Adrenergic and Cholinergic Receptors in Blood Vessels

Most arteries and veins in the body are innervated bysympathetic adrenergic nerves, which releasenorepinephrine (NE) as a neurotransmitter. Some blood vessels are innervated by parasympathetic cholinergic or sympathetic cholinergic nerves, both of which releaseacetylcholine (ACh) as their primary neurotransmitter. Neurotransmitter binding to the adrenergic and cholinergic receptors activatesignal transduction pathways that cause the observed changes in vascular function.

NE preferentially bindsα₁-adrenoceptors to cause smooth muscle contraction and vasoconstriction. Similar responses occur when NE binds to postjunctional α₂-adrenoceptors on some blood vessels. NE also binds weakly to postjunctionalβ₂-adrenoceptors, which causes vasodilation (this can be observed during alpha adrenoceptor blockade), although this vasodilator effect of NE is relatively minor and overwhelmed by alpha adrenoceptor-mediated vasoconstriction. Circulatingepinephrine (EPI) binds with high affinity to smooth muscle β₂-adrenoceptors to cause vasodilation in some organs; however, the effect of EPI is very concentration-dependent. While EPI has a higher affinity for β₂ than postjunctional α₁ or α₂-adrenoceptors, at high concentrations it does bind to the postjunctional α₁ and α₂-adrenoceptors, which can override the vasodilatory effects of β₂-adrenoceptor stimulation and produce vasoconstriction.

Some blood vessels in the body (e.g., coronary vessels) are innervated byparasympathetic cholinergic fibers. These nerves release ACh, which binds to muscarinic M₃ receptors on the smooth muscle and/or endothelium that are linked to Gq-proteins. It has been shown that endothelial M₃ receptors also stimulate the formation ofnitric oxide (NO), which causes smooth muscle relaxation. Note, however, that in the absence of NO, ACh causes smooth muscle contraction through smooth muscle M₃ receptors. This latter finding has been used to assess coronary vascular dysfunction in humans, in which NO production is diminished in diseased coronary arteries. In contrast to other arteries, cerebral arteries appear to have M₅ muscarinic receptors that produce vasodilation in response to ACh.

Some arterial blood vessels inskeletal muscle of dogs and cats (but not humans) are innervated bysympathetic cholinergic nerves that release ACh and cause vasodilation. In species having sympathetic cholinergic nerves, their activation may contribute toactive hyperemia in skeletal muscle, particularly at the onset of exercise. Sympathetic cholinergic nerves controlled by hypothalamic thermoregulatory nuclei play an important role in producing cutaneous active vasodilation when skin and body core temperature increase in warm environments.

Drugs are available for blocking vascular adrenergic receptors. Alpha-blockers, for example, are used in treating hypertension. Some of the alpha-blockers are relatively selective for a specific receptor subtype, whereas others as non-selective. Drugs such as atropine block muscarinic receptors.

Revised 12/9/2022