Final answer:
Phenylephrine functions as an α1-adrenergic receptor agonist, causing vasoconstriction and smooth muscle contraction through the activation of the G protein-coupled receptor pathway.
Step-by-step explanation:
The mechanism of phenylephrine on smooth muscle involves it acting as an α-adrenergic receptor agonist, specifically an α-1 adrenergic receptor agonist. When phenylephrine binds to these receptors, it stimulates a response that leads to vasoconstriction in various blood vessels, including those of the skin, gastrointestinal system, and in the kidneys and brain. This action is due to the activation of a G protein-coupled receptor that in turn activates phospholipase C, leading to an increase in intracellular calcium and subsequent smooth muscle contraction.