Some properties of acetylcholine receptors in human cultured myotubes

Properties of acetylcholine-receptor activation in human Duchenne muscular dystrophy myotubes

In human myotubes cultured from biopsies of normal subjects and dystrophic patients we investigated, with the patch-clamp technique, the activation properties of the nicotinic acetylcholine receptor (AChoR) in the presence of acetylcholine and suberyldicholine. The single-channel conductance and the lifetime of the openings were not found to differ. In contrast, the average frequency of openings was about four times higher in Duchenne muscular dystrophy (DMD) myotubes in the presence of equal amounts of acetylcholine, but not of suberyldicholine. The most reasonable conclusion from this observation is that the behaviour of the AChoR is not altered in DMD cells but that there is a greater average concentration of ACho molecules present around AChoRs. This leads to the tentative conclusion that the activity of the enzyme acetylcholinesterase (AChoE) is impaired by some unknown mechanism in the dystrophic myotube.

A role for acetylcholine receptors in the fusion of chick myoblasts

The role of acetylcholine receptors in the control of chick myoblast fusion in culture has been explored. Spontaneous fusion of myoblasts was inhibited by the nicotinic acetylcholine receptor antagonists alpha-bungarotoxin, Naja naja toxin and monoclonal antibody mcAb 5.5. The muscarinic antagonists QNB and n-methyl scopolamine were without effect. Atropine had no effect below 1 microM, where it blocks muscarinic receptors; at higher concentrations, when it blocks nicotinic receptors also, atropine inhibited myoblast fusion. The inhibitions imposed by acetylcholine receptor antagonists lasted for approximately 12 h; fusion stimulated by other endogenous substances then took over. The inhibition was limited to myoblast fusion. The increases in cell number, DNA content, the level of creatine phosphokinase activity (both total and muscle-specific isozyme) and the appearance of heavy chain myosin, which accompany muscle differentiation, followed a normal time course. Pre-fusion myoblasts, fusing myoblasts, and young myotubes specifically bound labeled alpha-bungarotoxin, indicating the presence of acetylcholine receptors. The nicotinic acetylcholine receptor agonist, carbachol, induced uptake of [14C]Guanidinium through the acetylcholine receptor. Myoblasts, aligned myoblasts and young myotubes expressed the synthetic enzyme Choline acetyltransferase and stained positively with antibodies against acetylcholine. The appearance of ChAT activity in myogenic cultures was prevented by treatment with BUDR; nonmyogenic cells in the cultures expressed ChAT at a level which was too low to account for the activity in myogenic cultures. We conclude that activation of the nicotinic acetylcholine receptor is part of the mechanism controlling spontaneous myoblast fusion and that myoblasts synthesize an endogenous, fusion-inducing agent that activates the nicotinic ACh receptor.

Activation of ion channels in the frog end-plate by high concentrations of acetylcholine

1. The equilibrium relationship between acetylcholine (ACh) concentration and response (fraction of channels open), corrected for the effects of desensitization, has been estimated by single-ion-channel recording at the adult frog skeletal neuromuscular junction. At high ACh concentration channel openings occur in well-defined clusters separated by long desensitized intervals. The response, po, was estimated as the proportion of time for which a single channel was open during a cluster. 2. At negative membrane potential (-120 mV) po reached a maximum value of 0.9 at 100 microM-ACh and was half-maximum at 15 microM with a Hill slope of 1.6 at this point. At concentrations higher than 200 microM-ACh, po declined as a result of open-channel block by free ACh itself. 3. At positive membrane potentials (+100 mV) there was little channel block by ACh; po reached a maximum value of 0.41 at 500 microM-ACh, with half-maximum activation at 50 microM and Hill slope of 1.2 at this point. 4. Particular mechanisms for channel activation by ACh were fitted to the data by the method of least squares. Fits were fully determinate only if the two binding sites for ACh were assumed to be equivalent with no co-operativity in the ACh binding reactions. At negative potential the microscopic equilibrium constant for binding was K1 = K2 = 77 microM and the equilibrium constant for channel opening (opening/closing rates, beta/alpha) was 32. At positive potential the affinity was slightly higher, K = 32 microM, which confirms the view that the binding sites for ACh are outside the membrane electric field. The equilibrium constant for channel opening was reduced to 0.7 mainly as a result of the much shorter open lifetime (increased closing rate alpha) at positive potentials. 5. The data were also fitted well by very high values of beta/alpha together with a high degree of negative co-operativity or non-equivalence in ACh binding affinity (K2 much greater than K1). A good fit could also be obtained with moderate positive co-operativity combined with non-equivalence of the binding sites. 6. A mechanism that postulates a receptor with two independent gating subunits provided a poor fit to the data at negative potential. 7. The rate constants for channel opening and ACh dissociation were estimated by constraining the fitted parameters so that the burst length for channel opening was equal to its observed value at low concentrations of ACh.(ABSTRACT TRUNCATED AT 400 WORDS)