Neuromuscular blocking action of verapamil in cats

Effects of nicardipine on the onset time and intubation conditions of rocuronium-induced neuromuscular blockade

STUDY OBJECTIVE

The objective of this study was to identify the effects of nicardipine on neuromuscular blockade of rocuronium, such as the onset time and intubation conditions, using a nicardipine dose that attenuates cardiovascular responses during endotracheal intubation.

DESIGN

Randomized, double-blinded, placebo-controlled clinical comparison was used as the design of this study

SETTING

The study was conducted at the operating room of a university hospital.

PATIENTS

Participants of this study comprise 78 American Society of Anesthesiologists physical status 1 and 2 patients, aged 18 to 60 years who were undergoing elective surgery under general anesthesia.

INTERVENTIONS

The nicardipine group was given an intravenous bolus of 20 μg/kg nicardipine before tracheal intubation: the control group was given an intravenous bolus of a comparable volume of normal saline before tracheal intubation.

MEASUREMENTS

Using a TOF-Watch SX monitor, the time from the end of the injection of rocuronium to maximum depression of T1 (onset time) was measured. Intubation was performed 1 minute after rocuronium administration, and the status of the intubation conditions was assessed. The mean blood pressure and heart rate were each measured after endotracheal intubation. Rate pressure product values were also calculated.

MAIN RESULTS

Intubation conditions were clinically acceptable in 37 (94.9%) of 39 patients in group N compared with 29 (74.4%) of 39 in group C (P < .05). The onset time of rocuronium was significantly faster in group N than in group C (P < .05). The mean blood pressure was significantly lower in group N than in group C (P < .05). The heart rate was significantly higher in group N than in group C (P < .05). Rate pressure product values showed no significant difference between the two groups (P > .05).

CONCLUSIONS

Pretreatment with 20 μg/kg nicardipine improves intubation conditions, shortens the onset time of rocuronium, and attenuates cardiovascular responses to tracheal intubation.

Epileptic seizure-induced hypertension and its prevention by calcium channel blockers: a real-time study in conscious telemetered rats

Epileptic seizures are accompanied by changes in autonomic function that in turn influence the cardiovascular system (hypertension and bradyarrhythmia). We have studied possible cardioprotective activity (during the ictal state in conscious animals) of valproic acid, nifedipine, and verapamil, alone and in combination, during pentylenetetrazole (PTZ)-induced seizures. Telemetry system was used for recording EEG, blood pressure, and heart rate in conscious, freely moving rats during seizures. We observed that PTZ-induced seizures were accompanied by hypertension and bradyarrhythmia. Pretreatment with valproic acid did not block seizure-induced hypertension and bradyarrhythmia. Nifedipine alone and in combination with valproic acid blocked seizure-induced hypertension and bradyarrhythmia significantly. We also observed that pretreatment with verapamil alone and in combination with valproic acid did not block seizure-induced hypertension and bradyarrhythmia significantly. Our results suggest that pretreatment with nifedipine alone or in combination with valproic acid provides protection against seizure-induced hypertension and bradyarrhythmia.

Verapamil-associated cardiogenic shock in a 71-year-old man with myasthenia gravis: a case report

Introduction

Myasthenia gravis is a rare neuromuscular disorder associated with a reduction in the availability of acetylcholine receptors at the post-synaptic membranes of skeletal muscles. This is caused by the production of anti-acetylcholine receptor antibodies at the neuromuscular junction due to an autoimmune insult, leading to a compromised neuromuscular transmission. Verapamil can influence, in a dose-dependent fashion, the neuromuscular transmission in myasthenia gravis.

Case presentation

We report a 71-year-old Caucasian man with myasthenia gravis suffering from a cardiogenic shock following a single dose of verapamil. The patient had uncontrolled atrial fibrillation with a heart rate of 120 beats/min. Atenolol 100 mg was started. The next day, verapamil SR 240 mg was started. Two hours after the first dose of verapamil, the patient complained of weakness and dyspnea with signs of shock; his blood pressure was 70/50 mm Hg and heart rate at 101 beats/min. An echocardiogram showed diffuse hypokinesis of both ventricles with an ejection fraction of 20%. Cardiac catheterization was performed and coronary arteries appeared without significant stenosis, but there was a diffuse hypokinesis. Verapamil was stopped and the patient received intravenous glucagon and calcium chloride. Both the anti-acetylcholine receptor and anti-striated muscle antibodies tested positive. A few hours later, another echocardiogram showed an improvement in the ventricular function, which returned to normal five days later.

