Activity-dependent differences between rat fast and slow neuromuscular systems.

Activity-dependent pharmacologic differences between the fast tibialis anterior and slow soleus neuromuscular systems of the rat were studied. The tibialis anterior was more sensitive than the soleus to d-tubocurarine tetanic fade (50 Hz), as determined from recordings of compound muscle action potentials. Pre-treatment with physostigmine prevented curare-induced tetanic fade in the tibialis anterior, but not the soleus. Additionally, when contractile tension was 80% blocked by d-tubocurarine, the tibialis anterior was more responsive than the soleus to the decurarizing action of tetanic stimulation (25, 50 and 100 Hz). These results disclose that activity-dependent pharmacologic differences exist between neuromuscular systems. Further, they indicate that the tibialis anterior and the soleus differ in their processes of transmitter release. It is speculated that differences in nerve terminal Ca2+ account for the observed pharmacologic differences between the tibialis anterior and soleus.

Frequency-dependence of neuromuscular blockade in the presence of d-tubocurarine.

The frequency-dependence of neuromuscular blockade in the presence of d-tubocurarine (dTC) was studied in the vascular perfused rat phrenic nerve-hemidiaphragm preparation by intracellular recording. Endplate potential (EPP) amplitude decreased as the frequency of stimulation increased (2.5-40 Hz). Additionally, the slopes of the regression lines of EPP amplitude versus the log of stimulation frequency decreased when the dTC concentration was increased. Since the dependence of neuromuscular blockade on frequency was not enhanced by higher concentrations of dTC, the postjunctional channel-blocking hypothesis does not sufficiently explain frequency-dependent neuromuscular blockade in the presence of dTC. However, the present results are compatible with the hypothesis that dTC influences neuromuscular blockade via a prejunctional action, affecting acetylcholine release.

d-Tubocurarine sensitivities of a fast and a slow neuromuscular system of the rat.

Fast and slow neuromuscular systems in the rat were compared with respect to their sensitivity to d-tubocurarine (dTC). The fast tibialis anterior was more sensitive than the slow soleus to dTC-induced block of contractile tension when stimulated at either 0.2 or 1.0 Hz. These results in the rat contrast those made by others in the cat. Thus, relative drug sensitivities are not simply related to neuromuscular type.

Bovine serum albumin does not completely block synaptosomal cholinergic activities of presynaptically acting snake venom phospholipase A2 enzymes.

Bovine serum albumin (BSA), which binds fatty acids, was used to test the contribution of free fatty acid to the presynaptic toxicity of phospholipase A2 (PLA2) enzymes. The effects of BSA on inhibition of [14C]choline uptake and stimulation of [14C]acetylcholine (ACh) release in synaptosomes by PLA2 enzymes that do not have a predominant presynaptic action at the neuromuscular junction (PS-) were compared with those on the cholinergic actions of PLA2 enzymes that do have a predominant presynaptic action at the neuromuscular junction (PS+). The inhibition of choline uptake by the Naja naja atra PLA2, a PS- PLA2, was completely antagonized by BSA (0.5%); whereas that by beta-bungarotoxin, a PS+ PLA2, was unaffected by BSA. The inhibition of choline uptake by two other PS+ PLA2 toxins (scutoxin and pseudexin) was partially antagonized by BSA. The effects of the PLA2 enzymes were antagonized in the same manner by BSA whether on Na(+)-dependent or on Na(+)-independent choline uptake. Likewise, the stimulation of ACh release by two PS- PLA2 enzymes (from Naja naja atra and Naja naja kaouthia snake venoms) was completely blocked by BSA; whereas that by beta-bungarotoxin was unaffected and that by scutoxin and pseudexin was only partially antagonized by BSA. The results suggest that the PS- PLA2 enzymes are completely dependent on fatty acid production for their cholinergic toxicity and that BSA can be used to investigate further the neurotoxic mechanisms of PS+ PLA2 enzymes in synaptosomes.

Preliminary evidence for a postsynaptic action of beta-bungarotoxin in mammalian skeletal muscle.

Two hours after treatment with beta-bungarotoxin (0.34-0.4 microM), when there was complete neuromuscular block, the peak contracture response to 50 microM succinylcholine was significantly reduced by about 35% in the mouse phrenic nerve-diaphragm preparation. Additionally, significant phospholipase A2 activity was detected on primary cell cultures from skeletal muscle which were incubated for 2 hr with concentrations of beta-bungarotoxin greater than or equal to 0.1 microM. Thus, beta-bungarotoxin appears to have pharmacologically and biochemically detectable postsynaptic actions in mammalian muscle systems.

Anti-curare effect of plasma from patients with thermal injury.

