A possible modulation of acetylcholine receptors of embryonic chick muscle cells by α‐bungarotoxin

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Abstract
Acetylcholine receptors were assayed with α‐bugarotoxin on embryonic chick skeletal muscle growing in primary cell culture. Toxin was bound specifically to muscle cells and could be competed with D‐tubocurarine. Two dissociation constants were obtained by equilibrium binding: 7.2 × 10−9M and 2.7 × 10−7M at 25°C. Two sets of rate constants were also obtained from dissociation kinetics. There are five times more low affinity sites on cells than high affinity sites. The average density of high‐affinity receptors is about 200/μm2. A time course of toxin binding to receptors at 37°C vs 25°C in growth medium revealed that under conditions permitting growth and metabolism, toxin bound to cells was lost. The possibility that the growth medium was in‐activating toxin molecules was ruled out by showing that unbound toxin molecules in the medium were fully capable of binding to fresh cultures.