Conclusion

Caution is needed when administering verapamil to patients with myasthenia gravis, especially when the anti-acetylcholine receptor and anti-striated muscle antibodies titres are positive.

Interactions of calcium channel blockers with non-depolarising muscle relaxants in vitro

The effects of two calcium channel blockers (verapamil and nicardipine) on indirectly elicited muscle twitch and possible interactions between these drugs and non-depolarising muscle relaxants (vecuronium, atracurium, pancuronium) were investigated using isolated rat phrenic nerve-hemidiaphragm preparations. Both verapamil 10(-5) M and nicardipine 10(-6) M caused significant depression of twitch amplitude. Verapamil significantly increased vecuronium- and atracurium-induced neuromuscular block, but not that produced by pancuronium. Nicardipine potentiated atracurium-induced neuromuscular block but had no effect on pancuronium- and vecuronium-induced twitch depression. Neostigmine 10(-6) M did not produce any significant changes in the maximal recovery of twitch depression induced with calcium channel blockers and muscle relaxants combinations; also, neostigmine had no effect on maximal recovery time of twitch depression.

In-vitro smooth muscle relaxant activity of a series of vecuronium analogues in the rat aorta

The ability of a series of 17-ester analogues of vecuronium to elicit a direct relaxant effect on vascular smooth muscle has been studied using rat isolated aortic rings contracted with 40 mM KCl. The IC50 for inhibition of KCl-induced contractions increased with increasing size of the 17-ester substituent, such that vecuronium (17 beta-acetate) was the least potent with an IC50 of around 50 microM and Org-9827 (17 alpha-pivalate) was the most potent with an IC50 of around 5 microM. In addition, for the weaker-acting compounds, the 17 alpha-esters were more potent than their corresponding 17 beta-esters, although this difference was lost as the size of the 17-ester substituent increased. From the results obtained here, it is concluded that the hypotensive activity of some of the newer neuromuscular blocking steroids seen in cats, pigs and dogs in-vivo is probably, at least in part, a consequence of a direct relaxant effect of the compound on vascular smooth muscle through inhibition of voltage-activated, L-type, calcium channels. This may have both advantageous and disadvantageous clinical consequences when using large doses of one of the newer vecuronium analogues with a low relative neuromuscular-blocking potency.

Nicardipine enhances diaphragmatic fatigue

The effects of nicardipine, a calcium channel blocker, on diaphragmatic fatigue were studied in 20 anaesthetized, mechanically ventilated dogs divided into two groups: control group (Group C, n = 10) and nicardipine group (Group N, n = 10). Diaphragmatic fatigue was induced by intermittent supramaximal electric stimulation to bilateral phrenic nerves at a frequency of 20 Hz for 30 min. In Group N, 5 micrograms.kg-1.min-1 nicardipine iv was infused continuously during this fatigue-producing period. Transdiaphragmatic pressure (Pdi) produced by electrical stimulation (10-100 Hz) of the phrenic nerves was used as an index of diaphragmatic contractility. After a fatigue-producing period, the Pdi in both groups decreased at low frequency (10-30 Hz) stimulation compared with pre-fatigue values (P < 0.05), whereas no change in Pdi was observed at high-frequency (50-100 Hz) stimulation. The decrease of Pdi at low-frequency stimulation was larger in Group N (P < 0.05). The integrated diaphragmatic electric activity (Edi) in both groups did not change at any frequency of stimulation throughout the experiment. We conclude that nicardipine enhances diaphragmatic fatigue.

Variable effect of calcium channel blockers on the decremental response in experimental autoimmune myasthenia gravis

We tested the effect of intravenous administration of verapamil and nimodipine on the decremental response in rabbits with experimental autoimmune myasthenia. Nimodipine produced an immediate augmentation of the decremental response to 3-Hz nerve stimulation, which lasted about 30 min. In contrast, verapamil caused marked amelioration of the decrement beginning 30 min after injection. Our findings are consistent with previous reports suggesting that verapamil has a presynaptic effect of enhanced acetylcholine release at the neuromuscular junction. Since evaluation of a drug effect in vivo in animals with experimental autoimmune myasthenia gravis may be more pertinent to its effect on patients with myasthenia gravis (MG), verapamil might prove to be safer in MG than nimodipine. However, due to the additional effects of calcium channel blockers, the safety of their use in myasthenia gravis cannot be inferred from the experimental results.

