Adrenergic control of skeletal muscle blood flow during chronic hypoxia in healthy males

Sympathetic transduction is reduced following chronic high-altitude (HA) exposure; however, vascular a-adrenergic signaling, the primary mechanism mediating sympathetic vasoconstriction at sea level (SL), has not been examined at HA. In nine male lowlanders, we measured forearm blood flow (Doppler ultrasound) and calculated changes in vascular conductance (DFVC) during 1) incremental intra-arterial infusion of phenylephrine to assess a₁-adrenergic receptor responsiveness and 2) combined intra-arterial infusion of b-adrenergic and a-adrenergic antagonists propranolol and phentolamine (a-b-blockade) to assess adrenergic vascular restraint at rest and during exercise-induced sympathoexcitation (cycling; 60% peak power). Experiments were performed near SL (344 m) and after 3 wk at HA (4,383 m). HA abolished the vasoconstrictor response to low-dose phenylephrine (DFVC: SL: —34 ± 15%, vs. HA; þ 3 ± 18%; P < 0.0001) and markedly attenuated the response to medium (DFVC: SL: —45 ± 18% vs. HA: —28 ± 11%; P = 0.009) and high (DFVC: SL: —47 ± 20%, vs. HA: —35 ± 20%; P = 0.041) doses. Blockade of b-adrenergic receptors alone had no effect on resting FVC (P = 0.500) and combined a-b-blockade induced a similar vasodilatory response at SL and HA (P = 0.580). Forearm vasoconstriction during cycling was not different at SL and HA (P = 0.999). Interestingly, cycling-induced forearm vasoconstriction was attenuated by a-b-blockade at SL (DFVC: Control: —27 ± 128 vs. a-b-blockade: þ 19 ± 23%; P = 0.0004), but unaffected at HA (DFVC: Control: —20 ± 22 vs. a-b-blockade: —23 ± 11%; P = 0.999). Our results indicate that in healthy males, altitude acclimatization attenuates a₁-adrenergic receptor responsiveness; however, resting a-adrenergic restraint remains intact, due to concurrent resting sympathoexcitation. Furthermore, forearm vasoconstrictor responses to cycling are preserved, although the contribution of adrenergic receptors is diminished, indicating a reliance on alternative vasoconstrictor mechanisms.