Final answer:
The stimulation of beta-2 (β2) adrenergic receptors causes bronchial smooth muscle relaxation. These receptors, when activated, increase cAMP, promoting relaxation of the smooth muscles in the bronchi.
Step-by-step explanation:
Beta-2 Adrenergic Receptors and Bronchial Relaxation
The stimulation of beta-2 (β2) adrenergic receptors produces bronchial smooth muscle relaxation. Activation of these receptors leads to an increase in intracellular cAMP (cyclic Adenosine Monophosphate), which then results in smooth muscle relaxation, including that of the bronchi in the lungs.
In contrast, alpha-adrenergic receptors, which come in the subtypes α₁ and α₂, commonly result in smooth muscle contraction and are not involved in bronchial relaxation. Beta-1 (β1) receptors primarily affect the heart, leading to an increase in heart muscle contraction and not directly involved in bronchial smooth muscle relaxation.
Therefore, the correct answer to the question of which receptor sites' stimulation produces bronchial smooth muscle relaxation is β2.