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Acetylcholine (ACh) receptors have been purified separately from normal rat diaphragm muscle (junctional receptors) and from extrajunctional regions of denervated diaphragm (extrajunctional receptors) in order to compare their properties. The toxin-receptor complexes of the two receptors were indistinguishable by gel filtration and by zone sedimentation in sucrose gradients, and showed identical precipitation curves with rabbit antiserum to the eel ACh receptor. Both toxin-receptor complexes bind concanavalin A and are therefore probably glycoproteins. Low concentrations of d-tubocuratine (dTC) were more effective in decreasing the rate of toxin binding to junctional than to extrajunctional receptors. The apparent dissociation constant for dTC binding to the junctional receptor was 4.5 X 10 minus 8 M, whereas the value for the extrajunctional receptor was 5.5 X 10 minus 7 M. When the complexes were analyzed by isoelectric focusing, the junctional complex focused at approximately 0.15 pH unit lower than the extrajunctional complex. This result was also found with crude preparations of receptor. We conclude that junctional and extrajunctional receptors are similar but distinct molecules. The properties of receptors present in neonatal diaphragm muscle were also examined and found to be similar to those of receptors in denervated muscle, as shown by dTC inhibition and isoelectric focusing.
Brockes et al. (Tue,) studied this question.