Following severe thermal injury, patients are resistant to non-depolarizing muscle relaxants. Although this resistance has been well documented clinically, little is known about its etiology. We have tested the hypothesis that circulating factors contribute to the decreased potency of neuromuscular blockers following burns. The potencies of d-tubocurarine (2 microM) or pancuronium (2 microM) dissolved in plasma from either burned or control human subjects were tested on the indirectly stimulated (0.2 Hz) rat phrenic nerve-hemidiaphragm preparation. The muscle relaxants produced less neuromuscular blockade when dissolved in plasma from burned patients than when they were dissolved in plasma from controls. Thus, circulating factors are involved in the decreased potency of non-depolarizing neuromuscular blocking drugs.

Approximate entropy and point correlation dimension of heart rate variability in healthy subjects.

The contribution of nonlinear dynamics to heart rate variability in healthy humans was examined using surrogate data analysis. Several measures of heart rate variability were used and compared. Heart rates were recorded for three hours and original data sets of 8192 R-R intervals created. For each original data set (n = 34), three surrogate data sets were made by shuffling the order of the R-R intervals while retaining their linear correlations. The difference in heart rate variability between the original and surrogate data sets reflects the amount of nonlinear structure in the original data set. Heart rate variability was analyzed by two different nonlinear methods, point correlation dimension and approximate entropy. Nonlinearity, though under 10 percent, could be detected with both types of heart rate variability measures. More importantly, not only were the correlations between these measures and the standard deviation of the R-R intervals weak, the correlation among the nonlinear measures themselves was also weak (generally less than 0.6). This suggests that in addition to standard linear measures of heart rate variability, the use of multiple nonlinear measures of heart rate variability might be useful in monitoring heart rate dynamics.

Atropine and neuromuscular fade in the mouse phrenic nerve-diaphragm.

PURPOSE: These studies were intended to resolve the conflict between the reasonable inference from the scientific literature that atropine might alter neuromuscular fade and the expectation from informal clinical experience that it does not. METHODS: We examined the effect of a high concentration of atropine (20 microM) on moderate neuromuscular block and fade produced by d-tubocurarine (dTC). Isometric twitch tension was measured in the mouse phrenic nerve-diaphragm preparation. In one set of experiments, the phrenic nerve was stimulated with trains of 5 pulses at 10 Hz every second. Block and fade were measured in two groups, control and with atropine (n = 6 each). In another set of experiments, the phrenic nerve was stimulated with standard train-of-four stimulation (TOF, 4 pulses at 2 Hz every 11.5 seconds). Block and fade were measured first in a control period and then in a treatment period with either saline (n = 4) or atropine (n = 4). RESULTS: During 10 Hz train stimulation, atropine had no significant effect on either the block of the first twitch (control: 62 +/- 17; atropine: 75 +/- 4) or fade (control: 55 +/- 12: atropine; 57 +/- 14) produced by dTC. Similarly, atropine did not differ significantly from saline in altering dTC-induced block of first twitch (saline: 92.5 +/- 14; atropine 92.5 +/- 9.6% control) or fade (saline 119 +/- 50; atropine 102 +/- 30% control) during TOF stimulation. CONCLUSIONS: While atropine may alter ACh release under some conditions, its action is not great enough to alter either block or fade.

Halothane and temperature interact to increase succinylcholine-induced jaw contracture in the rat.

BACKGROUND: The agonist actions of succinylcholine (SCh) have recently come under study because of their involvement in the clinical problem of masseter muscle rigidity, and their possible involvement in malignant hyperthermia. The authors investigated factors affecting SCh-induced contractures in an animal preparation. METHODS: Rats were anesthetized with either halothane (1-2%) or pentobarbital. Resting and twitch isometric tension were measured from the jaw muscles. Succinylcholine (500 or 750 micrograms/kg) was administered intravenously, producing increases in resting tension (i.e., contractures). Jaw muscle temperature was controlled by radiant heat. RESULTS: Succinylcholine increased jaw muscle tension for several seconds. These contractures exhibited tachyphylaxis, and were antagonized by vecuronium (0.8-1.5 mg/kg), indicating mediation by acetylcholine receptors (AChR). In the presence of 2% halothane, contractures were tenfold greater at a rectal temperature of 41 degrees C than at 37 degrees C. In contrast, under 50 mg/kg intraperitoneal pentobarbital anesthesia, contractures were not affected by rectal temperature. Neither the half-decay time of contracture nor twitch tension (0.2 Hz, preceding SCh) were increased in the presence of halothane at 41 degrees C. In a set of experiments in which rectal temperature was maintained at 37 degrees C but jaw temperature was varied between 36-41 degrees C, there was a significant regression of SCh-induced jaw contracture on temperature in the presence of halothane. In contrast, there was no significant relationship between jaw temperature and contracture in the presence of pentobarbital. CONCLUSIONS: These results in the rat demonstrate a temperature-dependent interaction between halothane and SCh that has not previously been described.

