Mechanisms for the paradoxical resistance to d-tubocurarine during immobilization-induced muscle atrophy.

This study investigated whether immobilization-induced hyposensitivity to d-tubocurarine (dTC), up-regulation of acetylcholine receptors (AChRs) and changes in fiber size and motor endplate size persist indefinitely and whether they are causally related. Unilateral disuse of the tibialis muscle was produced in adult rats by pinning the knee and ankle joints at 90 degrees flexion. The contralateral unpinned and a separate group of sham-pinned legs served as controls. After 7, 14 or 28 days of disuse, the in vivo dose of dTC that produced 50% depression of nerve-evoked twitch (ED50) in the tibialis muscle increased 3.0-, 3. 2- and 2.1-fold (P < .05), and membrane AChRs increased 6.0- (P < . 05), 6.3- (P > .05) and 1.2-fold (P > .395) relative to control, respectively. Disuse caused muscle fiber atrophy (P < .01) but did not affect endplate size. Hence, the ratio of endplate size to fiber size increased. There was a transient increase in gene expression of all (including de novo expression of the gamma) subunits of the AChR, peaking at day 7 and returning to normal by day 28 of immobilization. The ED50 of dTC correlated directly with AChRs (R2 = 0.51; P < .0001) or the ratio of endplate size to fiber size (R2 = 0. 30; P < .001), and inversely with fiber size (R2 = 0.43, P < .0001). It is proposed that acting together, but not singly, the changes in AChRs, fiber size and relative endplate size contribute to the magnitude and time course of the resistance to dTC produced by chronic disuse.

Decreased sensitivity to metocurine during long-term phenytoin therapy may be attributable to protein binding and acetylcholine receptor changes.

Long-term phenytoin therapy induces resistance to the neuromuscular blocking effects of metocurine. The hypothesis that this is attributable to increased plasma protein binding of the drug (decreased free fraction) related to increased concentrations of alpha 1-acid glycoprotein (AAG) or attributable to the proliferation of acetylcholine receptors (AChR) at the muscle membrane was tested in the rat. After 14 days of phenytoin 40 mg.kg-1, administered intraperitoneally twice daily (n = 12), the neuromuscular pharmacodynamics were evaluated and compared with those of time-matched controls (n = 10). Protein binding was measured by equilibrium dialysis, AAG concentrations by radial immunodiffusion assay, and AChR by 125I-alpha-bungarotoxin binding. The effective dose for 50% inhibition of baseline twitch height (ED50) was significantly greater in the phenytoin group than in the control group (15.03 +/- 1.65 micrograms.kg-1 vs. 9.98 +/- 0.69 micrograms.kg-1, respectively). The concentrations of AAG increased gradually from 133.8 +/- 7.8 micrograms.ml-1 at day 0, to 343.1 +/- 58.0 micrograms.ml-1 at day 7, to 1,729.5 +/- 422.3 micrograms.ml-1 at day 14 in the phenytoin group. The induction of AAG concentrations in plasma was dependent on plasma phenytoin concentrations and was most prominent after 14 days of phenytoin (r = 0.77; P less than 0.01; n = 22). The free fraction of metocurine was significantly decreased in the phenytoin group compared to the control group (67.2 +/- 0.18% vs. 74.5 +/- 2.5%). There was a significant negative correlation between increased AAG concentrations and decreased free fraction (r = 0.65).(ABSTRACT TRUNCATED AT 250 WORDS)

Tolerance and upregulation of acetylcholine receptors follow chronic infusion of d-tubocurarine.

The hypothesis that chronic competitive antagonism of nicotinic acetylcholine receptors (nAChR), even in the absence of immobilization or paralysis, induces proliferation of the receptor and tolerance to the competitive antagonist was tested. Chronic antagonism of the nAChR was achieved in rats by an infusion of d-tubocurarine (dTC) via subcutaneously placed osmotic pumps. After 2 wk of dTC or saline, the neuromuscular pharmacodynamics and nAChR number were examined. No differences in weight gain or mobility were observed between groups. Chronic dTC infusion at 2 wk resulted in a baseline concentration of 0.41 +/- 0.07 (SE) micrograms/ml, which, if achieved acutely, would cause a depression of the twitch tension to 60% of control twitch height. Moreover the experimental group was able to develop a baseline twitch tension of 50 g, similar to that of controls. Despite the baseline dTC concentration in the experimental group, the effective doses of dTC for twitch depression were similar to those of controls. The plasma dTC concentration required for steady-state twitch inhibition was significantly (P less than 0.05) higher in the experimental group (0.83 +/- 0.04 vs. 0.50 +/- 0.15 micrograms/ml) as were the extrajunctional nAChR (19.76 +/- 1.77 vs. 13.37 +/- 1.82 fmol/mg protein). The diaphragmatic nAChR were unaltered. This study confirms that chronic doses of dTC cause tolerance to its effects and proliferation of nAChR even in the absence of immobilization. The absence of nAChR changes in the diaphragm may be due to the higher margin of safety of the diaphragm for muscle relaxants than for peripheral muscles. Intensive Care Unit patients receiving chronic infusions of dTC to facilitate mechanical ventilation will require increased doses with time.

Resistance to d-tubocurarine in lower motor neuron injury is related to increased acetylcholine receptors at the neuromuscular junction.

The hypothesis that lower motor neuron injury, with its associated proliferation of acetylcholine receptors (AChR), induces resistance to the neuromuscular effects of d-tubocurarine (dTC) was tested in the rat. The left gastrocnemius was denervated by a 75-80% lesion of the sciatic nerve. The effective dose for 95% twitch depression (ED95) was studied in the denervated gastrocnemius and compared to the contralateral undenervated and sham-injured (control) gastrocnemius muscles approximately 2 weeks after injury. The AChR number was quantitated by the specific ligand 125I-alpha-bungarotoxin (125I-alpha-BT). Plasma dTC concentrations, measured by high-performance liquid chromatography (HPLC), were correlated to twitch tension during spontaneous recovery from neuromuscular blockade in the denervated animal. The ED95 (mean +/- SE) of dTC for the denervated leg was significantly (P less than 0.05) higher (0.26 +/- 0.06 mg.kg-1) than contralateral (0.16 +/- 0.03) and sham-operated left (0.13 +/- 0.03) legs. The twitch tension recovered to 50% of control twitch height at significantly (P less than 0.05) higher plasma dTC concentrations in the denervated (0.78 micrograms.ml-1) compared to contralateral (0.24 micrograms.ml-1) limb. The AChR number was significantly increased in the denervated limb (1041 +/- 96 fmol.mg protein-1) compared to contralateral right (109 +/- 4) and control left limb (113 +/- 11). There was a significant (P less than 0.05) positive correlation (R2 = 0.73) between ED95 and AChR number; that is, 73% of the variability in ED95 could be explained by changes in AChR. This study, therefore, confirms the hypothesis that proliferation of AChR after nerve denervation results in resistance to the neuromuscular effects of dTC.(ABSTRACT TRUNCATED AT 250 WORDS)