Baroreflex control of sympathetic vasomotor activity and resting arterial pressure at high altitude: insight from Lowlanders and Sherpa

Abstract

Exposure to high altitude (HA) is characterized by heightened muscle sympathetic neural activity (MSNA); however, the effect on arterial baroreflex control of MSNA is unknown. Furthermore, arterial baroreflex control at HA may be influenced by genotypic and phenotypic differences between lowland and highland natives. Fourteen Lowlanders (10 male) and 9 male Sherpa underwent haemodynamic and sympathetic neural assessment at low altitude (Lowlanders, LA; 344m, Sherpa, KT; 1400m) and following gradual ascent to 5050m. Beat-by-beat haemodynamics (photoplethysmography) and MSNA (microneurography) were recorded lying supine. Indices of vascular sympathetic baroreflex function were determined from the relationship of diastolic blood pressure (DBP) and corresponding MSNA at rest (i.e. DBP ‘operating pressure’ and MSNA ‘operating point’), and during a modified Oxford baroreflex test (i.e. ‘gain’). Operating pressure and gain were unchanged for Lowlanders during HA exposure; however, the operating point was reset upwards (48 ± 16 vs 22 ± 12 bursts·100HB-1, P=0.001). Compared to Lowlanders at 5050m, Sherpa had similar gain and operating pressure, but operating point was lower (30 ± 13 bursts·100HB-1P=0.02); MSNA burst frequency was lower for Sherpa (22 ± 11 versus 30 ± 9 bursts·min-1 P = 0.03). Breathing 100% oxygen did not alter vascular sympathetic baroreflex function for either group at HA. For Lowlanders, upward baroreflex resetting promotes heightened sympathetic vasoconstrictor activity and maintains blood pressure stability, at least during early HA exposure; mechanisms other than peripheral chemoreflex activation could be involved. Sherpa adaptation appears to favour lower sympathetic vasoconstrictor activity than Lowlanders for blood pressure homeostasis.