Relationships between block-of-twitch and train-of-four fade in the mouse phrenic nerve-diaphragm preparation.

The relationships between the block-of-twitch and train-of-four fade in the presence of nondepolarizing neuromuscular blocking drugs (d-tubocurarine, vecuronium and pancuronium) were examined in vitro by measuring the contractile tension from mouse phrenic nerve-diaphragm preparations. The slope of the block/fade relationship differed between onset of and recovery from neuromuscular block following single doses of d-tubocurarine, vecuronium or pancuronium. Decreasing the dose of d-tubocurarine or using a divided dose technique to accelerate onset (i.e., priming) increased the amount of fade for a given amount of block. In addition, the block/fade relationships for cumulative dosing and sequential dilution were the same when measurements were made at steady-state for several doses. It is concluded that the block/fade relationship in the mouse phrenic nerve-diaphragm preparation is variable, and is related to the time course of the neuromuscular block. In addition, the block/fade relationships for d-tubocurarine, vecuronium and pancuronium did not differ when determined at steady-state.

In vitro investigation of the priming principle for rapid neuromuscular block.

The priming principle was investigated in the rat phrenic nerve-diaphragm preparation stimulated continuously at 0.2 Hz. Tubocurarine was added to the organ bath as either a single (non-primed) or a divided (primed) dose. Priming consisted of 15% or 20% of the final dose, with priming intervals of 5 or 10 min. Priming decreased significantly the time to 80% block and was associated with mild neuromuscular block. A simple model adequately predicted the time to 50% and 80% block, using the same diffusion constant for both primed and non-primed conditions. The onset of neuromuscular block, with and without priming, depended mostly upon the distribution of the drug to its site(s) of action.

Differences among heart rate variability measures after anesthesia and cardiac surgery.

OBJECTIVE: To determine whether there are differences among measures of heart rate variability (HRV; traditional and nonlinear) after anesthesia and cardiac surgery. DESIGN: Prospective. SETTING: University hospital. PARTICIPANTS: Patients scheduled for cardiac surgery. INTERVENTIONS: None. Medical management was not varied as part of this study. MEASUREMENTS AND MAIN RESULTS: HRV was measured in 13 patients from electrocardiograms (ECGs) recorded before anesthesia, during anesthesia but before cardiac surgery, and on the first postoperative day. Anesthesia was induced with moderate-dose fentanyl. For each ECG, HRV was measured from series of 400 heartbeat intervals using standard deviation (SD), approximate entropy (ApEn), and point correlation dimension (PD2). Multivariate repeated-measures analyses of variance on ranks and Spearman correlations were performed. All HRV measures decreased significantly with anesthesia. Postoperatively, ApEn recovered to original values. PD2 and SD did not recover with consciousness and were significantly less than original values. Correlations among ApEn, PD2, and SD were weak. CONCLUSIONS: Nonlinear measures of HRV differ among themselves after anesthesia and cardiac surgery. The use of multiple nonlinear and traditional measures may improve the effectiveness of using HRV to assess the cardiovascular system.

A comparison of chemiluminescent and radioenzymatic methods for the measurement of acetylcholine released from a rat phrenic nerve-hemidiaphragm preparation.

A chemiluminescent assay coupled to a periodide extraction method is described for the measurement of acetylcholine release from the vascular perfused rat phrenic nerve-hemidiaphragm preparation. A direct comparison of the chemiluminescent assay with an established radioenzymatic assay for acetylcholine demonstrates that the two assays are quantitatively equivalent and yield similar limits of sensitivity of approximately 2 pmol, and that the periodide extraction/chemiluminescent assay method is more consistent than the tetraphenylboron extraction/radioenzymatic assay method. Additionally, cholinergic drug interference with the chemiluminescent assay is minimal. The absence of radioactivity and the reduced cost of the chemiluminescent assay make it an attractive alternative to the radioenzymatic assay.

Antagonism of suxamethonium-induced jaw muscle contracture in rats.

Masseter muscle rigidity (MMR) induced during general anaesthesia by suxamethonium is a clinical problem that may interfere with tracheal intubation. We have investigated the relation between twitch tension and contracture response to suxamethonium in rats. Rats were anaesthetized with 1% halothane (1.35 MAC). Jaw muscle temperature was maintained at either 37 or 41 degrees C while rectal temperature was kept at 37 degrees C by radiant heat. Twitch tension was produced by nerve stimulation at 0.2 Hz. Rats were pretreated with either a low dose of vecuronium (0.03 mg kg-1) or dantrolene (0.8 mg kg-1). Thereafter suxamethonium 750 micrograms kg-1 was administrated i.v. Low-dose vecuronium pretreatment significantly (90%) decreased suxamethonium-induced jaw muscle contracture (JMC) with minimal (3%) twitch block during local hyperthermia. Low-dose dantrolene pretreatment also reduced JMC (81% at 37 degrees C and 82% at 41 degrees C) while decreasing twitch by 30% at 37 degrees C and 31% at 41 degrees C. Both vecuronium and dantrolene at doses that minimally depressed the twitch response antagonized suxamethonium-induced JMC. We speculate that pretreatment with low-dose vecuronium decreases suxamethonium-induced MMR clinically.

