A 28-year-old woman presents with palpitations, tremor, and anxiety for 3 days. She reports recent upper respiratory infection treated with a nasal decongestant containing pseudoephedrine. On examination: HR 110 bpm, BP 145/92 mmHg, fine tremor of outstretched hands. Serum electrolytes and thyroid function are normal. ECG shows sinus tachycardia. She is counselled to discontinue the decongestant. Within 24 hours, symptoms resolve. Which second messenger system was primarily overstimulated in this patient?

Explanation

## Clinical Context Pseudoephedrine is a sympathomimetic amine that acts as an agonist at **α1- and β-adrenergic receptors**. The patient's acute presentation of tachycardia, tremor, and hypertension reflects excessive β-adrenergic stimulation. ## Second Messenger Pathway Involved **Key Point:** β-Adrenergic receptors are coupled to **Gs proteins**, which activate adenylyl cyclase, leading to increased **cAMP** production. ### cAMP Mechanism in Cardiac & Skeletal Muscle 1. **β-Adrenergic agonist** (pseudoephedrine) binds receptor 2. **Gs protein** activated → adenylyl cyclase stimulation 3. **cAMP↑** → PKA (protein kinase A) activation 4. **Cardiac effects:** increased heart rate, contractility, conduction velocity 5. **Skeletal muscle:** increased tremor via enhanced motor neuron firing and muscle sensitivity 6. **Vascular:** β2-mediated vasodilation (but α1 vasoconstriction dominates → net hypertension) **High-Yield:** cAMP is the primary second messenger for catecholamine-mediated sympathetic responses. Resolution of symptoms upon drug discontinuation confirms the reversible nature of this pathway. ## Why Other Pathways Are Not Involved | Pathway | Coupling | Role | Why Not Here | |---------|----------|------|---------------| | **IP3/DAG** | Gq protein | Phospholipase C activation; Ca²⁺ mobilization | α1-adrenergic receptors do contribute to hypertension, but tremor and tachycardia are primarily β-mediated | | **cGMP** | Guanylate cyclase (NO, natriuretic peptides) | Vasodilation, smooth muscle relaxation | Opposite effect; would reduce BP and HR | | **Ryanodine receptor** | Direct SR calcium release | Skeletal muscle contraction | Not a second messenger pathway; not the primary mechanism here | **Clinical Pearl:** Pseudoephedrine's sympathomimetic effects are **reversible and dose-dependent**. The rapid resolution upon discontinuation confirms the cAMP pathway was the culprit, not a structural cardiac or metabolic abnormality.