Differential potentiation by calcium antagonists of neuromuscular blockade induced by pancuronium and succinylcholine in cats in vivo

The effects of several calcium antagonists (verapamil, nicardipine and two diltiazem isomers, d-cis and l-cis diltiazem) alone and associated to non-depolarizing (pancuronium) and depolarizing (succinylcholine) neuromuscular blockers, were evaluated on sciatic nerve-tibialis anterior muscle preparations from cats in vivo. The calcium antagonists used (at 0.1 and 0.5 mg/kg iv) did not modify the height of muscular twitches elicited indirectly. However, these agents potentiated in a dose-dependent way the neuromuscular blockade induced by iv pancuronium (2-40 micrograms/kg) and succinylcholine (6-200 micrograms/kg). The order of potency in increasing the effects of pancuronium was nicardipine much greater than d-cis diltiazem greater than or equal to verapamil, whereas the order of potency in enhancing succinylcholine effects was d-cis diltiazem greater than or equal to verapamil much greater than nicardipine. The effects of diltiazem were stereoselective, thus the potentiation induced by d-cis diltiazem was significantly greater in all cases than that induced by l-cis diltiazem, which suggests that calcium channel blockade plays a role in these interactions. However, other mechanisms such as calcium antagonists-induced nicotinic receptor desensitization may also be involved.

In vivo potentiation of atracurium neuromuscular blockade by nimodipine in rabbits

The interaction between nimodipine, a calcium channel blocker, used for the treatment of subarachnoid haemorrhage, and atracurium, was studied in rabbits. An intravenous dose of 0.1 mg.kg-1 of nimodipine given over 3 min caused a potentiation of the neuromuscular blocking action of atracurium (administered at an infusion rate of 7.5 micrograms.kg-1.min-1), measured on the indirectly stimulated tibialis-anterior muscle of the animal. (ED50)inf and (ED95)inf were significantly reduced (30.7% and 23.3%) in nimodipine-treated animals (P less than 0.05 and P less than 0.02, respectively). No changes were observed in recovery rate.

Calcium-channel blockers and anaesthesia

Verapamil was the first calcium-channel blocker (CCB). It has been used since 1962 in Europe then in Japan for its antiarrhythmic and coronary vasodilator effects. The CCB have become prominent cardiovascular drugs during the last 15 years. Many experimental and clinical studies have defined their mechanism of action, the effects of new drugs in this therapeutic class, and their indications and interactions with other drugs. Due to the large number of patients treated with CCB it is important for the anaesthetist to know the general and specific problems involved during the perioperative period, the interactions with anaesthetics and the practical use of these drugs.

Neuromuscular blockade induced by flunarizine alone and in combination with pancuronium, suxamethonium or neomycin: studies in isolated rat phrenic-hemidiaphragm preparations

The in vitro effects of flunarizine on indirectly- and directly-elicited contractions in rat phrenic-hemidiaphragm preparations were studied. The interactions of flunarizine with non-depolarizing and depolarizing neuromuscular blocking drugs (pancuronium and suxamethonium) and with an aminoglycoside antibiotic (neomycin) were also evaluated. Flunarizine induced a slowly developing concentration-dependent reduction of indirectly-elicited diaphragm twitch height, but only slightly reduced directly-elicited contractions. Flunarizine 1 and 5 mumol.l-1 produced a concentration-dependent enhancement of pancuronium-induced neuromuscular blockade, whereas suxamethonium blockade was significantly increased by flunarizine 5 mumols.l.-1 only. Moreover, both flunarizine 1 and 5 mumols.l-1 also increased the neuromuscular blockade induced by neomycin. In conclusion, flunarizine induced neuromuscular blockade and enhanced the effects of several neuromuscular blocking agents to varying degrees in vitro.

Neuromuscular block by verapamil and diltiazem and inhibition of acetylcholine release

The effects of Ca2+-channel antagonists, verapamil, diltiazem and nifedipine, on the neuromuscular transmission were studied in the isolated mouse phrenic nerve-diaphragm preparations. All 3 drugs increased the twitch response evoked by direct single stimulation at 10-100 microM. The neuromuscular transmission at 0.1 Hz was blocked by verapamil and diltiazem, but not by nifedipine, only at very high concentrations (greater than or equal to 100 microM). In the time course of block, no endplate potential (e.p.p.) could be recorded, whenever the junction failed to elicit an action potential, suggesting that the block is due to an axonal conduction failure. Conduction block became apparent in both axon and muscle at low concentrations (greater than 10 microM) of verapamil and diltiazem at 100 Hz. When the safety margin of neuromuscular transmission was reduced by tubocurarine or low Ca2+ plus high Mg2+, verapamil and diltiazem, but not nifedipine, reduced the single twitch response to nerve stimulation at concentrations that did not cause axon conduction block. The inhibition was dependent on the frequency of nerve stimulation, enhanced by low-Ca2+ and antagonized by high-Ca2+. Verapamil (50 microM) inhibited the mean amplitude of the median size miniature e.p.p. by only 8%, whereas it increased the frequency by 4-5-fold and the proportion of both small and giant miniature e.p.p.s. The e.p.p. amplitude was inhibited by verapamil by about 67% in low-Ca2+ media and by about 38% in normal Tyrode. Similar but somewhat lesser effect was obtained with diltiazem. It is concluded that verapamil and diltiazem, but not nifedipine, inhibit the transmitter release.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of diltiazem and verapamil on responses to acetylcholine

