A 52-year-old man from Delhi presents to the emergency department with sudden onset severe headache, profuse sweating, and tremor for the past 2 hours. His blood pressure is 210/130 mmHg, heart rate 118/min, and he appears anxious. On examination, he has dilated pupils and piloerection. His wife mentions he stopped taking his antihypertensive medication 3 days ago. Urine metanephrine levels are elevated. Which autonomic receptor is primarily responsible for the piloerection and pupillary dilation observed in this patient?

Explanation

## Autonomic Receptor Physiology in Catecholamine Excess ### Clinical Context This patient presents with a hypertensive crisis secondary to pheochromocytoma (elevated metanephrines, classic triad of headache-sweating-palpitations). The catecholamine surge activates multiple adrenergic receptors. ### Alpha-1 Adrenergic Receptor Actions **Key Point:** Alpha-1 receptors are G~q~-coupled and mediate: 1. **Piloerection** — contraction of arrector pili muscles in skin via sympathetic innervation 2. **Pupillary dilation (mydriasis)** — contraction of radial muscle of iris 3. Vasoconstriction → hypertension 4. Increased cardiac contractility ### Why Alpha-1 Is the Answer **High-Yield:** Both piloerection and mydriasis are **classic alpha-1 mediated responses**. In catecholamine excess (pheochromocytoma, sympathomimetic overdose), alpha-1 activation dominates because: - Epinephrine and norepinephrine have high affinity for α1 receptors - These are the most visible autonomic signs in acute sympathetic surge ### Receptor Distribution Table | Receptor | Tissue | Effect | Mechanism | | --- | --- | --- | --- | | **α1** | Arrector pili | Piloerection | G~q~ → IP~3~/DAG → muscle contraction | | **α1** | Iris radial muscle | Mydriasis | Radial muscle contraction | | **β2** | Bronchial smooth muscle | Bronchodilation | G~s~ → ↑cAMP → relaxation | | **M3** | Ciliary muscle | Miosis + accommodation | G~q~ → contraction (parasympathetic) | | **Nicotinic** | NMJ | Skeletal muscle contraction | Ligand-gated ion channel | **Clinical Pearl:** Piloerection is so characteristic of sympathetic activation that it's colloquially called "goosebumps" — a vestigial reflex from our evolutionary past. In modern clinical practice, it's a reliable sign of catecholamine excess. ### Mnemonic: ALPHA-1 EFFECTS **A**rteries (vasoconstriction) | **L**idated pupils (mydriasis) | **P**iloerection | **H**eart (↑contractility) | **A**drenaline dominates [cite:Guyton & Hall Ch 60]