Effects of presynaptic muscarinic cholinoreceptor blockade on neuromuscular transmission as assessed by the train-of-four and the tetanic fade response to rocuronium

Abstract

This study investigated the effect of muscarinic M₁ and M₂ receptor antagonists on the rocuronium-induced train of four (TOF) fade and tetanic fade, respectively. Ex-vivo phrenic nerves and diaphragms were obtained from adult Sprague-Dawley rats and stabilized in Krebs buffer; the nerve-stimulated muscle TOF fade was observed at 20 s intervals. For the TOF study, phrenic nerves and diaphragms were incubated with pirenzepine (an M₁ blocker) at concentrations of 0 nmol L^-1 (control), 10 nmol L^-1 (PZP10), or 100 nmol L^-1 (PZP100). Rocuronium was then administered incrementally until the first twitch tension had depressed by >95% during TOF stimulation. The mean TOF ratios were compared when the first twitch tensions were depressed by 40%-50%. For the tetanic fade study, 50 Hz/5 s tetani was applied initially, 30 min after the administration of a loading dose of rocuronium and methoctramine (an M₂ receptor blocker, loaded at 0 μmol L^-1 [control], 1 μmol L^-1 [MET1], or 10 μmol L^-1 [MET10]). The EC₉₅ of rocuronium was significantly lower in the PZP10 group than in the control group. In the PZP10 group, the TOF ratios at 50% and first twitch tension depression were significantly lower than those in the control group (P=.02). During tetanic stimulation, the tetanic fade was significantly enhanced in the MET10 group compared to the other groups. This study shows that antagonists of muscarinic M₁ and M₂ receptors affect the rocuronium-induced neuromuscular block as demonstrated by the reduced EC₉₅ and TOF ratios (M₁ antagonist, pirenzepine) or the enhanced 50-Hz tetanic fade (M₂ antagonist, methoctramine).

Keywords: acetylcholine release; methoctramine; muscarinic cholinoreceptor; neuromuscular physiology; pirenzepine; rocuronium.

Figure 1

Actions of rocuronium on rat phrenic nerve/hemidiaphragm tissues. (A) Dose‐response curves of the effect of rocuronium on T1 depression and (B) train‐of‐four ratio (TOFR) values in the control, PZP10, and PZP100 groups. The R² values of each regression line were > 0.8. (C) The regression equation of T1 depression: y = 50 + 50sin(Ωx), where y represents the T1 depression, x represents the concentration of rocuronium, and Ω represents the slope of the regression curve. The dose‐response curves were shifted to the left in the PZP10 group (large mean Ω, P=0.001). (D) The regression equation for TOFR: y = 1 ‐ λx2, where y and x represent TOFR and concentration of rocuronium, respectively, and λ represents the slope of the regression curve. TOF fade was detected more rapidly in the PZP10 group than in the control (P=0.001). However, changes in these variables in the PZP100 group did not differ significantly from those observed in the other groups.

Figure 2

TOFR values at 40%‐50% of T1 twitch tension depression. The TOFR value in the PZP10 group was significantly lower than that in the control group (*P=.02). However, the corresponding value in the PZP100 group did not differ significantly from that in the control or PZP10 groups

Figure 3

(A) Comparison of the changes in the tetanic fade ratio after incubation with MET (R _{post‐incubation}/R _{pre‐incubation}). During the initial and pre‐incubation (Pre) periods, tetanic fade ratios did not significantly differ between the groups. However, the tetanic fade ratios in the post‐incubation (Post) period in the MET10 group decreased more than in the other groups (*P=.005). (B) Comparison of the changes in the percentage of the tetanic fade ratio (%R) after incubation with MET (R _{post‐incubation}/R _{pre‐incubation}). During the initial and pre‐incubation periods, tetanic fade ratios did not significantly differ between the groups. However, the %R was significantly larger in the MET10 group than in the other groups (*P=.005)

Figure 4

Study protocol. (A) Diagram of the TOF study. The baseline stimulation pattern is the TOF stimulation, which was repeated every 20 s. (B) Diagram of the tetanic fade study. The baseline stimulation pattern is the TOF stimulation, which was repeated every 20 s. For the tetanic fade study, we used 50‐Hz tetanic stimulation for 5 s; the interval between each tetanic stimulation was 30 min. The inset figure shows twitch tension of a single tetanic stimulation. A, the initial tetanic twitch tension; B, the last tetanic twitch tension; R, the ratio of these twitch tensions (R=B/A)