Vascular adrenergic responsiveness is inversely related to tonic activity of sympathetic vasoconstrictor nerves in humans

Abstract

In humans, sympathetic nerve activity (SNA) at rest can vary several-fold among normotensive individuals with similar blood pressures. We recently showed that a balance exists between SNA and cardiac output, which may contribute to the maintenance of normal blood pressures over the range of resting SNA levels. In the present studies, we assessed whether variability in vascular adrenergic responsiveness has a role in this balance. We tested the hypothesis that forearm vascular responses to noradrenaline (NA) and tyramine (TYR) are related to SNA such that individuals with lower resting SNA have greater adrenergic responsiveness, and vice-versa. We measured multifibre muscle SNA (MSNA; microneurography), arterial pressure (brachial catheter) and forearm blood flow (plethysmography) in 19 healthy subjects at baseline and during intrabrachial infusions of NA and TYR. Resting MSNA ranged from 6 to 34 bursts min −1 , and was inversely related to vasoconstrictor responsiveness to both NA (r = 0.61, P = 0.01) and TYR (r = 0.52, P = 0.02), such that subjects with lower resting MSNA were more responsive to NA and TYR. We conclude that interindividual variability in vascular adrenergic responsiveness contributes to the balance of factors that maintain normal blood pressure in individuals with differing levels of sympathetic neural activity. Further understanding of this balance may have important implications for our understanding of the pathophysiology of hypertension.

Resting sympathetic activity is associated with the sympathetically mediated component of energy expenditure following a meal

“…2006). Contrary to our hypothesis, we found that young healthy individuals with high baseline MSNA and plasma norepinephrine have a larger sympathetically mediated component of energy expenditure following a meal ( β ‐REE, %REE) when compared with individuals with lower baseline sympathetic activity.…”

“…2006). This led us to hypothesize that high levels of basal MSNA would be associated with lower β ‐adrenergic receptor thermogenic responsiveness to a meal.…”

“…2006). Briefly, the peroneal nerve was localized with transcutaneous stimulation, looking for a motor response.…”

“…2006). Furthermore, individuals with high resting sympathetic nervous system activity (determined by plasma norepinephrine levels) have a smaller reduction in REE following adrenergic receptor blockade with propranolol (Bell et al.…”

Functional effects of cardiac sympathetic denervation in neurogenic orthostatic hypotension

“…[16][17][18][19][20] Sympathetic nerves take up tyramine via the cell membrane norepinephrine transporter. Tyramine in the axoplasm is then taken up into vesicles via the vesicular monoamine transporter, and in the vesicles tyramine displaces norepinephrine.…”