We measured brachial and femoral artery flow velocity in eight subjects and
peroneal and median muscle sympathetic nerve activity (MSNA) in five subje
cts during tilt testing to 40 degrees. Tilt caused similar increases in MSN
A in the peroneal and median nerves. Tilt caused a fall in femoral artery f
low velocity, whereas no changes in flow velocity were seen in the brachial
artery. Moreover, with tilt, the increase in the vascular resistance emplo
yed (blood pressure/flow velocity) was greater and more sustained in the le
g than in the arm. The ratio of the percent increase in vascular resistance
in leg to arm was 2.5:1. We suggest that the greater vascular resistance e
ffects in the leg were due to an interaction between sympathetic nerve acti
vity and the myogenic response.