Calcitonin gene-related peptide and muscle activity regulate acetylcholine receptor alpha-subunit mRNA levels by distinct intracellular pathways

In cultured chicken myotubes, calcitonin gene-related peptide (CGRP), a peptide present in spinal cord motoneurons, increased by 1.5-fold the number of surface acetylcholine receptors (AChRs) and by threefold AChR alpha-subunit mRNA level without affecting the level of muscular alpha- actin mRNA. Cholera toxin (CT), an activator of adenylate cyclase, produced a similar effect, which did not add up with that of CGRP. In contrast, tetrodotoxin, a blocker of voltage-sensitive Na+ channels, elevated the level of AChR alpha-subunit mRNA on top of the increase caused by either CGRP or CT. 12-O-Tetradecanoyl phorbol-13-acetate (TPA), an activator of protein kinase C, markedly decreased the cell surface and total content of [125I]alpha BGT-binding sites and reduced the rate of appearance of AChR at the surface of the myotubes without reducing the level of AChR alpha-subunit mRNA. Moreover, TPA inhibited the increase of AChR alpha-subunit mRNA caused by tetrodotoxin without affecting that produced by CGRP or CT. Under the same conditions, TPA decreased the level of muscular alpha-actin mRNA and increased that of nonmuscular beta- and gamma-actins mRNA. These data suggest that distinct second messengers are involved in the regulation of AChR biosynthesis by CGRP and muscle activity and that these two pathways may contribute to the development of different patterns of AChR gene expression in junctional and extrajunctional areas of the muscle fiber.