Describe the action of vasoconstrictors like phenylephrine, that act on smooth muscles.

Phenylephrine is a pure alpha-1 agonist, and very powerfully constricts arterial smooth muscle, as a result you get a marked “pressor” response when this drug is injected. The body reacts by decreasing sympathetic nerve traffic from the CNS to the arteries and heart, and an increase in vagal nerve impulses to the heart and blood vessels. So in the figure, the reflex that occurs with that pressor response to phenylephrine is a reflex bradycardia. So the direct effect of the drug is on the alpha-1 receptor, you get arterial vasoconstriction. Sympathetic traffic is diminished and vagal activity is increased as a compensatory mechanism, and as a result you get a reflex bradycardia; the pressure is lower. This doesn’t cancel out the effect of phenylephrine. Phenylephrine is used as a very powerful specific arterial constrictor… many cold medicines contain phenylephrine to constrict the vessels in the nasal cavities.

If you inject histamine into a patient, you will get a drop in systolic pressure due to a direct action of histamine on the histamine blood receptors on the blood vessels (we’ll talk about these receptors later). The opposite occurs with sympathetic and vagal stimulation, so you get a reflex tachycardia. So there are direct effects of drugs and reflex consequences of drugs—you must be able to distinguish between these two effects on exams!!

Category: Pharmacology Notes