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. 2020 Mar 21;12(3):197. doi: 10.3390/toxins12030197

Figure 2.

Figure 2

αM-MIIIJ blocks excitatory synaptic responses but not action potentials in X. laevis longitudinal pectoralis (LP) muscle. Extracellular recording from adult muscle was performed as described in Methods. Graphs plot peak amplitudes of responses versus time, where presence of αM-MIIIJ is represented by black bar above each plot and insets show representative traces at the enumerated time points. Arrows in insets of panels A and C show when stimulus was applied. A and B. Muscle action potentials in response to indirect (A) and direct (B) stimulation. Ten μM αM-MIIIJ completely blocked action potentials evoked by nerve stimulation (A); in contrast, a five-times higher concentration of αM-MIIIJ had essentially no effect on action potentials evoked by direct stimulation (B). As a positive control, μ-PIIIA (10 μM), a conotoxin that irreversibly inhibits muscle V-gated Na channels, completely blocked action potentials within 10 min. (flat dashed trace of panel B’s inset). C. Motor-nerve stimulation evoked synaptic responses (without confounding muscle action potentials) in muscle preparation treated with μ-PIIIA (10 μM). The synaptic responses were reversibly blocked by 10 μM αM-MIIIJ. Each trace in inset of panel C represents the average of ~10 responses. Results replicating those illustrated here (i.e., complete block by toxin of nerve stimulation-evoked action potentials and synaptic responses, and conversely no block by toxin of direct stimulation-evoked action potentials) were obtained in three other LP muscle preparations.