PLM provides a primarily NO-dependent assessment of vascular function (16, 19, 20). Conditions associated with vascular dysfunction including aging, heart failure, and spinal cord injury exhibit reductions in PLM-induced vasodilation (21, 22). In the current study, acute elevations in MSNA directly impaired the vasodilatory response to PLM. Specifically, during metaboreceptor activation following HG exercise at 25% and 40% MVC (i.e., ExPECO 25% and ExPECO 40%) the change in LVC was reduced by 16% and 44%, respectively, when compared to CONTROL (Fig. 5 and Table 3). This reduction in the change in LVC coincided with significant increases in MSNA (Fig. 6). These findings support recent findings from our group of stepwise reductions in PLM-induced vasodilation during progressive exercise-mediated increases in MSNA (15) and extend these findings to local activation of this reflex loop by the metaboreceptors. The concomitant elevation in MSNA and reduction in the hyperemic response to PLM helps to elucidate factors, in addition to diminished NO bioavailability, that may contribute to previous reports of reduced PLM-induced vasodilation with aging and heart disease (22, 33, 34).
The mechanisms contributing to the reduced vasodilation during elevated sympathetic activation are not entirely clear. During PECO, MSNA increases systemically in response to localized metaboreceptor activation (35). This global increase in sympathetic outflow and stimulus for vasoconstriction may directly oppose the vasodilatory response to PLM by some general, yet unrecognized, mechanism. Conversely, elevated sympathetic outflow may directly inhibit NO. Hijmerring et al. (13) reported a reduction in FMD following acute sympathetic activation (via lower body negative pressure) and attributed the reduction in FMD to a specific inhibitory effect of sympathetic activation on shear-mediated NO release. In vitro evidence suggests that norepinephrine may lead to inactivation of NO (36) lending support to this potential mechanism, however, evidence of this occurring in vivo is lacking (37). A third potential mechanism may involve an imbalance between NO bioavailability and MSNA. Physiologically, NO and MSNA are antagonistic, as NO promotes vasodilation whereas increased MSNA evokes vasoconstriction. Therefore, increasing MSNA without an increase in NO (as would be expected given the peak hyperemic response was not altered by PECO (Table 3)), would shift the balance toward vasoconstriction and reduction in LVC during PLM. Both animal and human models demonstrate an interaction between NO and α-adrenergic function such that NO acts to attenuate α-adrenergic vasoconstriction (38, 39), however, how such a mechanism may manifest in humans during PLM is not certain. Based on the current findings we are unable to determine if the reduction in LVC during PLM is due to a general increase in sympathetically-mediated vasoconstriction, a direct impact on NO, or a combination of these aforementioned mechanisms.
Handgrip exercise without subsequent PECO and heightened MSNA yielded unexpected reductions in the vasodilatory response to PLM ( ExCON 25% and ExCON 40%, Fig. 7). At the cessation of HG exercise, MSNA and MAP returned to pre-HG levels indicating that sympathetic activation likely does not account for the observed reductions in LVC (Fig. 3). Alternatively, the reduced LVC during PLM may be explained by the increase in LBF during HG exercise, which may have resulted in activation of endothelial nitric oxide synthase (eNOS) and a subsequent increase in NO bioavailability (40). Increasing LBF prior to PLM with heating effectively reduced the PLM-induced hyperemic response (41). Recent human and animal work suggests that vascular responsiveness, as measured by vasodilation, is reduced with repeated stimulation due to potential 'resetting' of the endothelium (42) or alterations in tissue oxygenation (43). In keeping with this notion, activation of eNOS prior to PLM may have decreased NO bioavailability leading to the observed and marked reduction in LVC at the initiation of PLM. Importantly, this does not negate our finding that MSNA plays a significant role in the LVC response as the increase in LBF during and after HG was nearly identical between ExPECO 25% and ExPECO 40%, yet LVC was reduced during ExPECO 40% when compared to ExPECO 25%.
The impact of acute sympathoexcitation on vascular function, assessed by FMD, has been examined previously during lower body negative pressure, mental stress, cold pressor test, and metaboreceptor engagement (12–14, 44). The findings of these investigations are equivocal, as FMD has been reported to be unchanged, improved, or impaired due to increased sympathetic activity. Reductions in FMD ranging from 40 to 60% have been reported during sympathetic activation induced by lower body negative pressure and cold pressor test (13, 14). Conversely, metaboreceptor activation reportedly increased FMD by nearly 2-fold (12). The variability in the FMD response to acute sympathetic activation in these previous investigations is likely attributed to disparate techniques used to evoke sympathetic activation, as well as uncertainty regarding the magnitude of changes in MSNA. Moreover, uncertainty with regard to the mechanisms contributing to vasodilation during FMD may further confound interpretation of these findings (45, 46). Thus, while differences in methodology between these former studies and the current work preclude a direct comparison, the present findings provide new evidence for the capacity of the sympathetic nervous system to diminish lower limb hyperemic and vasodilatory responsiveness in young, healthy adults.