### Date : 2024-10-31 13:29 ### Topic : Adrenergic receptors ---- Adrenergic receptors are a group of G-protein-coupled receptors (GPCRs) that are targets for the catecholamines *epinephrine (adrenaline)* and *norepinephrine (noradrenaline)*. They play a crucial role in the sympathetic nervous system, mediating various physiological responses like heart rate, vascular tone, and metabolic regulation. These receptors are broadly classified into **alpha (α)** and **beta (β)** receptors, each with further subtypes. ### 1. **Alpha (α) Receptors** Alpha receptors are primarily involved in *vasoconstriction* and *smooth muscle contraction*. They are further divided into α1 and α2 receptors. - **α1 Receptors** - **Location**: Blood vessels (arterioles and veins), bladder sphincter, radial muscle of the iris, and prostate. - **Function**: Causes smooth muscle *contraction* leading to: - *Vasoconstriction*: Increases peripheral resistance and raises blood pressure. - *Mydriasis*: Pupil dilation due to contraction of the radial muscle of the iris. - *Urinary retention*: Contraction of the bladder sphincter. - **Mechanism**: Works through the *Gq protein* pathway, which activates phospholipase C (PLC), leading to an increase in intracellular calcium and smooth muscle contraction. - **α2 Receptors** - **Location**: Presynaptic nerve terminals, pancreas, and some blood vessels. - **Function**: Inhibits norepinephrine release, causing: - *Decreased sympathetic outflow*: This reduces blood pressure. - *Inhibition of insulin release*: Reduces insulin secretion from pancreatic β-cells. - **Mechanism**: Works through the *Gi protein* pathway, inhibiting adenylate cyclase, reducing cyclic AMP (cAMP), leading to a decreased release of norepinephrine. ### 2. **Beta (β) Receptors** Beta receptors are mainly involved in *smooth muscle relaxation*, *cardiac stimulation*, and *metabolic effects*. They are divided into β1, β2, and β3 receptors. - **β1 Receptors** - **Location**: Heart (myocardium, SA node), kidneys. - **Function**: Enhances cardiac function by: - *Increased heart rate* (positive chronotropy). - *Increased force of contraction* (positive inotropy). - *Increased renin release* from the kidneys, leading to the activation of the renin-angiotensin-aldosterone system (RAAS), which increases blood pressure. - **Mechanism**: Works through the *Gs protein* pathway, which stimulates adenylate cyclase, increasing cAMP levels and enhancing cardiac contractility. - **β2 Receptors** - **Location**: Smooth muscle of the lungs (bronchi), blood vessels (skeletal muscle), uterus, liver. - **Function**: Causes smooth muscle *relaxation* leading to: - *Bronchodilation*: Useful for asthma management. - *Vasodilation*: Decreases peripheral resistance, especially in skeletal muscles. - *Relaxation of uterine smooth muscle*: Prevents preterm labor. - *Glycogenolysis and gluconeogenesis*: Increases blood glucose levels. - **Mechanism**: Works through the *Gs protein* pathway, increasing cAMP levels, leading to smooth muscle relaxation. - **β3 Receptors** - **Location**: Adipose tissue, bladder. - **Function**: Involved in: - *Lipolysis*: Breakdown of fat in adipose tissue. - *Bladder relaxation*: Reduces bladder overactivity. - **Mechanism**: Also works through the *Gs protein* pathway, increasing cAMP, which stimulates lipolysis in adipocytes. ### Summary Table | Receptor | Location | Function | Pathway | |----------|----------|----------|---------| | **α1** | Blood vessels, bladder | Vasoconstriction, pupil dilation | Gq (↑Ca²⁺) | | **α2** | Presynaptic terminals, pancreas | ↓ Norepinephrine release, ↓ insulin | Gi (↓cAMP) | | **β1** | Heart, kidneys | ↑ Heart rate, ↑ renin release | Gs (↑cAMP) | | **β2** | Lungs, skeletal muscle vessels, uterus | Bronchodilation, vasodilation | Gs (↑cAMP) | | **β3** | Adipose tissue, bladder | Lipolysis, bladder relaxation | Gs (↑cAMP) | ### Clinical Relevance - **Agonists** of α1 receptors are used to treat hypotension (e.g., *phenylephrine*). - **α1 Blockers** like *prazosin* are used for hypertension and benign prostatic hyperplasia (BPH). - **β1 Agonists** (e.g., *dobutamine*) are used in heart failure to increase cardiac output. - **β1 Blockers** (e.g., *metoprolol*) are used to manage hypertension and heart failure by reducing cardiac workload. - **β2 Agonists** (e.g., *albuterol*) are used for asthma as they promote bronchodilation. ### Reference: - ### Connected Documents: -