Exercise intensity-dependent contribution of beta-adrenergic receptor-mediated vasodilatation in hypoxic humans.
We previously reported that hypoxia-mediated reductions in alpha-adrenoceptor sensitivity do not explain the augmented vasodilatation during hypoxic exercise, suggesting an enhanced vasodilator signal. We hypothesized that beta-adrenoceptor activation contributes to augmented hypoxic exercise vasodilatation. Fourteen subjects (age: 29 +/- 2 years) breathed hypoxic gas to titrate arterial O(2) saturation (pulse oximetry) to 80%, while remaining normocapnic via a rebreath system. Brachial artery and antecubital vein catheters were placed in the exercising arm. Under normoxic and hypoxic conditions, baseline and incremental forearm exercise (10% and 20% of maximum) was performed during control (saline), alpha-adrenoceptor inhibition (phentolamine), and combined alpha- and beta-adrenoceptor inhibition (phentolomine/propranolol). Forearm blood flow (FBF), heart rate, blood pressure, minute ventilation, and end-tidal CO(2) were determined. Hypoxia increased heart rate (P < 0.05) and minute ventilation (P < 0.05) at rest and exercise under all drug infusions, whereas mean arterial pressure was unchanged. Arterial adrenaline (P < 0.05) and venous noradrenaline (P < 0.05) were higher with hypoxia during all drug infusions. The change (Delta) in FBF during 10% hypoxic exercise was greater with phentolamine (Delta306 +/- 43 ml min(-1)) vs. saline (Delta169 +/- 30 ml min(-1)) or combined phentolamine/propranolol (Delta213 +/- 25 ml min(-1); P < 0.05 for both). During 20% hypoxic exercise, DeltaFBF was greater with phentalomine (Delta466 +/- 57 ml min(-1); P < 0.05) vs. saline (Delta346 +/- 40 ml min(-1)) but was similar to combined phentolamine/propranolol (Delta450 +/- 43 ml min(-1)). Thus, in the absence of overlying vasoconstriction, the contribution of beta-adrenergic mechanisms to the augmented hypoxic vasodilatation is dependent on exercise intensity.
Wilkins
Brad W
BW
Department of Human Physiology, 122 Esslinger Hall, 1240 University of Oregon, Eugene, OR 97403-1240, USA. bwilkins@uoregon.edu
Pike
Tasha L
TL
Martin
Elizabeth A
EA
Curry
Timothy B
TB
Ceridon
Maile L
ML
Joyner
Michael J
MJ
D000328:Adult; D000959:Antihypertensive Agents; D001784:Blood Gas Analysis; D001794:Blood Pressure / Q000187:drug effects / Q000502:physiology; D004837:Epinephrine / Q000097:blood; D015444:Exercise / Q000502:physiology*; D005260:Female; D005542:Forearm / Q000098:blood supply; D006339:Heart Rate / Q000187:drug effects / Q000502:physiology; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D009638:Norepinephrine / Q000097:blood; D010646:Phentolamine / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D011943:Receptors, Adrenergic, beta / Q000502:physiology*; D012039:Regional Blood Flow / Q000187:drug effects / Q000502:physiology; D014664:Vasodilation / Q000187:drug effects / Q000502:physiology*; D014665:Vasodilator Agents / Q000494:pharmacology
D000959:Antihypertensive Agents; D011943:Receptors, Adrenergic, beta; D014665:Vasodilator Agents; D011433:Propranolol; D009638:Norepinephrine; D004837:Epinephrine; D010646:Phentolamine