Pancuronium, vecuronium, mivacurium and rocuronium are nondepolarizing neuromuscular blocking agents. Although neuromuscular blocking drugs are designed to specifically block nicotinic cholinergic receptors at the neuromuscular junction, many bind to muscarinic cholinergic receptors on ganglia, nerve endings and smooth muscle . The principal action of nondepolarizing muscle relaxants is competitive antagonism against acetylcholine at nicotinic receptors in the neuromuscular junction and these agents are considered to have no direct actions on vascular smooth muscle .
Although they have a similar amino steroidal three-dimensional structure, experimental studies performed in different species have shown that these compounds affect the cardiovascular system with different potencies [3,4]. Pancuronium, which has been reported to have more sympathomimetic properties than vecuronium , is reported to increase heart rate and arterial blood pressure (BP) [6,7] by its positive inotropic and chronotropic effects [3,4]. Although rocuronium did not produce significant cardiac effects  except minor haemodynamic changes , vecuronium has been reported to induce bradycardia [9–11]. Mivacurium injection caused transient tachycardia and hypotension in healthy individuals . The mechanisms of action of these compounds have been explained by cardiac antimuscarinic effects, noradrenaline release and inhibition of noradrenaline reuptake [13–16].
The aim of this study was to investigate the relaxant effects and possible underlying mechanisms of pancuronium, vecuronium, mivacurium and rocuronium on isolated rat thoracic aorta along with their effects on the BP in clinical doses and compare their vascular actions with each other.
All experimental procedures were performed in accordance with the recommendations of the Guide for the Care and Use of Laboratory Animals and the experiments were approved by Cumhuriyet University Medical Faculty, Animal Care Committee. Twenty male albino Wistar rats (weighing 180–220 g) were injected with pentobarbital sodium (100 mg kg−1) and sodium heparin (5000 IU kg−1) intraperitoneally. The thoracic aorta was removed carefully and cleaned of fat and adherent connective tissues. It was cut into segments of 2–3 mm in length and mounted onto two stainless-steel stirrups immersed in a 10 ml organ chamber, containing Krebs–Henseleit solution (composition in mmol l−1: NaCl 120.0, KCl 4.6, CaCl2 2.5, MgSO4 1.2, NaHCO3 22.0, NaH2PO4 1.2, CaCl2 2.5, glucose 11.5) that was gassed continuously with 95% O2 and 5% CO2 and maintained at 37°C. The upper ends of the preparations were tied to an isometric transducer (Grass FT 03, Quincy, Massachusetts, USA) and preloaded with 1.5 g resting tension. They were allowed to achieve equilibrium for 60 min with the bath fluid being changed every 15 min. In some rings (n = 5), the endothelium was removed mechanically by inserting a polyethylene tube (PE50) into the lumen of the ring and rolling the vessel with the tube gently onto moistened gauze.
The aortic rings were precontracted with prostaglandin F2α (PGF2α) (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) were added to organ baths at cumulative concentrations in the presence or absence of a prostaglandin synthesis inhibitor, indomethacin (10−6 mol l−1), and a nitric oxide synthesis inhibitor, N(omega)-nitro-L-arginine methylester (L-NAME, 3 × 10−5) . The same protocol was applied to both endothelia (+) and endothelia (−) aortic rings.
Preparations precontracted with PGF2α (10−7 mol l−1) were stimulated with two parallel platinum electrodes at a frequency of 10 Hz as square-wave pulses of 50 V (0.2 ms) delivered by a current amplifier and a stimulator. Before electrical field stimulation (EFS), the tissues were treated with a noradrenaline reuptake inhibitor, desipramine (10−7 mol l−1), and a nonselective beta-blocker, propranolol (10−6 mol l−1) . Pancuronium, rocuronium, vecuronium and mivacurium were added to the organ bath at ineffective concentration of 10−7 mol l−1. Tetrodotoxin (10−7 mol l−1) was added to test whether the changes were depending on the neuronal response.
