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5 results on '"Smith, Erica G."'

1. Mechanical effects of muscle contraction do not blunt sympathetic vasoconstriction in humans

Author

Kirby, Brett S., Markwald, Rachel R., Smith, Erica G., and Dinenno, Frank A.

Subjects
Muscles -- Research, Nervous system, Sympathetic -- Research, Blood flow -- Research, Biological sciences
Abstract

Sympathetic vasoconstrictor responses are blunted in the vascular beds of contracting muscle (functional sympatholysis), but the mechanism(s) have been difficult to elucidate. We tested the hypothesis that the mechanical effects of muscle contraction blunt sympathetic vasoconstriction in human muscle. We measured forearm blood flow (Doppler ultrasound) and calculated the reductions in forearm vascular conductance (FVC) in response to reflex increases in sympathetic activity evoked via lower body negative pressure (LBNP). In protocol 1, eight young adults were studied under control resting conditions and during simulated muscle contractions using rhythmic forearm cuff inflations (20 inflations/min) with cuff pressures of 50 and 100 mmHg with the arm below heart level (BH), as well as 100 mmHg with the arm at heart level (HL). Forearm vasoconstrictor responses (%[DELTA]FVC) during LBNP were -26 [+ or -] 2% during control conditions and were not blunted by simulated contractions (range = -31 [+ or -] 3% to -43 [+ or -] 6%). In protocol 2, eight subjects were studied under control conditions and during rhythmic handgrip exercise (20 contractions/min) using workloads of 15% maximum voluntary contraction (MVC) at HL and BH (similar metabolic demand, greater mechanical muscle pump effect for the latter) and 5% MVC BH alone and in combination with superimposed forearm compressions of 100 mmHg (similar metabolic demand, greater mechanical component of contractions for the latter). The forearm vasoconstrictor responses during LBNP were blunted during 15% MVC exercise with the arm at HL (-1 [+ or -] 3%) and BH (-2 [+ or -] 3%) compared with control (-25 [+ or -] 3%; both P < 0.005) but were intact during both 5% MVC alone (-24 [+ or -] 4%) and with superimposed compressions (-23 [+ or -] 4%). We conclude that mechanical effects of contraction per se do not cause functional sympatholysis in the human forearm and that this phenomenon appears to be coupled with the metabolic demand of contracting skeletal muscle. blood flow; sympathetic nervous system

Published
2005

4. Ageing and leg postjunctional α-adrenergic vasoconstrictor responsiveness in healthy men

Author

Smith, Erica G, Voyles, Wyatt F, Kirby, Brett S, Markwald, Rachel R, and Dinenno, Frank A

Subjects
Adult, Male, Aging, Leg, Sympathetic Nervous System, Tyramine, Middle Aged, Femoral Artery, Norepinephrine, Phenylephrine, Receptors, Adrenergic, alpha-2, Reference Values, Vasoconstriction, Receptors, Adrenergic, alpha-1, Adrenergic alpha-2 Receptor Agonists, Humans, Vasoconstrictor Agents, Cellular, Adrenergic alpha-1 Receptor Agonists, Adrenergic alpha-Agonists, Blood Flow Velocity, Dexmedetomidine, Aged
Abstract

Muscle sympathetic vasoconstrictor nerve activity increases with advancing age, but does not result in elevated forearm vasoconstrictor tone because of a selective reduction in α1-adrenoceptor responsiveness. In contrast, the leg circulation of older adults is under greater tonic sympathetic vasoconstriction, but it is unclear whether α-adrenoceptor responsiveness is altered with age. In the present study, we tested the hypothesis that postjunctional α-adrenergic vasoconstrictor responsiveness is reduced in the leg circulation with age. We measured femoral blood flow (Doppler ultrasound) and calculated the femoral vascular conductance (FVC) responses to α-adrenoceptor stimulation during local blockade of β-adrenoceptors in 12 young (24 ± 1 year) and seven healthy older men (62 ± 2 year). Whole-leg vasoconstrictor responses to local intrafemoral artery infusions of tyramine (evokes noradrenaline (NA) release), phenylephrine (α1-agonist) and dexmedetomidine (α2-agonist) were assessed. Consistent with previous data, resting femoral blood flow and FVC were ∼30% lower in older compared with young men (P < 0.05). Maximal vasoconstrictor responses to tyramine (−30 ± 3 versus−41 ± 3%), phenylephrine (−25 ± 4 versus−45 ± 5%), and dexmedetomidine (−22 ± 4 versus−44 ± 3%) were all significantly lower in older compared with young men (all P < 0.05). Our results indicate that human ageing is associated with a reduction in leg postjunctional α-adrenoceptor responsiveness to endogenous NA release, and this reduction is evident for both α1- and α2-adrenoceptors. However, given that basal leg vascular conductance is reduced with age and is primarily mediated by sympathetic vasoconstriction, impaired α-adrenoceptor responsiveness does not negate the ability of the sympathetic nervous system to evoke greater tonic vasoconstriction in the leg vasculature of older men.

Published
2007

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