Receptors – Physiology · FCPS Part 1

Overview & Second Messengers

Receptor	Primary G-Protein	Second Messenger / Mechanism	Main Overall Function
alpha_1	G_q	Increases IP3 / DAG and intracellular Ca2+	Smooth muscle contraction (Vasoconstriction)
alpha_2	G_i	Decreases cAMP	Inhibitory (Presynaptic feedback & decreases secretions)
beta_1	G_s	Increases cAMP	Cardiac stimulation & Renin release
beta_2	G_s	Increases cAMP	Smooth muscle relaxation (Vasodilation & Bronchodilation)
beta_3	G_s	Increases cAMP	Lipolysis in adipose tissue

Primary Locations: Vascular smooth muscle (skin, splanchnic, renal), pupillary dilator muscle, intestinal and bladder sphincters.
Key Effects:
- Vasoconstriction: Increases systemic vascular resistance and blood pressure.
- Mydriasis: Contraction of radial muscle of iris (dilates pupil).
- Sphincter Contraction: Bladder internal sphincter and GI sphincters close.
- Glycogenolysis / Gluconeogenesis: Promotes hepatic glucose release.
Key MCQ Point: Cortisol upregulates alpha_1 receptors to maintain baseline vascular tone and blood pressure.

Primary Locations: Presynaptic sympathetic nerve terminals, pancreatic beta-cells, platelets.
Key Effects:
- Presynaptic Inhibition: Decreases norepinephrine release (negative feedback).
- Pancreas: Inhibits insulin secretion.
- Platelets: Promotes platelet aggregation.
Key MCQ Point: Stimulation of alpha_2 receptors lowers sympathetic output and reduces insulin release.

Primary Locations: Heart (SA node, AV node, myocardium) and Juxtaglomerular (JG) cells of the kidney.
Key Effects:
- Heart: Increases heart rate (positive chronotropy), increases conduction velocity (dromotropy), and increases contractility (inotropy).
- Kidney: Increases Renin secretion activates Renin-Angiotensin-Aldosterone System (RAAS).
Key MCQ Point: beta_1 is mainly cardiac-specific ("1 Heart, 2 Lungs").

Primary Locations: Smooth muscle of bronchioles, blood vessels supplying skeletal muscle, uterus, liver, and skeletal muscle cells.
Key Effects:
- Bronchodilation: Relaxes airway smooth muscle.
- Vasodilation: Dilates blood vessels in skeletal muscle.
- Uterine Relaxation: Relaxes uterine smooth muscle (tocolytic effect).
- Metabolism: Stimulates hepatic glycogenolysis and potassium uptake into skeletal muscle Na+/K+ ATPase pump activation).
Key MCQ Point: Epinephrine has a higher affinity for beta2 than Norepinephrine. Activated beta_2 drives potassium into cells, potentially causing hypokalemia.

Primary Locations: Adipose tissue and detrusor muscle of the bladder.
Key Effects:
- Lipolysis: Stimulates breakdown of triglycerides in brown and white fat.
- Bladder: Relaxes detrusor muscle to allow urine storage.