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

Train-of-four fade and neuromuscular block in rats: a comparison between pancuronium, vecuronium, and rocuronium

PURPOSE

To clarify the relationship between neuromuscular block and train-of-four fade and to investigate the causes of these drug-dependent differences, we compared the neuromuscular block and TOF fade after pancuronium, vecuronium and rocuronium.

METHODS

In 24 anesthetized rats, the sciatic nerve was stimulated, and the twitch of left tibialis anterior muscle was recorded. After T1 (first twitch response) was kept constant at 95% block by administration of pancuronium, vecuronium, or rocuronium (n=8, in each), the TOF fade was measured when T1 block was decreased to 40% and 20%. In addition, using 24 phrenic nerve-diaphragm preparations, the fade was measured when the T1 block increased to 20% and 40% by titrating of either one of the three drugs (n=8, in each).

RESULTS

In in vivo experiments, the fade produced by pancuronium was greater than that by vecuronium or rocuronium when T1 block was at 40% (81 +/- 9 vs 63 +/- 15 and 63 +/- 6%, respectively) and at 20% (66 +/- 13 vs 34 +/- 17 and 40 +/- 6%, respectively). In contrast, in in vitro experiments, the differences did not reach significant levels among the three drugs either at 20% (32 +/- 19 vs 33 +/- 10 and 32 +/- 17%) or 40% of block (62 +/- 29 vs 65 +/- 14 and 55 +/- 14%).

CONCLUSIONS

For vecuronium and rocuronium, the results were similar in vivo and in vitro. For pancuronium, fade was greater in vivo. These results suggest that different neuromuscular blocking agent have different relationships between the fade and the block. In vitro results might not be the same as in vivo, possibly due to pharmacokinetic differences.

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.

Fading responses in the evoked EMG after rocuronium in cats

This study was performed to evaluate the inhibitory effect on motor nerve terminals by rocuronium using recovery curves of muscle compound action potentials (CAPs) and train-of-four ratios (TOFRs) in anaesthetized cats, and to compare the results with other relaxants reported previously. Recovery curves were derived from the amplitude of the CAP induced in the gastrocnemius muscle by the second of a paired stimulus (test response) to the sciatic nerve and compared with results evoked by the first component (conditioning response). The interval between the paired stimuli was increased stepwise from 7 to 1,000 msec, and the differences in amplitude of the test and conditioning responses were plotted on a graph by relating the changes in paired intervals. The recovery curve after rocuronium was less inhibited than after pancuronium, (100.4 +/- 5.9%, 82.3 +/- 6.7% and 68.5 +/- 6.7% at 60, 100 and 500 msec intervals, compared with 70.3 +/- 3.3%, 59.0 +/- 4.7% and 46.7 +/- 4.3% after pancuronium (P < 0.05). The recovery curves with d-tubocurarine were more depressed than with pancuronium; however, the RC with vecuronium was similar to that of rocuronium. The degree of fade in TOF by rocuronium was also less than those seen with d-tubocurarine and pancuronium. The results obtained suggest that rocuronium has less inhibitory effect on motor nerve terminals than do d-tubocurarine and pancuronium, and has a similar effect to that of vecuronium.

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.