Lecture 16 — Adrenomimetic Drug Actions
Vascular actions
Alpha-mediated vasoconstriction occurs in skin, splanchnic and nasal mucosal vessels. Nasal vessel constriction explains decongestant effects.
Central alpha-2 action
Systemic alpha-2 agonists can reduce sympathetic outflow and lower blood pressure. The lecture lists clonidine, methyldopa and guanfacine.
Cardiac beta effects
Beta receptor stimulation increases cardiac contractility (inotropy), heart rate (chronotropy) and AV nodal conduction (dromotropy), and shortens refractoriness.
Skeletal muscle circulation
Epinephrine stimulates beta-2 receptors in skeletal-muscle vessels, producing vasodilation that can reduce total peripheral resistance.
Direct versus indirect action
Direct-acting drugs stimulate adrenoceptors themselves. Indirect-acting agents such as amphetamine promote norepinephrine release from nerve storage sites.
Epinephrine
Activates alpha and beta receptors; increases contractility and heart rate while causing vasoconstriction in several vascular beds. These effects contribute to increased systolic pressure.
Norepinephrine
Strong alpha-mediated vasoconstriction increases peripheral resistance and blood pressure. Reflex vagal slowing can counteract its direct heart-rate stimulation.
Phenylephrine and methoxamine
Phenylephrine is relatively selective for alpha-1 and can produce vasoconstriction, mydriasis and nasal decongestion. The lecture compares methoxamine with phenylephrine.
Other agonists
Oxymetazoline is used topically for nasal decongestion; excessive systemic absorption may cause hypotension. Beta-2 agonists discussed include albuterol and terbutaline for bronchodilation; terbutaline is also discussed in relation to delaying preterm labor.
