Figure 3.

Action and synaptic potentials in R. pipiens cutaneous pectoralis (CP) muscle are unaffected by αM-MIIIJ (100 μM). Muscle preparation and recording were as described in Methods. Format of presentation of results are as in Figure 2 (except traces in panel B were acquired with a low pass-filter setting of 0.1, instead of 1, Hz). Indirectly-evoked action potentials (A) and synaptic responses (B) are essentially unaffected by 100 μM αM-MIIIJ. In B, the muscle was treated with μ-PIIIA as in Figure 2C. As a positive control, the preparation was exposed to the muscle nAChR antagonist d-tubocurare (10 μM), which blocked within 15 min. (largely flat, dashed trace of panel B’s inset). These results indicate that αM-MIIIJ does not block the muscle action potential in R. pipiens (like in X. laevis) muscle, nor does αM-MIIIJ block (unlike in X. laevis) the synaptically-evoked response. Results replicating those illustrated here (i.e., no block of either action or synaptic potentials by toxin) were obtained in three other CP muscle preparations.