Interactions between tetraethylammonium and methohexitone at the chick neuromuscular junction: Experiments with the moving fluid electrode technique

Abstract

The effect of methohexitone on the depolarization and contracture responses produced by tetraethylammonium (TEA), acetylcholine (ACh) and repetitive indirect stimulation were investigated, using the moving fluid electrode technique, in the chick biventer cervicis (BVC) nerve muscle preparation. TEA (4.8 × 10 −4−4.8 × 10 −2 M) produced contracture and depolarization responses which were concn-dependent. These responses were potentiated by methohexitone (8.8 × 10 −5 M). The mean ED 50 S for the contracture responses in the control Krebs solution and with methohexitone were (mean ± S.E.M.) 6.5 ± 0.03 × 10 −3 M and 1.3 ± 0.04 × 10 −3 M (N = 6) respectively. The mean ED 50 S for the depolarizations were (mean ± S.E.M.) 5.9 ± 0.1 × 10 −3 and 1.5 ± 0.06 × 10 −3 M (N = 6) respectively. ACh (5.5 × 10 −6−1.1 × 10 −2 M) produced contracture and depolarization responses which were concn-dependent. These responses were reduced by methohexitone (8.8 × 10 −5 M). The mean ED 50 S for the contracture responses in the control Krebs solution and with methohexitone were (mean ± S.E.M. 2.4 ± 0.21 × 10 −4 and 2.3 ± 0.1 × 10 −3 M (N = 6) respectively. The mean (± S.E.M.) ED 50 S for the depolarizations were 8.4 ± 0.33 × 10 −4 and 3.7 ± 0.14 × 10 −3 M (N = 6), respectively. Repetitive indirect stimulation, at 1–20 Hz, produced contraction and depolarization responses which were frequency-dependent. These responses were slightly potentiated by methohexitone (8.8 × 10 −5 M). The mean (± S.E.M.) frequency 50 S for the contractions produced in the control Krebs solution and with methohexitone were 9.2 ± 0.1 and 8.5 ± 0.2 Hz (N = 6) respectively. The mean frequency 50 S for the depolarizations were (mean ± S.E.M.) 7.2 ± 0.1 and 5.8 ± 0.19 Hz (N = 6) respectively. It is concluded that TEA may have a direct post-synaptic action, in addition to releasing ACh from the presynaptic nerve terminals. TEA produces more contracture tension than does ACh for a given level of membrane depolarization. Methohexitone, non-competitively, reduces the responses produced by applied ACh whereas it potentiates those produced by TEA and repetitive nerve stimulation.