1. The calcium channel antagonists diltiazem and verapamil were found to alter the average lifetime of ion channels activated by acetylcholine (ACh). 2. Average channel lifetime was determined from the decay phase of miniature endplate currents at the neuromuscular junction of mouse hemidiaphragms and from direct recording of single channel currents activated by ACh from BC3H1 mouse tumour cells in culture. 3. Both diltiazem and verapamil reduced average channel lifetime in a dose-dependent manner. For each drug, concentrations as high as 20 microM-100 microM were required to decrease channel lifetime by 50%. 4. Single channel recording experiments also showed that both diltiazem and verapamil greatly decreased the frequency of opening events at concentrations as low as 2 microM to 5 microM. This finding is consistent with an enhancement of receptor desensitization.

Interaction of verapamil with d-tubocurarine and cholinergic agonists at the avian neuromuscular junction

The effect of verapamil on neuromuscular transmission and muscle contraction was studied in the skeletal muscle of chick in vitro. The interactions of verapamil with d-tubocurarine (d-TC)-induced neuromuscular blockade, acetylcholine (ACh) and tetraethylammonium (TEA)-induced contractures were also studied. The purpose of the present investigation was to see if verapamil: intensified the neuromuscular blockade produced by d-TC; modified the cholinergic responses to ACh, TEA; and inhibited both directly and indirectly elicited twitch contractions. The results showed that verapamil (1-100 micrograms/ml, 2-200 mumol/l) had a neuromuscular blocking activity on its own; i.e. it reduced both directly and indirectly evoked twitch contractions, and intensified the neuromuscular blockade produced by d-TC. In addition, verapamil reduced the contractures produced by ACh and TEA in the chick muscle. The results are in favour of the possibility that verapamil acts by a mixture of pre- and post-junctional effects at the chick neuromuscular junction.

Effects of calcium channel blockers on neuromuscular blockade induced by aminoglycoside antibiotics

The effects of several calcium channel blockers (nifedipine, verapamil and diltiazem) on rat phrenic-hemidiaphragm preparations were studied. The calcium channel blockers were used either alone or associated with two aminoglycoside antibiotics, neomycin and streptomycin. All drugs investigated produced a concentration-dependent decrease in indirectly elicited diaphragmatic contractions. The order of potency was: verapamil greater than neomycin congruent to nifedipine greater than diltiazem greater than streptomycin. Moreover, neomycin-induced neuromuscular blockade was significantly increased by nifedipine (1 and 10 microM), verapamil (1 and 10 microM) and diltiazem (10 microM), whereas the streptomycin-induced neuromuscular blockade was increased only by nifedipine (1 and 10 microM) and verapamil (10 microM).

Verapamil intensifies neuromuscular blockade produced by gallamine and pancuronium at the chick neuromuscular junction

Experiments were performed to study the effect of verapamil on neuromuscular transmission and muscle contraction at a chick skeletal muscle-nerve preparation. In addition, the effects and interactions of verapamil with some muscle relaxants were studied in the same preparation. These effects were explored by studying the effects of verapamil on: directly-and indirectly-elicited twitch contractions, and neuromuscular blockade produced by gallamine and pancuronium. The results showed that verapamil (2-200 microM) had a differential effect on the twitch responses; more reductions occurred in the indirectly-elicited twitch tension, whereas the directly-elicited twitch response was reduced only by 20-30% of maximum indirectly-elicited twitch tension. Furthermore, in low concentrations (1-20 microM), verapamil significantly increased the neuromuscular blockade produced by gallamine (28-1280 nM) and pancuronium (18-573 nM). In high concentrations (greater than 200 microM), verapamil completely blocked the indirectly-elicited twitch response and produced a marked contracture in the chick skeletal muscle (1.0 +/- 0.1 g, n = 6). It was concluded that by reducing twitch tension and inhibiting neuromuscular transmission, verapamil increases (intensifies) neuromuscular blockade produced by muscle relaxants, e.g. gallamine and pancuronium.

