Six weeks after a surgical postganglionic sympathetic denervation of the hand, the blood vessels supplying the denervated skin show an exaggerated constrictor response to a test dose of intravenous norepinephrine compared with the normal contralateral side. The most likely explanation is:

• A Loss of neuronal norepinephrine reuptake at the junction
• B Proliferation of collateral parasympathetic fibres
• C Upregulation of postsynaptic adrenergic receptors on the effector cells ✓
• D Increased activity of vascular nitric oxide synthase

Explanation

Denervation supersensitivity follows loss of postganglionic innervation: the transmitter-deprived effector cells upregulate their receptors, so exogenous agonist produces a greatly exaggerated response. Loss of reuptake (option A) potentiates released transmitter but does not explain heightened sensitivity to injected norepinephrine. Parasympathetic collaterals do not grow into sympathetic targets, and increased NO would reduce, not increase, constriction.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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