Journal of Neuroscience, Vol 14, 3922-3933, Copyright © 1994 by Society for Neuroscience
Comparison of mammalian adult and fetal nicotinic acetylcholine receptors stably expressed in fibroblasts
C Kopta and JH Steinbach
Department of Anesthesiology, Washington University School of Medicine, St. Louis, Missouri 63130.
Cells from a line of transformed quail fibroblasts (QT-6) were transfected
with cDNAs coding for subunits of the mouse muscle nicotinic ACh receptor
(AChR). Stable clones were selected that expressed subunits of the
fetal-type AChR (alpha, beta, gamma, delta) or the adult-type AChR (alpha,
beta, epsilon, delta). The receptors had the appropriate burst durations
and single-channel conductances for the fetal or adult type, respectively.
Each type of receptor had a dose- response relationship that was close to a
square law at low concentrations of ACh, implying that they contained two
ACh-binding subunits. The metabolic stability of surface fetal and adult
receptors was identical (about 10 hr half-life), for two independent clones
expressing fetal and two expressing adult AChR. The metabolic stability was
unaffected by treatment with okadaic acid, which enhanced receptor
phosphorylation. d-Tubocurarine (dTC) blocked both the binding of alpha-
bungarotoxin (BTX) to the cells and the ACh-elicited current. dTC blocked
BTX binding with indistinguishable efficacy for both fetal and adult AChR.
However, it was sixfold less effective at blocking ACh- elicited current
from fetal AChR. At least part of the difference results from the ability
of fetal receptor channels to open when the receptor has one ACh and one
dTC molecule bound, whereas channels of heteroliganded adult receptors do
not open. The data indicate that the subunit composition directly affects
physiological and pharmacological properties of muscle AChR, but has little
effect by itself on metabolic stability.