Learning Objective
At the end of this note, the learner should be able to identify the locations, mechanisms, and physiological effects of α, β, and dopamine adrenergic receptors, and predict clinical responses to drugs acting on each receptor subtype.
Alpha Receptor Activation
α₁ Receptors (Gq-coupled → ↑IP₃/DAG/Ca²⁺)
| Location |
Response |
| Eye – radial (dilator) muscle |
Contraction → mydriasis |
| Arterioles (skin, viscera) |
Contraction → ↑ TPR, ↑ diastolic BP, ↑ afterload |
| Veins |
Contraction → ↑ venous return, ↑ preload |
| Bladder trigone/sphincter & prostatic urethra |
Contraction → urinary retention |
| Male sex organs |
Vas deferens contraction → ejaculation |
| Liver |
↑ Glycogenolysis |
| Kidney |
↓ Renin release |
α₂ Receptors (Gi-coupled → ↓cAMP)
| Location |
Response |
| Prejunctional nerve terminals |
↓ NE release (negative feedback) |
| Platelets |
Aggregation |
| Pancreas |
↓ Insulin secretion |
β₁ Receptors (Gs → ↑cAMP)
| Location |
Response |
| SA node |
↑ HR (positive chronotropy) |
| AV node |
↑ Conduction velocity (positive dromotropy) |
| Atrial/ventricular muscle |
↑ Contractility & CO (positive inotropy) |
| His-Purkinje system |
↑ Automaticity & conduction |
| Kidney |
↑ Renin release |
β₂ Receptors (Gs → ↑cAMP) — mostly not innervated
| Location |
Response |
| Blood vessels (all) |
Vasodilation → ↓ TPR, ↓ diastolic pressure, ↓ afterload |
| Uterus |
Relaxation |
| Bronchioles |
Dilation |
| Skeletal muscle |
↑ Glycogenolysis; ↑ contractility (tremor) |
| Liver |
↑ Glycogenolysis |
| Pancreas |
↑ Insulin secretion |
β₃ Receptors (Gs → ↑cAMP)
| Location |
Response |
| Detrusor muscle |
Relaxation |
D1 Receptors (Gs → ↑cAMP) – Peripheral
| Location |
Response |
| Renal, mesenteric, and coronary vasculature |
Vasodilation → ↑ RBF, ↑ GFR, ↑ Na⁺ excretion |
Activity
Signaling Mechanisms
| Receptor |
Mechanism |
Effect |
| α₁ |
Gq |
↑ IP₃, DAG, Ca²⁺ |
| α₂ |
Gi |
↓ cAMP |
| β₁, β₂, β₃, D1 |
Gs |
↑ cAMP |
Important Notes
Adrenoceptor Sensitivity
- β receptors are more sensitive to agonists than α receptors.
- Low doses → β effects dominate, especially vasodilation.
- High doses → α effects dominate, especially vasoconstriction.
Dopamine in Shock
- Dopamine activates D1 → β1 → α1 in a dose-dependent sequence.
- Fenoldopam, a selective D1 agonist, is used for severe hypertension due to renal vasodilation.
Activity
