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1 Department of Anesthesiology, Mayo Clinic, Rochester, Minnesota, United States
2 Department of Anesthesiology, Mayo Clinic, Rochester, Minnesota, United States; Department of Cardiology, College of Medicine, Zhejiang University
* To whom correspondence should be addressed. E-mail: wilkins.brad{at}mayo.edu.
Exercise blunts sympathetic alpha-adrenergic vasoconstriction (functional sympatholysis). We hypothesized that sympatholysis would be augmented during hypoxic exercise compared to exercise alone. Fourteen subjects were monitored with ECG and pulse oximetry. Brachial artery and antecubital vein catheters were placed in the non-dominant (exercising) arm. Subjects breathed hypoxic gas to titrate arterial O2 saturation to 80% while remaining normocapnic via a re-breath system. Baseline and two 8-minute bouts of rhythmic forearm exercise (10 and 20% of maximum) were performed during normoxia and hypoxia. Forearm blood flow (FBF), blood pressure, heart rate, minute ventilation and end tidal CO2 were measured at rest and during exercise. Vasoconstrictor responsiveness was determined by responses to intra-arterial tyramine at rest and each exercise bout. Heart rate was higher during hypoxia (P<0.01) while blood pressure was similar (P=0.84). Forearm blood flow was higher during hypoxia compared to normoxia at rest (85±9 vs. 66±7ml/min), at 10% exercise (276±33 vs. 217±27ml/min) and at 20% exercise (464±32 vs. 386±28ml/min; P<0.01). Arterial epinephrine was higher during hypoxia (P<0.01), however v-a norepinephrine difference was similar between hypoxia and normoxia prior to (P=0.47). Vasoconstriction to tyramine (% decrease from pre-tyramine values) was blunted in a dose dependent manner with increasing exercise intensity (P<0.01). Interestingly, vasoconstrictor responsiveness tended to be greater (P=0.06) at rest (-37±6% vs. -33±6%), at 10% (-27±5% vs. -22±4%), and at 20% (-22±5% vs. -14±4%) exercise between hypoxia and normoxia respectively. Thus, sympatholysis is not augmented by moderate hypoxia nor does it contribute to the increased blood flow during hypoxic exercise.
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