Preserved α-Adrenergic Tone in the Leg Vascular Bed of Spinal Cord–Injured Individuals

Abstract

Background— Supraspinal sympathetic control of leg vascular tone is lost in spinal cord–injured individuals, but this does not result in a reduced leg vascular tone: Leg vascular resistance is even increased. The aim of this study was to assess the α-adrenergic contribution to the increased vascular tone in the lower extremity in patients without central sympathetic control of leg circulation. Methods and Results— Upper-leg vascular resistance responses to local infusion of incremental doses of phentolamine (a competitive antagonist of the α-adrenoceptor) into the femoral artery were determined in 10 spinal cord–injured individuals (SCI) and 8 healthy age-matched control subjects during local β-adrenergic receptor blockade with propranolol. Basal leg vascular resistance was higher in SCI than in control subjects (41±6 arbitrary units [AU] versus 24±4 AU; P =0.034). The same accounts for minimal leg vascular resistance, assessed during reactive hyperemia, which was higher in SCI compared with control subjects (6.9±1.0 AU versus 2.5±0.2 AU; P <0.01). The maximal phentolamine-induced reduction in leg vascular resistance normalized to each individual’s minimal resistance did not differ between the groups (68±17% and 51±4% for SCI and control subjects, respectively; P >0.1). A decline in mean arterial pressure was observed in both groups with increasing dosage of phentolamine. In response, baroreceptor-mediated vasoconstriction was observed in the noninfused leg of the control subjects, whereas in SCI individuals this reaction was absent. Conclusions— These results indicate that the α-adrenoceptor–mediated vascular tone in the leg is preserved in spinal cord–injured individuals without sympathetic supraspinal control.