Muscle sympathetic reactivity to apneic and exercise stress in high-altitude Sherpa

Abstract

Lowland-dwelling populations exhibit persistent sympathetic hyperactivity at altitude that may alter vascular function. High altitude populations, such as Sherpa, exhibit greater peripheral blood flow in response to acute stress, suggesting Sherpas may exhibit lower sympathetic activity and reactivity to stress than Lowlanders. Muscle sympathetic activity (MSNA; microneurography) including frequency (bursts/min), incidence (bursts/100HB), amplitude (% of max burst), was measured at rest in Lowlanders (n=14; age=27±6yrs) at 344m and following a 8- 9 days of graded ascent to 5050m. Sherpa (age=32±11yrs) were tested at 5050m (n=8). Neurovascular reactivity (i.e., change in MSNA patterns) was measured during maximal end expiratory apnea, isometric hand-grip (IHG; 30% maximal voluntary contraction for 2 minutes) and post exercise circulatory occlusion (PECO; 3 minutes). Total normalized SNA (au/min) was calculated over 10 cardiac cycles during baseline and pre-volitional apnea breakpoint. Lowlander burst frequency (11±5 bursts/min to 30±7 bursts/min; Mean±SD; p<0.001) and burst incidence (25±13 bursts/100HB to 53±15 bursts/100HB; p<0.001) increased at 5050m. In contrast, Sherpas had lower burst frequency (23±11 bursts/min; p<0.05) and incidence (30±13 bursts/100HB; p<0.05) at 5050m. MSNA increases in Lowlanders and Sherpa during apnea at 5050m were significantly lower than Lowlanders at 344m (both P<0.05), with a possible sympathetic ceiling reached in Lowlanders at 5050m. MSNA increased similarly during the IHG/PECO in Lowlanders at 334m and 5050m altitude and Sherpa at 5050m. Sherpa demonstrate overall lower sympathetic activity and reactivity during severe stress. This may be a result of improved systemic hemodynamic function associated with evolutionary adaptations to permanent residency at altitude.