Which of the following relaxes bronchial smooth muscles?

Introduction. A major symptom of asthma is an excessive contraction of airway smooth muscle cells (SMCs) which results in airway hyper-reactivity. To allete this acute and chronic airway constriction, b-adrenergic agonists, that relax SMCs, are commonly administered Ref: guyton and hall textbook of medical physiology 12 edition page number:331,332,333