Effects of calcium antagonists on mechanical responses of mammalian skeletal muscles

Rodent muscles were exposed to several organic calcium antagonists, and mechanical responses to direct electrical stimulation were recorded. Verapamil and D600, at 25 microM, depressed twitch and tetanus tension and caused fading of the tetanus plateau. These effects increased with frequency of stimulation, and were not reversed by doubled extracellular calcium. Depression of tension progressed to complete paralysis after 60-90 min exposure to verapamil. Bepridil and diltiazem both caused depression of tension and tetanus fade. Nifedipine caused marked, and nitrendipine caused slight, potentiation of twitch tension but did not alter tetanic tension. The magnitude of the observed effects on tension (either depression or potentiation) correlated with neither the relative calcium antagonist potencies of the drugs in other tissues nor with the ability of the drugs to cross the cell membrane. The continued decline in tension observed on prolonged exposure indicates that chronic exposure to low levels in vivo might lead to significant muscle weakness.

Anaesthetic implications of calcium channel blockers

Clinical uses of calcium channel blockers are expanding. In addition to the established uses in patients with arrhythmias, angina pectoris or hypertension, newer and to some extent investigational uses indicate widespread application. For instance, their use has been reported in hypertrophic cardiomyopathy and cold cardioplegia, as well as in pulmonary hypertension, antiplatelet therapy, asthma, achalasia and oesophageal spasm, increased intraocular pressure and in cerebral vasospasm. Their use in obstetrical practice has been proposed. Thus, the presentation of a patient who is treated with calcium channel blockers and who requires anaesthesia will become more common. Calcium channel blockers may, under certain circumstances, potentiate haemodynamic and MAC depressive effects of inhalation agents. There is also evidence that the effects of neuromuscular blocking agents may be potentiated. The anaesthetist should be aware that the potential for interactions exists with digoxin, propranolol, quinidine, theophylline or dantrolene. Of interest and some significance are the anaesthetic implications of pathophysiological alterations that can be induced by calcium channel blockers, by affecting lower oesophageal tone, intracranial hypertension, bronchomotor tone (asthma), muscular dystrophy, neuromuscular function, hypoxic pulmonary vasoconstriction, malignant hyperthermia, inhibition of platelet aggregation and hyperkalemia. Despite these significant potential anaesthetic implications and because, at this time, in some instances withdrawal has clearly demonstrated increase in the signs of myocardial ischaemia, it would not seem necessary to recommend preoperative discontinuation of calcium channel blocker medication in patients presenting for anaesthesia. It is, however, appropriate that there is a high index of awareness of potential problems, unless there is some modification in inhalation anaesthetic concentrations and neuromuscular blocker dosage. Monitoring of cardiovascular and neuromuscular functions is essential. Calcium channel blockers would appear to be currently the drugs of choice for angina pectoris, arrhythmias or hypertension in patients with associated chronic obstructive pulmonary disease.

Verapamil alters the amplitude and time course of miniature endplate current

The effect of verapamil on neuromuscular transmission was examined by recording miniature endplate currents (mepcs) in voltage-clamped frog sartorius muscle fibres. In the presence of 100 microM verapamil, the amplitude and time constant of decay of the mepcs (tau D) were reduced to 68% and 55% of control respectively, and the normal voltage dependency of tau D was decreased. Part of the decrease in amplitude of the mepc was independent of changes in tau D because, on washout, recovery of tau D was more rapid than that of amplitude, and in some cells smaller concentrations of verapamil (1 and 10 microM) decreased amplitude without affecting tau D. Evidence of open and closed channel blockade by verapamil (5-20 microM) was obtained from ionophoretic end-plate current trains and it is proposed that these effects are mediated via an allosteric mechanism.

Effective treatment of verapamil intoxication with 4-aminopyridine in the cat

To study the value of 4-aminopyridine as an antidote to verapamil intoxication, we subjected 12 adult cats to verapamil poisoning by administering doses of 4.0-25.0 mg/kg verapamil by intravenous infusion. Six animals were given 4-aminopyridine 2 X 0.5 mg/kg i.v. after the verapamil infusion was stopped and the other six animals (the control group) were not. Verapamil caused profound cardiovascular depression and also partial neuromuscular block, both of which were completely reversed by 4-aminopyridine within 50 min, in spite of extremely high serum verapamil concentrations (ranging between 3,700 and 13,500 ng/ml). The six animals that received 4-aminopyridine survived the verapamil intoxication, whereas four of the six animals in the control group died. The results suggest that 4-aminopyridine may be useful in the treatment of verapamil intoxication.