What is the main therapeutic action of phenylephrine?

Phenylephrine primarily acts as an alpha-1 adrenergic agonist, leading to vasoconstriction of blood vessels. This mechanism occurs through the stimulation of alpha-1 receptors located on vascular smooth muscle. When phenylephrine binds to these receptors, it causes an increase in intracellular calcium concentration, which results in smooth muscle contraction and subsequent vasoconstriction. Phenylephrine is commonly used as a decongestant to alleviate nasal congestion and as a treatment for hypotension during surgical procedures due to its ability to raise blood pressure by increasing systemic vascular resistance.

While it does have some weak activity on alpha-2 receptors, its predominant action is through alpha-1 stimulation, making it effective in conditions where increased vascular tone is desired. The other choices pertain to mechanisms not associated with phenylephrine; for instance, bronchodilation is related to beta-2 adrenergic agonists, and inhibition of Na/K ATPase is characteristic of cardiac glycosides like digoxin, while calcium channel blockade relates to medications such as calcium channel blockers that lower blood pressure through different actions.