Pharmacology and Biophysical Properties of α7 and α7 ‐ α8 α‐Bungarotoxin Receptor Subtypes Immunopurified from the Chick Optic Lobe

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Abstract
Two chick optic lobe α‐bungarotoxin receptor subtypes (α7 and α7 ‐ α8) were immunopurified using polyclonal antibodies raised against synthetic peptides of chick α7 and α8 α‐bungarotoxin receptor subunits. The α7 subtype contained the Mr 57 000 α7 subunit, and represented 60 ‐ 70% of the α‐bungarotoxin receptors; the α7‐α8 subtype contained the Mr 57 000 α7 and α8 subunits, and represented only 20 ‐ 25% of the receptors. Both subtypes also had an additional Mr 52 000 subunit. The affinity of these subtypes for α‐bungarotoxin as well as antagonists was similar. However, the α7 ‐ α8 subtype displayed consistently higher affinities for agonists. When reconstituted in planar lipid bilayers, the α7 ‐ α8 subtype displayed several conductance states of 10 ‐ 50 pS; the α7 subtype had only one conductance state of 45 pS. The α7 ‐α8 subtype was activated by lower agonist concentrations than the α7 subtype. When expressed in Xenopus oocytes, the α8 subunit formed functional homomeric receptors that desensitized rapidly. These channels were blocked by α‐bungarotoxin and displayed a higher affinity for agonists than the α7 homomeric receptor. Taken together, these data indicate that at least two α‐bungarotoxin subtypes are present in the chick optic lobe. They operate as ligand‐gated channels and display different agonist sensitivities and kinetics/conductance properties.