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R2278 --> Introduction: Populations suffering from polycythemia such as polycythemia vera, Chuvash polycythemia, and chronic mountain sickness (CMS) suffer exercise fatigue and reduced exercise capacity. Potential mechanisms for reduced exercise capacity include the negative influences of high hematocrit, elevated blood viscosity, and increased sympathetic activity on convective and diffusive oxygen transport. However, it remains unclear how sympathetic activity impacts blood flow during isolated forearm exercise in Andeans suffering with polycythemia. Purpose: Therefore, using the isolated forearm handgrip model, the present study sought to examine the role of α‐adrenergic vasoconstrictor signaling in regulating skeletal muscle vasodilator function in seven Andeans with polycythemia (CMS+; n=7) and nine of their healthy townsmen (CMS‐; n=9) living at ~4350m. Methods: Forearm blood flow (FBF; Doppler ultrasound), mean arterial pressure (MAP, intra‐arterial catheter), and changes in forearm vascular conductance (∆FVC), calculated via FBF and MAP, were measured at rest and during graded handgrip exercise (HGE; 5, 15, 25% maximal voluntary contraction; MVC), before and after local +α‐β adrenergic blockade (phentolamine; propranolol). Blood samples were taken via arterial and venous catheter at rest and each stage of handgrip exercise to determine oxygen delivery, uptake, and extraction. All data are quantified as mean±SD. Results: During the control trial, progressive HGE increased FVC (CMS‐: P<0.01; CMS+: P<0.01), oxygen delivery (CMS‐: P<0.01; CMS+: P<0.01) and extraction (CMS‐: P<0.01; CMS+: P<0.01) similarly in both CMS‐ and CMS+ Andeans. However, after +α‐β adrenergic blockade, the FVC response to HGE at 25% was greater in CMS+ (∆FVC: control: 92.8±31.1 vs. +α‐β blockade: 137.5±66.7 ml·min‐1·100mmHg‐1; P<0.01) whereas the FVC response remained unchanged in CMS‐ (∆FVC: control: 109.0±32.8 vs. +α‐β blockade: 95.5±57.9 ml·min‐1·100mmHg‐1; P=0.15). In CMS+, increased FVC during +α‐β adrenergic blockade also increased oxygen delivery during 25% HGE (∆DO2: control: 21.9±5.9 vs. +α‐β blockade: 29.7±9.9 ml·min‐1; P<0.01); however, oxygen extraction remained unchanged (∆a‐VO2: control: 9.6±4.7 vs. +α‐β blockade: 8.3±4.1 ml·dl‐1; P=0.46). Conclusion: CMS+ individuals demonstrate heightened α‐adrenergic restraint of vasodilation during HGE. However, oxygen uptake in the forearm during exercise remains intact due to elevated arterial oxygen content from increased hemoglobin. Perhaps, a phenotypical trade‐off to polycythemia; whereby heightened vasoconstrictor signaling restrains blood flow to match the needed metabolic demand, or elevated whole blood viscosity elicits greater shear signaling requiring higher sympathetic restraint. [ABSTRACT FROM AUTHOR]