The drugs used were PGF2α, indomethacin, L-NAME, desipramine, propranolol, tetrodotoxin (Sigma Chemical, St Louis, Missouri, USA); vecuronium, rocuronium (N.W. Organon, Oss, Holland); mivacurium (Glaxo Smith Kline, Farma, Italy) and Pancuronium (Organon Teknika, Istanbul, Turkey). All drugs were prepared by dissolving in deionized water and were prepared freshly on the day of the experiment. The stock solutions were stored at −20°C.
All data are expressed as means ± SEM. Groups were compared statistically using general linear models of analysis of variance (ANOVA) followed by the Newman–Keuls test and t test when appropriate. A P value of less than 0.05 was considered significant. All analyses were performed with Statistica for Windows 6.0. (Statsoft, Inc., Tulsa, USA).
Pancuronium (10−7–10−4 mol l−1) and rocuronium (10−7–10−4 mol l−1) relaxed the preparations precontracted with PGF2α (10−7 M) in a dose-dependent manner but vecuronium and mivacurium did not. The relaxation effect of pancuronium was statistically more than rocuronium (P < 0.05) (Fig. 1a). There was no significant difference between their relaxation effects in the endothelium-denuded preparations (Fig. 1b). L-NAME (3 × 10−5) did not change the pancuronium (10−7–10−4 mol l−1) or rocuronium (10−7–10−4 mol l−1) effects but indomethacin (10−6 mol l−1) decreased their effect significantly (P < 0.05) (Fig. 2a and b).
EFS (10 Hz, 50 V, 0.2 ms) relaxed the preparations precontracted with PGF2αa (10−7 mol l−1); this relaxation effect was increased significantly by pancuronium (10−7 mol l−1), vecuronium (10−7 mol l−1), mivacurium (10−7 mol l−1) and rocuronium (10−7 mol l−1) (P < 0.05). Although desipramine, at a concentration sufficient to inhibit the uptake of catecholamine (10−7 M) by sympathetic nerve terminals , did not change EFS-induced relaxations, a nonselective beta-blocker, propranolol (10−6 mol l−1), decreased them significantly (P < 0.05). Tetrodotoxin (10−7 mol l−1), which blocks voltage-dependent Na+ channels, abolished EFS-induced relaxations (Fig. 3). Emax and pD2 values of pancuronium and rocuronium were calculated and are shown in Table 1.
The vasomotor effects of anaesthetics have been widely evaluated by the isolated vessel ring model. Various animal preparations have been used (rat aorta, canine mesenteric artery, porcine coronary artery and others). There are some studies with intravenous anaesthetics (propofol, thiopental and others) using this model .
Although neuromuscular blocking drugs are widely used with general anaesthetics, there are few studies evaluating the effects of them on vascular tone. Sai et al.  found that pancuronium caused relaxations in canine coronary and renal arteries partially contracted with PGF2α, whereas these arteries did not significantly respond to vecuronium.
The relaxant responses were independent of endothelium and abolished by treatment with indomethacin, a cyclooxygenase inhibitor, or tranylcypromine, a prostaglandin I2 (PGI2) synthesis inhibitor [20,21]. Among available prostanoids, only PGI2 relaxes canine coronary and renal arteries [21,22]. They concluded that these results might indicate that pancuronium releases vasodilator prostaglandins, possibly PGI2, from subendothelial tissues.
It has been reported that pancuronium stimulates the release of norepinephrine from nerves and simultaneously inhibits the neural uptake in canine-isolated saphenous veins  and in anesthetized dogs . Sai et al.  thought that the potentiation by pancuronium and vecuronium of the response to adrenergic stimulation appears to be due to the inhibition of the neural uptake of noradrenaline rather than the increased release of noradrenaline. In some studies, it was reported that, although vecuronium in clinical doses does not have the vascular action that pancuronium does [25,26], high doses of vecuronium may work as a sympathomimetic agent . Because of the endothelium-independent action, pancuronium might have some beneficial clinical effect, especially under pathologic conditions such as atherosclerosis and hypertension, in which the synthesis or action of nitric oxide [27,28] and endothelium-derived hyperpolarizing factor (EDHF) [28–30] are reduced. Sai et al.  reported that, in patients with heightened sympathetic activity, pancuronium and vecuronium also might elicit coronary vasodilatation through a potentiated action of noradrenaline on beta-adrenoceptors, whereas renal vascular resistance may be increased.
