A 57-year-old woman visits her primary care provider (PCP) c…

A 57-year-old woman visits her primary care provider (PCP) complaining of chronic shortness of breath, at rest and with exertion, and a productive cough with sputum.  The patient reports having smoked since age 16, and is currently smoking 1.5 pack/week. She denies drinking alcohol.  Past medical history is significant for bouts of untreated essential hypertension.  Vital signs reveal blood pressure 165/100 mm Hg. 88 HR, 21 RR. Physical exam reveals pronounced wheezing on lung auscultation.  ECG is normal, and CXR shows no lung consolidation.  Spirometry reveals low FEV1 & FEV1/FVC.  Inhalation of a bronchodilator does not improve FEV1 or FEV1/FVC.  Her peak flow meter reading is 110 L/min. The patient’s PCP prescribes Ipratropium bromide (Atrovent®), administered through a metered dose inhaler.  After 1 week of treatment, the patient’s peak flow velocity improves to 220 L/min. 14. Which of the following mechanisms explains the therapeutic effect of Ipratropium bromide in this patient?