Adrenoceptors


Next in our series about the heart, here's an article about the role of adrenoceptors in controlling the cardiovascular system.

Adrenergic receptors (adrenoceptors)

The autonomic nervous system serves as the master control centre for the cardiovascular system. It monitors the effectiveness of the latter system in providing nutrients and oxygen to the rest of the body and appropriately adjusts the heart rate, blood pressure, and blood flow. These adjustments are made via nerves that serve the heart, blood vessels, and kidneys. Adrenergic receptors form the interface between these nerves (of the sympathetic subsystem) and the organs they innervate. Catecholamines (adrenaline, noradrenaline and isoprenaline) released from sympathetic nerve terminals bind to adrenergic receptors on the surface of target cells, and the activated receptors modify the function of these cells. Adrenoceptors are classified into alpha(a-) and beta(ß-) subtypes. The main subtypes for each kind of adrenoceptors are given below:

(1) a-adrenoceptors: a1 and a2

(2) ß-adrenoceptors: ß1, ß2 and ß3

Receptor: Order of agonist potency

a : Noradrenaline > Adrenaline > Isoprenaline

ß : Isoprenaline> Adrenaline >Noradrenaline

The main effects of receptor activation are summarised below:

a1-receptors: Vasoconstriction, relaxation of gastro-intestinal smooth muscle, salivary secretion and hepatic glycogenolysis

a2-receptors: Inhibition of transmitter release, platelet aggregation, contraction of vascular smooth muscle, and inhibition of insulin release

ß1-receptors: Increased cardiac rate and force

ß2-receptors: Bronchodilation, vasodilation, relaxation of visceral smooth muscle, hepatic glycogenolysis and muscle tremor

ß3-receptors: Lipolysis

Note: (a) Norepinephrine is released from adrenergic sympathetic nerve endings and they act primarily on a-adrenergic receptors. (b) At low levels, epinephrine acts as a ß receptor agonist.

The copyright of the article Adrenoceptors in Medical Student Resources is owned by Yasser Anathallee. Permission to republish Adrenoceptors in print or online must be granted by the author in writing.

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