On muscarinic control of neurogenic mucus secretion in ferret trachea

1. Muscarinic receptor subtypes mediating neurogenic mucus secretion in ferret trachea were characterized in vitro and in vivo using (SO4)-S-35 as a label for secreted mucus, and the muscarinic receptor antagonists telenzepine for the M(1) receptor subtype, methoctramine for the M(2) subtype and 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP) for the M(3) receptor. We also performed receptor binding and mapping studies. 2. Each muscarinic antagonist displaced [N-methyl-H-3]scopolamine binding with high-affinity binding constant (K-H) values of 1.9, 2.7 and 5.0 nM for telenzepine, methoctramine and 4-DAMP, respectively. Muscarinic M(1) and M(3) receptors localized to submucosal glands, whereas M(2) receptors did not. 3. In vitro, electrical stimulation (50 V, 10 Hz, 0.5 ms for 5 min) increased (SO4)-S-35 output by 160%. Telenzepine did not inhibit the neurogenic secretory response at concentrations two- or twentyfold its K-H value, nor did it inhibit secretion induced by acetylcholine (ACh). 4-DAMP inhibited neurogenic secretion by 80 and 95%, respectively, at concentrations two- and twenty-fold its K-H value, and also inhibited ACh-induced secretion. Methoctramine potentiated neurogenic secretion induced at 2.5 Hz (50 V, 0.5 ms for 5 min) in a dose-related (5.4-100 nM) manner with increases of 33-451% above electrically stimulated values. Methoctramine did not potentiate secretion induced at 10 Hz and did not have any effect on ACh-induced secretion. 4. In vivo, vagal stimulation (10 V, 10 Hz, 2 ms for 8 min) increased output of (SO4)-S-35 by similar to 120%. Telenzepine had no significant effect on neurogenic secretion. Methoctramine approximately doubled the stimulated response, whereas 4-DAMP abolished the stimulated secretory response. 5. We conclude that in ferret trachea, cholinergic nerve stimulation increases mucus secretion via muscarinic M(3) receptors on the submucosal glands. The magnitude of the secretory response is regulated by neuronal M(2) muscarinic receptors. The muscarinic M(1) receptors localized to the submucosal glands do not appear to be involved with mucus secretion.