Transcriptional and translational requirements for developmental alterations in acetylcholine receptor channel function in Xenopus myotomal muscle

Two functionally distinct types of acetylcholine (ACh) receptor channels are present on embryonic Xenopus myotomal muscle. During differentiation of this muscle, both in vivo and in dissociated cell culture, the occurrence of the high conductance "fast" channel type increases relative to the low conductance "slow" channel type. In order to ascertain whether new receptor synthesis is required for this rapid switch in channel types we examined the effects of inhibitors of transcription (α-amanitin and actinomycin D) and translation (cycloheximide) on developing muscle in culture. Inhibition of protein synthesis resulted in >95% reduction in ACh receptor incorporation and also reversibly blocked the developmental appearance of the high conductance channel type. Inhibition of mRNA synthesis only slightly reduced the rate of receptor incorporation into muscle membrane over a 24-hr period but reversibly blocked appearance of the high conductance channel. These findings suggest that the high conductance Ach receptor channel type does not result from post-translational modifications of the low conductance type, but rather from transcription of a different mRNA encoding one or more of the ACh receptor subunits.