In another study , pancuronium produced a positive chronotropic effect. Vecuronium and rocuronium induced a positive inotropic effect; furthermore, vecuronium shortened the effective refractory period. All the changes in cardiac activity observed were induced by relatively high concentrations of the drugs.
There is only one study on the cardiovascular effects of mivacurium and in that study the authors reported that a slow injection rate of mivacurium (>30 s) might be used in hypertensive patients.
The plasma concentration of pancuronium administered to humans during the first 3 h is reportedly  500–100 ng ml−1 (about 90% free form , which is equivalent to 7 × 10−7 and 1.4 × 10−7 mol l−1, respectively). According to these data in this study, we used all of neuromuscular blocking drugs at concentrations similar to those in the study of Sai et al. . In this study, pancuronium and rocuronium relaxed aortic rings precontracted by PGF2α 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. Their relaxation effects may be due to an increase in prostaglandin synthesis. The increased relaxation effect of pancuronium, rocuronium, vecuronium and mivacurium at EFS may be by decreasing the 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 EFS-induced relaxations.
In conclusion, pancuronium and rocuronium may have some clinical effect, especially under pathologic conditions such as atherosclerosis and hypertension in which endothelium is damaged. The mediation of prostaglandins in the relaxation effect of pancuronium and rocuronium may decrease their effect before surgical operations in the presence of prostaglandin synthesis inhibitors.
1 Bowman WC. Nonrelaxant properties of neuromuscular blocking drugs. Br J Anaesth 1982; 54:147–160.
2 Savarese JJ, Lowenstein E. The name of the game: no anesthesia by cookbook. Anesthesiology 1985; 62:703–705.
3 Iwatsuki N, Hashimoto Y, Amaha K, et al
. Inotropic effects of nondepolarizing muscle relaxants in isolated canine heart muscle. Anesth Analg 1980; 59:717–721.
4 Alvarez L, Escudero C, Silva L, Castillo-Olivares JL. Electrophysiological effects of atracurium and vecuronium
on normal and denervated hearts. J Cardiothorac Vasc Anesth 1992; 6:304–307.
5 Narita M, Furukawa Y, Ren LM, et al
. Cardiac effects of vecuronium
and its interaction with autonomic nervous system in isolated perfused canine hearts. J Cardiovasc Pharmacol 1992; 19:1000–1008.
6 Krol T, Siondalska-Kunicka E. Haemodynamic side-effects of pancuronium
. Anaesth Resusc Intensive Ther 1974; 2:161–165.
7 Cornet JP, Abiad M, Coriat P, et al
. Evaluation of the effects of rocuronium
bromide on haemodynamic and left ventricular function in patients under going abdominal aortic surgery. Eur J Anesth 1994; 11:78–81.
8 Marshall RJ, Muir AW, Sleigh T, Savage DS. An overview of the pharmacology of rocuronium
bromide in experimental animals. Eur J Anesth 1994; 11:9–15.
9 Milligan KR, Beers HT. Vecuronium
associated cardiac arrest. Anaesthesia 1985; 42:192–194.
10 Inoue K, El-Banayosy A, Stolarski L, Reichelt W. Vecuronium
induced bradycardia following induction of anaesthesia with etomidate or thiopentone, with or without fentanyl. Br J Anaesth 1988; 60:10–17.
11 Conzanitis DA, Lindgren L, Rosenberg PH. Bradycardia in patients receiving atracurium or vecuronium
in conditions of low vagal stimulation. Anaesthesia 1989; 44:303–305.
12 Salt PJ, Barnes PK, Conway CM. Inhibition of neuronal uptake of noradrenalin in the isolated perfused rat heart by panctrophysiological uronium and its homologues Org 6368, Org 7268, and NC45. Br J Anaesth 1980; 52:313–317.
13 Kobayashi O, Nagashima H, Duncalf D, et al
. Direct evidence that pancuronium
and gallamine enhance the release of norepinephrine from the atrial sympathetic nerve by inhibiting prejunctional muscarinic receptors. J Auton Nerv Syst 1987; 18:55–60.
14 Foldes FF, Kobayashi O, Kinjo M, et al
. Presynaptic effect of muscle relaxants on the release of 3H-norepinephrine controlled by endogenous acetylcholine in guinea pig atrium. J Neural Transm 1989; 76:169–180.
