Acetylcholine receptor γ-subunits mRNA isoforms expressed in denervated rat muscle

The fetal acetylcholine (ACh) receptor, composed of the alpha beta gamma delta subunits, is expressed in fetal, neonatal, and denervated muscle. Single-channel recording has revealed three kinetically distinct classes in neonatal and denervated muscle, suggesting that at least three forms of the gamma-subunit are required. To account for the kinetic classes observed, we compared the messenger ribonucleic acid (mRNA) forms expressed in neonatal and denervated muscle using reverse transcriptase polymerase chain reaction, cloning, and RNAse protection assays. We found five novel forms arising from alternative splicing, which we named gamma 5-gamma 9. The forms gamma 5, gamma 6, and gamma 7 lack exon 4 and 63-, 89-, and 136 bp of exon 5, respectively. A gamma 8 form lacks exons 3 and 4 and 19 bp of exon 5. The last, gamma 9, lacks exons 3, 4, and 5. Results indicate that gamma 4 predominates in fetal muscle and gamma 7 in denervated adult muscle. Some of the gamma-subunit mRNAs found may generate the receptors observed in muscle.