Train-of-four and tetanic fade are not always a prejunctional phenomenon as evaluated by toxins having highly specific pre- and postjunctional actions

Molecular mechanisms of muscular and non-muscular actions of neuromuscular blocking agents in critical illness: a narrative review

Despite frequent use of neuromuscular blocking agents in critical illness, changes in neuromuscular transmission with critical illness are not well appreciated. Recent studies have provided greater insights into the molecular mechanisms for beneficial muscular effects and non-muscular anti-inflammatory properties of neuromuscular blocking agents. This narrative review summarises the normal structure and function of the neuromuscular junction and its transformation to a 'denervation-like' state in critical illness, the underlying cause of aberrant neuromuscular blocking agent pharmacology. We also address the important favourable and adverse consequences and molecular bases for these consequences during neuromuscular blocking agent use in critical illness. This review, therefore, provides an enhanced understanding of clinical therapeutic effects and novel pathways for the salutary and aberrant effects of neuromuscular blocking agents when used during acquired pathologic states of critical illness.

Anesthesia with Disuse Leads to Autophagy Up-regulation in the Skeletal Muscle

BACKGROUND

It has been known that skeletal muscles show atrophic changes after prolonged sedation or general anesthesia. Whether these effects are due to anesthesia itself or disuse during anesthesia has not been fully clarified. Autophagy dysregulation has been implicated in muscle-wasting conditions. This study tested the hypothesis that the magnitude of skeletal muscle autophagy is affected by both anesthesia and immobility.

METHODS

The extent of autophagy was analyzed chronologically during general anesthesia. In vivo microscopy was performed using green fluorescent protein-tagged LC3 for the detection of autophagy using sternomastoid muscles of live mice during pentobarbital anesthesia (n = 6 and 7). Western blotting and histological analyses were also conducted on tibialis anterior muscles (n = 3 to 5). To distinguish the effect of anesthesia from that due to disuse, autophagy was compared between animals anesthetized with pentobarbital and those immobilized by short-term denervation without continuation of anesthesia. Conversely, tibialis anterior and sternomastoid muscles were electrically stimulated during anesthesia.

RESULTS

Western blots and microscopy showed time-dependent autophagy up-regulation during pentobarbital anesthesia, peaking at 3 h (728.6 ± 93.5% of basal level, mean ± SE). Disuse by denervation without sustaining anesthesia did not lead to equivalent autophagy, suggesting that anesthesia is essential to cause autophagy. In contrast, contractile stimulation of the tibialis anterior and sternomastoid muscles significantly reduced the autophagy up-regulation during anesthesia (85% at 300 min). Ketamine, ketamine plus xylazine, isoflurane, and propofol also up-regulated autophagy.

CONCLUSIONS

Short-term disuse without anesthesia does not lead to autophagy, but anesthesia with disuse leads to marked up-regulation of autophagy.

Block of postjunctional muscle-type acetylcholine receptors in vivo causes train-of-four fade in mice

BACKGROUND

Train-of-four (TOF) fade during nerve-mediated muscle contraction is postulated to be attributable to inhibition of prejunctional nicotinic α3β2 acetylcholine receptors (nAChRs), while decrease of twitch tension is attributable to block of postjunctional muscle nAChRs. The validity of these presumptions was tested using specific prejunctional and postjunctional nAChR antagonists, testing the hypothesis that fade is not always a prejunctional phenomenon.

METHODS

Pentobarbital anaesthetized mice had TOF fade measured after administration of: either 0.9% saline; the prejunctional α3β2 nAChR antagonist, dihydro-β-erythroidine (DHβE); the postjunctional nAChR antagonists, α-bungarotoxin (α-BTX) or α-conotoxin GI; and a combination of α-BTX and DHβE; or a combination of α-conotoxin GI and DHβE.

RESULTS

Saline caused no neuromuscular changes. Administration of muscle nAChR antagonists, α-BTX or α-conotoxin GI caused significant decrease of twitch tension and TOF fade compared with baseline (P<0.01). DHβE alone caused no change of twitch tension or fade even after 90 min, but its coadministration with α-BTX or α-conotoxin GI significantly accelerated the onset of paralysis and degree of fade compared with α-BTX or α-conotoxin GI alone (P<0.01).

CONCLUSIONS

Occupation of postjunctional nAChRs alone by α-BTX or α-conotoxin GI causes fade. As the prejunctional effects of DHβE on fade became manifest only when co-administered with α-BTX or α-conotoxin GI, specific inhibition of prejunctional nAChR alone is not necessary and sufficient to cause fade. Fade observed during repetitive nerve stimulation can be because of block of either postjunctional nAChRs alone, or block of prejunctional and postjunctional nAChRs together.