Influence of acute and chronic ethanol treatment on muscarinic responses and receptor expression in Chinese hamster ovary cells

The influence of ethanol on the muscarinic receptor-mediated release of inositol phosphate from Chinese hamster ovary (CHO) cells stably transfected with one of the five subtypes of muscarinic acetylcholine receptor was determined. In CHO cells expressing M3 muscarinic receptors (CHO-M3), carbamylcholine increased muscarinic receptor-induced release of inositol phosphate by 150–350% following a 15-min incubation with an EC50 of ~30 μM. Maximal responses were obtained with 1 mM carbamylcholine, while responses to 10 mM carbamylcholine were somewhat less than maximal. Preincubation with atropine for 10 min inhibited the response with an IC50 of ~30 nM. CHO cells transfected with M1, M3, and M5 receptors displayed a similar pattern of activity; CHO cells transfected with M2 and M4, as well as untransfected cells, were unresponsive to carbamylcholine. Ethanol acutely inhibited the response of CHO-M3 cells to carbamylcholine by 15% at 18 mM and by 47% at 180 mM (the highest concentration examined). CHO-M3 cells were incubated with 50 mM ethanol for 48 hr. This treatment did not affect the number of cells or their protein content (113 pg/cell). The expression of M3 muscarinic receptors (determined using [3H]N-methylscopolamine) increased from 1.34 ± 0.23 to 1.75 ± 0.16 pmol/mg protein (P < 0.05). In contrast, carbamylcholine-stimulated release of inositol phosphate was depressed by 40–70% in four experiments. Concentration-response analyses indicated a non-competitive inhibitory mechanism. This dissociation of muscarinic receptor expression and muscarinic signaling suggests a compensatory increase in receptor expression in response to chronic inhibition of muscarinic signaling by ethanol.