Effect of a M1-selective muscarinic receptor antagonist (pirenzepine) on basal bronchomotor tone in young women.

To examine the role of muscarinic M1-receptors in vagally mediated basal bronchomotor tone, we studied the effects of inhaled pirenzepine, which is a M1-receptor antagonist, on basal bronchomotor tone in 8 young women. To obtain dose-response curves to inhaled pirenzepine, we measured partial and full flow-volume curves before and 30 min after inhalation of each concentration of pirenzepine (3.25-200 micrograms/ml). Maximum expiratory flow on partial flow-volume curve at 25% forced vital capacity (PEF25) was measured as an index showing basal bronchomotor tone. The dose-response curve of the percent increase in PEF25 by pirenzepine was biphasic in shape. PEF25 was significantly increased by 3.12 (14.5 +/- 5.8%), 6.25 (15.5 +/- 4.2%), 12.5 (20.0 +/- 0.29%) and 200 micrograms/ml (17.3 +/- 5.9%) of pirenzepine, but not by 25 (9.6 +/- 5.6%), 50 (9.4 +/- 6.2%) and 100 micrograms/ml (14.4 +/- 6.2%) of pirenzepine. The percent increase in PEF25 by 40 micrograms of ipratropium bromide was 66.2 +/- 14.2%. These results suggest that the role of M1-receptors in vagally mediated basal bronchomotor tone may be significant but little compared with M3-receptors in normal humans.