Effects of drug-induced paralysis and depolarisation on acetylcholine receptor and cyclic nucleotide levels of chick muscle cultures

Abstract

Neuromuscular interactions play an important role in the development and maintenance of several myofiber properties including the regulation of its chemosensitivity. In adult innervated skeletal muscle, receptors for the neuromuscular transmitter, acetylcholine, are confined to the endplate region of the myofiber membrane. By contrast, non-innervated embryonic myotubes or denervated adult muscle fibers bear acetylcholine receptors (AChR) on their entire ‘extrasynaptic’ cell surface (reviewed [ 1,2]). Different investigators showed that the developmental localisation of AChR to the postsynaptic part of the sarcolemma involves: (i) The aggregation of receptors under the nerve terminal [3,4]; (ii) The repression of the synthesis of extrasynaptic AChR [S-7]. Several lines of evidence suggest the latter process to be mediated by neurally-induced muscle activity: (1) Electrical stimulation of adult denervated muscle prevents appearance of extrasynaptic AChd [8] and blocks the incorporation of radiolabelled amino acids into the receptor protein [9]; (2) Chronic paralysis of the motor endplate by cholinergic antagonists increases the synthesis of extrasynaptic AChR in adult and embryonic myofibers [6,10,11]; (3) In muscle cell cultures, the rate of AChR synthesis is decreased by electrical stimulation or depolarising agents [ 12,131; under the same conditions, paralysis of the spontaneously contracting myotubes by tetrodoxin leads to an increase of newly synthesized receptor molecules. At present, the molecular events underlying the regulation of extrasynaptic AChR by muscle activity are unknown. In experiments with chick myotubes in tissue culture, dibutyryl cGMP added to the medium was found to reduce the rate of AChR synthesis [ 131, whereas CAMP derivatives increased the latter [ 13,141. Moreover, both cholinergic and electrical stimulation of adult muscle are known to induce transient increases in intracellular cGMP which depend on the presence of extracellular Ca*+ [ 15-171. It, therefore, was postulated that Ca*+ and cGMP may constitute the intracellular messengers of the activity-mediated control of AChR [13]. In this letter, it is shown that there is no obvious correlation between the AChR and the cyclic nucleotide levels of chick muscle cell cultures under conditions of drug-induced paralysis and depolarisation.