Evidence for nitric oxide-mediated sympathetic forearm vasodiolatation in humans

1. Our aim was to determine if sympathetic vasodilatation occurs in the human forearm, and if the vasodilating substance nitric oxide contributes to this dilatation. We also sought to determine if the nitric oxide might be released as a result of cholinergic stimulation of the vascular endothelium. 2. Blood flow was measured in the resting non-dominant forearm with venous occlusion plethysmography. To increase sympathetic traffic to the resting forearm, rhythmic handgrip exercise to fatigue followed by post-exercise ischaemia was performed by the dominant forearm. A brachial artery catheter in the non-dominant arm was used to selectively infuse drugs. 3. During control conditions, there was mild vasodilatation in the resting forearm during exercise followed by constriction during post-exercise ischaemia. When exercise was performed after brachial artery administration of bretylium (to block noradrenaline release) and phentolamine (an alpha-adrenergic antagonist), profound vasodilatation was seen in the resting forearm during both exercise and post-exercise ischaemia. 4. When the nitric oxide synthase blocker NG-monomethyl-L-arginine (L-NMMA) was administered in the presence of bretylium and phentolamine prior to another bout of handgripping, little or no vasodilatation was seen either during exercise or post-exercise ischaemia. Atropine also blunted the vasodilator responses to exercise and post-exercise ischaemia after bretylium and phentolamine. 5. These results support the existence of active sympathetic vasodilatation in the human forearm and the involvement of nitric oxide in this phenomenon. They also suggest nitric oxide might be released as a result of cholinergic stimulation of the vascular endothelium.