15 Appadu BL, Lambert DG. Studies on the interaction of steroidal neuromuscular blocking drugs with cardiac muscarinic receptors. Br J Anaesth 1994; 72:86–88.
16 Plaud B, Marty J, Debaene B, et al
. The cardiovascular effects of mivacurium
in hypertensive patients. Anesth Analg 2002; 95:379–384.
17 Sai Y, Ayajiki K, Okamura T, et al
. Comparison of the effects of pancuronium
in canine coronary and renal arteries. Anesthesiology 1998; 88:165–171.
18 Toda N. Interactions of bretylium and drugs that inhibit the neuronal membrane transport of norepinephrine in isolated rabbit atria and aortae. J Pharmacol Exp Ther 1972; 181:318–327.
19 Park WK, Lynch C, Johns RA. Effects of propofol and thiopental in isolated rat aorta
and pulmonary artery. Anesthesiology 1992; 77:956–963.
20 Gryglewski RJ, Bunting S, Moncada RJ, et al
. Arterial walls are protected against deposition of platelet thrombi by a substance (prostaglandin X) which they make from prostaglandin endoperoxides. Prostaglandins 1976; 12:685–713.
21 Toda N, Miyazaki M. Angiotensin-induced relaxation in isolated dog renal and cerebral arteries. Am J Physiol 1981; 240:H247–H254.
22 Toda N, Bian K, Akiba T, Okamura T. Heterogeneity in mechanisms of bradykinin action in canine isolated blood vessels. Eur J Pharmacol 1987; 135:321–329.
23 Vercruysse P, Bossuyt P, Hanegreefs G, et al
. Gallamine and pancuronium
inhibit pre and postjunctional muscarinic receptors in canine saphenous veins. J Pharmacol Exp Ther 1979; 209:225–230.
24 Domenech JS, Garcia RC, Sasiain JMR, et al
bromide: an indirect sympathomimetic agent. Br J Anaesth 1976; 48:1143–1148.
25 Marshall IG, Agoston S, Booij LH, et al
. Pharmacology of ORG NC 45 compared with other nondepolarizing neuromuscular blocking drugs. Br J Anaesth 1980; 52:11S–19S.
26 Morris RB, Cahalan MK, Miller RD, et al
. The cardiovascular effects of vecuronium
(ORG NC45) and pancuronium
in patients undergoing coronary artery bypass grafting. Anesthesiology 1983; 58:438–440.
27 Najibi S, Cowan CL, Palacino JJ, Cohen RA. Enhanced role of potassium channels in relaxations to acetylcholine in hypercholesterolemic rabbit carotid artery. Am J Physiol 1994; 266:H2061–H2067.
28 Van de Voorde J, Vanheel B, Leusen I. Endothelium-dependent relaxation and hyperpolarization in aorta from control and renal hypertensive rats. Circ Res 1992; 70:1–8.
29 Hayakawa H, Hirata Y, Suzuki E, et al
. Mechanisms for altered endothelium-dependent vasorelaxation in isolated kidneys from experimental hypertensive rats. Am J Physiol 1993; 264:H1535–H1541.
30 Fukao M, Hattori Y, Kanno M, et al
. Evidence for selective inhibition by lysophosphatidylcholine of acetylcholine-induced endothelium-dependent hyperpolarization and relaxation in rat mesenteric artery. Br J Pharmacol 1995; 116:1541–1543.
31 Melnikov AL, Malakhov KY, Helgesen KG, Lathrop DA. Cardiac effects of nondepolarizing neuromuscular blocking agents pancuronium
, and rocuronium
in isolated rat atria. Gen Pharmacol 1999; 33:313–317.
32 McLeod K, Watson MJ, Rawlins MD. Pharmacokinetics of pancuronium
in patients with normal and impaired renal function. Br J Anaesth 1976; 48:341–345.
33 Wood M. Neuromuscular blocking agents. In: Wood M, Wood AJJ, editors. Drugs and anesthesia: pharmacology for anesthesiologists.
Baltimore: Lippincott Williams and Wilkins; 1982. pp. 299–340.