Sex differences in limb vasoconstriction responses to increases in transmural pressures

Abstract

Women compared with men are more likely to have orthostatic intolerance. The purpose of this study was to examine whether sex affects limb vasoconstrictor response to increases in transmural pressure. Brachial and femoral mean blood velocity (MBV) and diameter (Doppler Ultrasound) were measured in 10 women and 10 men as transmural pressure was altered by applying local suction (Ϫ25, Ϫ50, Ϫ75, and Ϫ100 mmHg) via pressurized-limb tanks for 1 min to a single arm and leg. With the abrupt application of forearm suction (Ϫ75 and Ϫ100 mmHg), women compared with men had a greater initial rise in MBV (peak), followed by a quicker dynamic rate of velocity reduction. In the leg, women had a tendency for higher peak MBV but had similar dynamic velocity reductions compared with men. After 60 s of suction, women compared with men had attenuated reductions in brachial flow and conductance (Ϫ8.05 Ϯ 1.71 vs. Ϫ16.25 Ϯ 1.71 ml/min; Ϫ0.12 Ϯ 0.03 vs. Ϫ0.20 Ϯ 0.03 ml ⅐ min Ϫ1 ⅐ mmHg Ϫ1 ; main effect, P Ͻ 0.05), as well as attenuated femoral flow and conductance to sustained leg negative pressure at Ϫ100 mmHg (P Ͻ 0.05). When the data were expressed as percent change, women compared with men continued to have attenuated brachial flow responses (Ϫ24 Ϯ 2 vs. Ϫ36 Ϯ 2%, main effect, P Ͻ 0.05), with a trend toward attenuation at the highest leg pressure (Ϫ25 Ϯ 11 vs. Ϫ46 Ϯ 4%; P ϭ 0.08). These sex differences remained after normalizing the flow responses by limb volume (percent change). Our findings suggest that young women compared with men have attenuated brachial and femoral vasoconstrictor responses to increases in transmural pressure, which may have implications for the greater incidence of orthostatic intolerance in women.gender; myogenic response; autoregulation; sympathetic nervous system SYNCOPE, ESPECIALLY DURING postural stress, is more common in women then men (6, 22, 52). Most studies (4,7,16,19,21,28,46,55,61,63), but not all (26), have demonstrated that orthostatic tolerance is impaired in women compared with men. The suggested causes of the reduced orthostatic tolerance in women compared with men include altered central volume adjustments (20,21) and vasoconstriction responses (56). As human subjects rise from lying or sitting to the standing position, transmural pressure in the levels of the lower limbs rise. Postural stress evokes sympathetic engagement and autoregulatory constrictor responses to the rise in transmural pressure. In contrast to the effect of sex on neurally mediated cardiovascular responses to orthostatic stress, very few studies have examined the impact of sex on autoregulatory constrictor responses. A component of the autoregulatory adjustment is termed the myogenic response, which was first identified by Bayliss in 1902 (15, 38). This response can occur independent of sympathetic and endothelium influences (13, 50).In vitro animal studies have yielded mixed findings regarding the effects of sex on the myogenic response.Step increases in transmural pressure (i.e., 20 up to 140 mmHg) evoked attenuated myo...

Limb suction evoked during arterial occlusion causes systemic sympathetic activity in humans

“…The present data show that the limb suction in the absence of baroreceptor unloading evokes increases in sympathetic outflow. Thus, the present findings are different from the previous reports by Lott et al (19,20).…”

“…It was speculated that the increases in transmural pressure would evoke vasoconstriction, probably via both myogenic and sympathetic mechanisms. Because the suction on an unoccluded limb could induce fluid shift from central to the treated limb, the roles of the cardiopulmonary baroreceptors could not be excluded in those studies (19,20). The present data show that the limb suction in the absence of baroreceptor unloading evokes increases in sympathetic outflow.…”

“…Previous reports by Lott et al (19,20) showed that suction of an unoccluded upper or lower limb caused vasoconstriction in the treated limb. It was speculated that the increases in transmural pressure would evoke vasoconstriction, probably via both myogenic and sympathetic mechanisms.…”

“…All subjects were tested in the supine position in a temperature-controlled room (ϳ23°C). The subject's nondominant forearm and ipsilateral leg were inserted into the airtight pressure tanks (19,20). The lower aspect of the nondominant arm was sealed above the elbow, and the lower aspect of the ipsilateral leg was sealed above the knee with neoprene cuffs for sealing.…”

“…Thus, we developed the method in the present study. Previous studies in our group (19,20) employed the suction of an unoccluded upper or lower limb with airtight pressure tanks to raise the transmural pressure. To avoid fluid shift from central to the treated limb, in the present study, we employed airtight pressure tanks to apply suction on an arterially occluded limb (occlusion and suction, O&S).…”