1. Acetylcholine relaxes human resistance vessels and releases nitric oxide and other factors from the endothelium. Comparison of responses to acetylcholine with those to an endothelium-independent vasodilator (such as nitroprusside) forms the basis of the so-called acetylcholine test of endothelial function. However, when this test is applied in vivo by intraarterial infusion, the metabolic instability of acetylcholine may result in differential responses to these drugs arising from anatomical rather than functional differences.

2. Vasodilator responses to brachial Artery infusions of acetylcholine (41 and 83 nmol/min) and sodium nitroprusside (11 and 38 nmol/min) were measured in 30 healthy human subjects using venous occlusion plethysmography.

3. Responses to acetylcholine showed a greater dependence on resting blood flow (P < 0.05) and on forearm length (P < 0.05) than those to sodium nitroprusside (results predicted by a simple blood flow model).

4. Correction for forearm length abolished an apparent difference of 59% between responses to acetylcholine in men and women. Conduit vessel geometry and resting blood flow influence the acetylcholine test of endothelial function.

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