COMPARISON OF MAMMALIAN ADULT AND FETAL NICOTINIC ACETYLCHOLINE-RECEPTORS STABLY EXPRESSED IN FIBROBLASTS

Cells from a line of transformed quail fibroblasts (QT-6) were transfe cted with cDNAs coding for subunits of the mouse muscle nicotinic ACh receptor (AChR). Stable clones were selected that expressed subunits o f the fetal-type AChR (alpha, beta, gamma, delta) or the adult-type AC hR (alpha, beta, epsilon, delta). The receptors had the appropriate bu rst durations and single-channel conductances for the fetal or adult t ype, respectively. Each type of receptor had a dose-response relations hip that was close to a square law at low concentrations of ACh, imply ing that they contained two ACh-binding subunits. The metabolic stabil ity of surface fetal and adult receptors was identical (about 10 hr ha lf-life), for two independent clones expressing fetal and two expressi ng adult AChR. The metabolic stability was unaffected by treatment wit h okadaic acid, which enhanced receptor phosphorylation. d-Tubocurarin e (dTC) blocked both the binding of alpha-bungarotoxin (BTX) to the ce lls and the ACh-elicited current. dTC blocked BTX binding with indisti nguishable efficacy for both fetal and adult AChR. However, it was six fold less effective at blocking ACh-elicited current from fetal AChR. At least part of the difference results from the ability of fetal rece ptor channels to open when the receptor has one ACh and one dTC molecu le bound, whereas channels of heteroliganded adult receptors do not op en. The data indicate that the subunit composition directly affects ph ysiological and pharmacological properties of muscle AChR, but has lit tle effect by itself on metabolic stability.