Evidence that the human cutaneous venoarteriolar response is not mediated by adrenergic mechanisms

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Abstract
The venoarteriolar response causes vasoconstriction to skin and muscle via local mechanisms secondary to venous congestion. The purpose of this project was to investigate whether this response occurs through α‐adrenergic mechanisms. In supine individuals, forearm skin blood flow was monitored via laser‐Doppler flowmetry over sites following local administration of terazosin (α1‐antagonist), yohimbine (α2‐antagonist), phentolamine (non‐selective α‐antagonist) and bretylium tosylate (inhibits neurotransmission of adrenergic nerves) via intradermal microdialysis or intradermal injection. In addition, skin blood flow was monitored over an area of forearm skin that was locally anaesthetized via application of EMLA (2.5 % lidocaine (lignocaine) and 2.5 % prilocaine) cream. Skin blood flow was also monitored over adjacent sites that received the vehicle for the specified drug. Each trial was performed on a minimum of seven subjects and on separate days. The venoarteriolar response was engaged by lowering the subject's arm from heart level such that the sites of skin blood flow measurement were 34 ± 1 cm below the heart. The arm remained in this position for 2 min. Selective and non‐selective α‐adrenoceptor antagonism and presynaptic inhibition of adrenergic neurotransmission did not abolish the venoarteriolar response. However, local anaesthesia blocked the venoarteriolar response without altering α‐adrenergic mediated vasoconstriction. These data suggest that the venoarteriolar response does not occur through adrenergic mechanisms as previously reported. Rather, the venoarteriolar response may due to myogenic mechanisms associated with changes in vascular pressure or is mediated by a non‐adrenergic, but neurally mediated, local mechanism.