Chloroquine is a muscarinic antagonist. Binding and dose-response studies with chick embryo cells

A muscarinic acetylcholine receptor is present on undifferentiated cells of the chick embryo. We show that, in the chick embryo, chloroquine binds to the muscarinic receptor site and behaves as a muscarinic antagonist. In competition studies performed with cell suspensions, chloroquine displaced the specific muscarinic ligand [3H]quinuclidinylbenzilate ([3H]QNB) from the receptor. The dissociation constant (KD) of chloroquine was calculated to be 6.0 X 10(-6) M. In saturation studies performed in chick embryo homogenate, chloroquine shifted the binding curve of [3H]QNB to the right (KD = 4.8 X 10(-6) M). Dose-response curves were established by measuring the acetylcholine-triggered Ca2+ mobilization in cell suspensions by means of a fluorometric assay with chlorotetracycline. Chloroquine shifted the dose-response curve to the right. The inhibitory constant (KI) of chloroquine calculated from the dose-response curve was 1.5 X 1 10(-5) M. Our observations provide an explanation for the known interference of chloroquine with the muscarinic cholinergic systems in a variety of adult organs.