Oxytocin plays a key role in parturition and lactation. Which of the following accurately describes the molecular mechanism by which oxytocin increases myometrial contractility during labour?

• A Oxytocin binds Gq-coupled receptors, activating phospholipase C, increasing intracellular calcium ✓
• B Oxytocin binds Gs-coupled receptors, increasing cAMP, causing myometrial relaxation
• C Oxytocin binds Gi-coupled receptors, decreasing cAMP, causing myometrial contraction
• D Oxytocin binds nuclear receptors, upregulating oxytocin receptor gene transcription

Explanation

Oxytocin acts via Gq-coupled oxytocin receptors on myometrial cells, activating phospholipase C, generating IP3 and DAG, which increases intracellular calcium and stimulates contraction. This mechanism underlies oxytocin's use as a uterotonic in labour induction. Options B and C describe wrong G-protein coupling. Option D describes steroid hormone signalling, not peptide hormone action.

Reference: Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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