Effect of different volatile anaesthetics on suxamethonium-induced jaw muscle contracture in rats.

Previous work has demonstrated that the interaction of hyperthermia and halothane may greatly increase the jaw muscle contracture produced by suxamethonium. We have compared the interaction of temperature and suxamethonium in the presence of halothane with the suxamethonium/temperature interaction of two other volatile anaesthetics, isoflurane and desflurane. Rats were anaesthetized with 1.35 MAC of halothane, isoflurane or desflurane. The jaw area was heated to 36-41 degrees C by a heating lamp while rectal temperature was maintained at 37 degrees C. Isometric tension was recorded from the jaw muscles. Suxamethonium 750 micrograms kg-1 i.v. induced a transient jaw muscle contracture (JMC) during halothane, isoflurane and desflurane anaesthesia. JMC exhibited significant dependence on jaw muscle temperature with all three volatile anaesthetics. Increasing the temperature of the jaw area from 37 degrees C to 41 degrees C increased JMC 8.7-fold with halothane, 8.8-fold with isoflurane and 3.1-fold with desflurane. The difference between halothane and desflurane was significant. While suxamethonium-induced JMC was dependent on temperature for all three volatile anaesthetic, the temperature dependence appeared to be less with desflurane.

Ventilatory compliance after three sufentanil-pancuronium induction sequences.

Poor ventilatory compliance, a predictable side effect of high-dose opioid induction techniques, is purportedly blunted by pretreatment with nondepolarizing muscle relaxant. This study used both total compliance and a subjective compliance score to compare three different sequences of opioid induction using a 2-min infusion of sufentanil 3 micrograms.kg-1. Nineteen patients in each of three groups received a total of 100 micrograms.kg-1 of pancuronium, in the following randomized double-blinded fashion: control, all pancuronium 1 min after sufentanil; pretreated, 1 mg pancuronium 1 min before sufentanil and the balance of pancuronium 1 min after sufentanil; and mixed, all pancuronium mixed with sufentanil. Topical lidocaine prior to induction permitted early oral airway insertion midway through the sufentanil infusion. Immediately at the conclusion of sufentanil infusion, a tightly fitted mask, anterior jaw thrust, and mechanical ventilator permitted measurement of plateau airway pressure and exhaled volume in five replicates. Pressure and volume measurements were repeated 5 min later. Total compliance was calculated as the median plateau airway pressure divided into its associated exhaled volume. Groups did not differ in demographics. In one control patient and two pretreated patients hemoglobin oxygen saturation as measured by pulse oximetry decreased below 90%. Immediately after sufentanil infusion, the total compliance for control patients of 4.1 ml.cmH2O-1 (mean [2.6-6.5, 95% confidence interval] ) did not differ from that of the pretreated group (6.3 [3.5-11.4] ml.cmH2O-1), but the mixed group exhibited higher compliance (40.3 [33.8-47.9] ml.cmH2O-1) than the other groups (P less than 10(-8]. All groups achieved similar total compliances several minutes after a total of 100 micrograms.kg-1 pancuronium had been administered.(ABSTRACT TRUNCATED AT 250 WORDS)

Upper airway closure: a primary source of difficult ventilation with sufentanil induction of anesthesia.

Large-dose opioid induction of anesthesia can lead to difficult ventilation via a mask. Poor ventilatory compliance (VC) may be secondary to "rigid" chest and abdominal wall musculature, glottic closure, or upper airway obstruction. This double-blind study assessed the contribution of the upper airway to poor VC by inducing sufentanil anesthesia in patients undergoing cardiac surgery who are ventilated via a mask (Group M) or endotracheal tube fiberoptically inserted (Group E). After induction of anesthesia with sufentanil 3 microgram/kg from time (T) = 0 min to T = 2 in Group M (n = 17) or Group E(n = 23), VC and adductor pollicis (AP) twitch tension was measured continuously. Immediately prior to muscle relaxant (pipecuronium or doxacurium) administration at T = 3, Group E demonstrated significantly better VC (46 mL/cm H2O [39-55 interquartile range (IQR)]) than Group M (19 mL/cm H2O [7-24 IQR]). The effect of muscle relaxant administration on VC preceded its effect at the AP. After complete relaxation of the AP at T = 9, both groups had similar VC. Difficult ventilation during sufentanil induction of anesthesia lies at the level of the glottis or above. Bypassing these structures with an endotracheal tube overcomes the usual decreased VC.