The role of the alpha-adrenergic receptor in the leg vasoconstrictor response to orthostatic stress

Aim: The prompt increase in peripheral vascular resistance, mediated by sympathetic α‐adrenergic stimulation, is believed to be the key event in blood pressure control during postural stress. However, despite the absence of central sympathetic control of the leg vasculature, postural leg vasoconstriction is preserved in spinal cord‐injured individuals (SCI). This study aimed at assessing the contribution of both central and local sympathetically induced α‐adrenergic leg vasoconstriction to head‐up tilt (HUT) by including healthy individuals and SCI, who lack central sympathetic baroreflex control over the leg vascular bed. Methods: In 10 controls and nine SCI the femoral artery was cannulated for drug infusion. Upper leg blood flow (LBF) was measured bilaterally using venous occlusion strain gauge plethysmography before and during 30° HUT throughout intra‐arterial infusion of saline or the non‐selective α‐adrenergic receptor antagonist phentolamine respectively. Additionally, in six controls the leg vascular response to the cold pressor test was assessed during continued infusion of phentolamine, in order to confirm complete α‐adrenergic blockade by phentolamine. Results: During infusion of phentolamine HUT still caused vasoconstriction in both groups: leg vascular resistance (mean arterial pressure/LBF) increased by 10 ± 2 AU (compared with 12 ± 2 AU during saline infusion), and 13 ± 3 AU (compared with 7 ± 3 AU during saline infusion) in controls and SCI respectively. Conclusion: Effective α‐adrenergic blockade did not reduce HUT‐induced vasoconstriction, regardless of intact baroreflex control of the leg vasculature. Apparently, redundant mechanisms compensate for the absence of sympathetic α‐adrenoceptor leg vasoconstriction in response to postural stress.