by the M4 muscarinic receptor. M4 receptor activation shifted the voltage dependence and depressed maximal activation of Ca2+ channels but had no effect on the steady-state inactivation of Ca2+ channels. Peak Ca2+ channel tail current amplitude was reduced >/=30% at +90 mV in the presence of ACh, indicating a voltage-independent component to the muscarinic receptor-mediated inhibition. Both dihydropyridine- and omega-conotoxin GVIA-sensitive and -insensitive Ca2+ channels were inhibited by ACh, suggesting that the M4 muscarinic receptor is coupled to multiple Ca2+ channel subtypes in these neurons. Inhibition of IBa amplitude by muscarinic agonists was also observed after cell dialysis using the conventional whole cell recording configuration. However, internal perfusion of the cell with 100 microM guanosine 5'-O-(2-thiodiphosphate) trilithium salt (GDP-beta-S) or incubation of the neurons in Pertussis toxin (PTX) abolished the modulation of IBa by muscarinic receptor agonists, suggesting the involvement of a PTX-sensitive G-protein in the signal transduction pathway. Given that ACh is the principal neurotransmitter mediating vagal innervation of the heart, the presence of this inhibitory mechanism in postganglionic intracardiac neurons suggests that it may serve for negative feedback regulation.

Side effects of nondepolarizing muscle relaxants: relationship to their antinicotinic and antimuscarinic actions

Since acetylcholine (ACh) is the 'master key' to different subtypes of nicotinic and muscarinic receptors, and muscle relaxants (MRs) available in clinical practice are structurally related to it, MRs may exert their unwanted effects through inhibition of these receptors. Since the subunit composition of nicotinic ACh receptors (nAChRs) of pre- and/or postsynaptic location and the binding potency of MRs to these and muscarinic receptors are different, a search for selective muscle nAChR antagonists without or with less side effects seems to be promising and important for clinical practice.

Presynaptic modulation of the release of noradrenaline from electrically stimulated bicuspid valve leaflet of the rabbit heart

The bicuspid (mitral) valves were obtained from male albino New Zealand rabbits. The noradrenaline (NA) content (12.93 +/- 1.14 nmol/g) of the valve tissue was determined by high pressure liquid chromatography (HPLC) combined with electrochemical detection. After incubation with tritiated NA for 45 min, the tissues were mounted in perfusion baths and superfused with Krebs solution at a constant perfusion rate. After a 90 min washing period, the tissues were stimulated three times (S1; S2; S3) at a frequency of 1 or 10 Hz, and the release of NA was expressed as the stimulus-induced overflow of radioactivity. Using a constant number of impulses, the release of NA was significantly higher when the frequency applied was 10 Hz than in the case of 1 Hz. The release of NA was inhibited by stimulating the presynaptic alpha 2-adrenoceptors with xylazine or by stimulating the presynaptic muscarinic receptors with oxotremorine. Yohimbine (1 microM) not only overcame the effect of the alpha 2-adrenoceptor stimulation caused by xylazine, but increased it over the control level, whereas atropine blocked the inhibitory effect of oxotremorine. It is concluded that the adrenergic nerves in the valve tissue release NA in a frequency-dependent fashion, and the release of NA can be modulated through presynaptic alpha 2- and muscarinic receptors. This is the first case that neurochemical evidence was obtained showing that NA is released from the mitral valve and is subject to presynaptic modulation.

Presynaptic interaction between vagal and sympathetic innervation in the heart: modulation of acetylcholine and noradrenaline release

It is generally accepted that there is a functional antagonism between the sympathetic and parasympathetic (vagal) effects on the heart. In this study guinea-pig right atria loaded either with [3H]noradrenaline or [3H]choline were used and the release of [3H]noradrenaline or [3H]acetylcholine in response to field stimulation was measured under conditions when the negative feedback modulation was excluded. Strong evidence was obtained for a one-sided interaction between the sympathetic and vagal nerves at the level of the prejunctional axon terminals that send the final chemical message to the heart muscle affecting heart rate and force. Acetylcholine released from the vagal nerve inhibited its own release and also decreased the release of noradrenaline from the sympathetic axon terminals through muscarinic receptor stimulation. But muscarinic receptors located on cholinergic axon terminals are different from those present on the noradrenergic axon terminals. There is a significant difference in the dissociation constants (Kd) of different antimuscarinic drugs: The Kd values of pancuronium on vagal and sympathetic axon terminals were 5.68 +/- 0.41 and 7.20 +/- 0.25, respectively. By contrast, noradrenaline released from the sympathetic nerves or exogenous noradrenaline were not able to modulate the release of acetylcholine from the cholinergic axon terminals even under condition when the negative feedback modulation of acetylcholine release was excluded. These findings indicate that vagal axon terminals are not equipped with alpha 2- or alpha 1-adrenoceptors. However, noradrenaline released from the sympathetic axon terminals was able to inhibit its own release via alpha 2-adrenoceptor stimulation.

Neuromuscular and cardiovascular effects of pipecuronium

Pipecuronium bromide (Arduan) is a bisquaternary, steroid-type neuromuscular blocking agent in clinical use in Eastern Europe. Before its introduction into clinical practice in the USA, in the first phase of this study the neuromuscular potency of pipecuronium was determined under "balanced" and enflurance anaesthesia by the cumulative log dose-response method in 30 patients each. In the second phase the intubation and onset times, clinical duration of the first and repeated doses, spontaneous recovery index, reversibility of its residual neuromuscular effect by an anticholinesterase and its effect on heart rate and blood pressure was compared with the same variables observed in patients, anaesthetized with identical techniques but who had received vecuronium or pancuronium. The neuromuscular potency of pipecuronium was greater under enflurane (ED95 = 23.6 +/- 1.1 micrograms.kg-1 (mean +/- SEM)] than under balanced (ED95 = 35.1 +/- 17 micrograms.kg-1) anaesthesia. Pipecuronium was more potent than vecuronium under both balanced (ED95 = 45.8 micrograms.kg-1) and enflurane anaesthesia (ED95 = 27.4 micrograms.kg-1). Following the administration of 2 x ED95 doses there were no clinically significant differences in the intubation or onset times of pipecuronium, vecuronium and pancuronium. Under balanced anaesthesia the clinical duration of 2 x ED95 dose of pipecuronium (110.5 +/- 0.3 min) or pancuronium (115.8 +/- 8.1 min) were similar and about three times longer than that of vecuronium (36.3 +/- 2.1 min). The recovery indices of pipecuronium (44.5 +/- 8.2 min) and pancuronium (41.3 +/- 4.2 min) were also similar and about three times longer than that of vecuronium (14.3 +/- 1.4 min).(ABSTRACT TRUNCATED AT 250 WORDS)