Impaired modulation of postjunctional α1 - but not α2 -adrenergic vasoconstriction in contracting forearm muscle of postmenopausal women.

Abstract

Contraction-mediated blunting of postjunctional α-adrenergic vasoconstriction (functional sympatholysis) is attenuated in skeletal muscle of ageing males, brought on by altered postjunctional α1 - and α2 -adrenergic receptor sensitivity. The extent to which postjunctional α-adrenergic vasoconstriction occurs in the forearms at rest and during exercise in postmenopausal women remains unknown. The novel findings indicate that contraction-mediated blunting of α1 - (via intra-arterial infusion of phenylephrine) but not α2 -adrenergic (via intra-arterial infusion of dexmedetomidine) vasoconstriction was attenuated in postmenopausal women compared to young women. Additional important findings revealed that postjunctional α-adrenergic vasoconstrictor responsiveness at rest does not appear to be affected by age in women. Collectively, these results contribute to our understanding of local neurovascular control at rest and during exercise with age in women. Contraction-mediated blunting of postjunctional α-adrenergic vasoconstriction (functional sympatholysis) is attenuated in older males; however, direct confirmation of this effect remains unknown in postmenopausal women (PMW). The present study examined whether PMW exhibit augmented postjunctional α-adrenergic receptor vasoconstriction at rest and during forearm exercise compared to young women (YW). Eight YW (24±1years) and eight PMW (65±1years) completed a series of randomized experimental trials: (1) at rest, (2) under high flow (adenosine infusion) conditions and (3) during 6min of forearm exercise at relative (20% of maximum) and absolute (7kg) intensities. Phenylephrine (α1 -agonist) or dexmedetomidine (α2 -agonist) was administered during the last 3min of each trial to elicit α-adrenergic vasoconstriction. Forearm vascular conductance (FVC) was calculated from blood flow and blood pressure. Vasoconstrictor responsiveness was identified as the change in FVC (%) during α-adrenergic agonist infusions from baseline (resting trial) or from steady-state conditions (high flow and exercise trials). During resting and high flow trials, the %FVC during α1 - and α2 -agonist stimulation was similar between YW and PMW. During exercise, α1 -mediated vasoconstriction was blunted in YW vs. PMW at relative (-6±2% vs. -15±3%) and absolute (-4±2% vs. -14±5%) workloads, such that blood flow and FVC were lower in PMW (P<0.05 for all). Conversely, α2 -mediated vasoconstriction was similar between YW and PMW at relative (-22±3% vs. -22±4%; P>0.05) and absolute (-19±3% vs. -18±4%; P>0.05) workloads. Collectively, these findings demonstrate that despite similar α-adrenergic vasoconstrictor responsiveness at rest, PMW have a decreased ability to attenuate α1 -adrenergic vasoconstriction in contracting skeletal muscle.