- Alpha-1 (α1) Receptors
- G-Protein: Gq (activates phospholipase C → ↑IP3, DAG → ↑Ca²⁺)
- Location: Vascular smooth muscle, pupillary dilator muscle, prostate, internal urethral sphincter.
- Function:
- ↑ Vasoconstriction → ↑ Systemic vascular resistance → ↑ Blood pressure (esp. diastolic).
- Mydriasis (pupil dilation).
- Contraction of prostatic and bladder neck smooth muscle.
- Alpha-2 (α2) Receptors
- G-Protein: Gi (inhibits adenylyl cyclase → ↓cAMP)
- Location: Presynaptic adrenergic nerve terminals, pancreatic β-cells, platelets.
- Function:
- Inhibits norepinephrine release (negative feedback).
- ↓ Insulin secretion.
- ↑ Platelet aggregation.
- ↓ Aqueous humor production.
- Beta-1 (β1) Receptors
- G-Protein: Gs (activates adenylyl cyclase → ↑cAMP)
- Location: Heart (SA/AV nodes, ventricular myocardium), juxtaglomerular (JG) cells of the kidney.
- Mnemonic: “You have 1 heart.”
- Function:
- ↑ Heart rate (chronotropy).
- ↑ Contractility (inotropy).
- ↑ Conduction velocity (dromotropy).
- ↑ Renin release from JG cells.
- Beta-2 (β2) Receptors
- G-Protein: Gs (activates adenylyl cyclase → ↑cAMP)
- Location: Bronchial smooth muscle, vascular smooth muscle of skeletal muscle and heart, liver, uterus.
- Mnemonic: “You have 2 lungs.”
- Function:
- Bronchodilation.
- Vasodilation → ↓ Systemic vascular resistance → ↓ Blood pressure (esp. diastolic).
- ↑ Glycogenolysis & Gluconeogenesis in the liver.
- Uterine relaxation (tocolysis).
- ↑ Aqueous humor production.
- Beta-3 (β3) Receptors
- G-Protein: Gs (activates adenylyl cyclase → ↑cAMP)
- Location: Adipose tissue, detrusor muscle of the bladder.
- Function:
- ↑ Lipolysis.
- Relaxation of the detrusor muscle (promotes urine storage).