α-Adrenergic Regulation of Blood Pressure in Acclimatizing Lowlanders and Andean Highlanders at High Altitude

Elevated muscle sympathetic nerve activity (MSNA) at high altitude is associated with blunted transduction of sympathetic signals to blood pressure in lowlanders and indigenous Andean highlanders. However, it is unclear whether this is due to reduced adrenergic communication or other factors (e.g. augmented dilatory signaling). Therefore, we quantified the contribution of α-adrenoreceptor activity to 1) resting systemic sympathetic transduction and 2) pressor responses to sympathoexcitation in acclimatizing lowlanders (9M, 4F) and Andean highlanders (15M) at 4,300m. MSNA (microneurography) and mean arterial pressure (MAP; finger photoplethysmography) were measured at rest, during maximal voluntary apnea, and with an α1-adrenergic agonist (phenylephrine) prior to and following partial α-adrenergic blockade (phentolamine). Sympathetic transduction was quantified as the slope of the relationship between MAP and total MSNA associated with sequences of sympathetic bursts. Transduction was attenuated in both lowlanders (0.0041±0.0037 to 0.0017±0.0017mmHg∙%-1, P=0.026) and highlanders (0.0033±0.0024 to 0.0008±0.0007mmHg∙%-1, P=0.005) under α-adrenergic blockade (main effect P