Autonomic control of bronchial blood flow and airway dimensions during strenuous exercise in sheep.
BACKGROUND
During exercise and recovery the transient and steady-state changes in autonomic activity regulating lower airway blood flow and dimensions are unknown. The aim of this study was to define changes in bronchial blood flow (Q(br)) and dimensions during moderate and strenuous exercise, and to analyse the role of vagal and sympathetic nerves.


METHODS
Nine ewes (34-44kg) underwent left thoracotomy during general anaesthesia (thiopentone/isoflurane) and either (5 sheep=Group 1) a pulsed Doppler transducer was placed on the bronchial artery, or (4 sheep=Group 2) a pulsed Doppler transducer was placed on the bronchial artery, and transit-time and single crystal sonomicrometers were mounted on the left main bronchus. These measured continuously Q(br), bronchial circumference (Circ(br)) and wall thickness (Th(br)). Aortic pressure (P(a)) and central venous pressure catheters were placed in the superficial cervical artery and vein. Trained sheep exercised on a horizontal treadmill, i.e. Group 1, moderate exercise 2.2mph over 1.6, 6min recovery, for analysis of changes in Q(br) before and after cholinoceptor blockade; Group 2, strenuous exercise 4.4mph over 2, 10min recovery for analysis of changes in Q(br) and airway dimensions, before and after cholinoceptor plus alpha(1)-, alpha(2)-adrenoceptor blockade. beta-adrenoceptor systems were intact.


RESULTS
In Group 1 during moderate exercise P(a) and heart rate (HR) rose. Q(br) and blood flow conductance (C(br)) fell immediately to 83% (P<0.001) before returning toward resting levels, but fell when exercise ceased to 89% (P<0.01) before recovering. Prior cholinoceptor blockade abolished the immediate fall in Q(br) and C(br), but not the recovery vasoconstriction. Later in recovery the bronchial bed dilated progressively over 6min (P<0.05). In Group 2 during strenuous exercise P(a) and HR rose substantially. Q(br) and C(br) fell to 68% and 54% (P<0.001), respectively, and there was early vasoconstriction in recovery. Circ(br) fell immediately and remained at 93% (P<0.01), and did not recover fully when exercise ceased. Th(br) did not change during or after exercise. Prior cholinoceptor plus alpha-adrenoceptor block caused P(a) and Q(br) to fall slightly during exercise, but the bronchovascular constriction during and after exercise was abolished, as was circumferential shortening in the airway.


CONCLUSIONS
At exercise onset and steady-state, resetting the arterial baroreflex upward in sheep increases parasympathetic cholinergic vasoconstrictor activity and causes bronchial wall and bronchovascular smooth muscle contraction in concert with sympathetic adrenergic constriction of systemic vascular beds. Whether the known sigmoid baroreflex control of tracheal smooth muscle tension at rest is extended to tracheobronchial smooth muscle and its circulation during exercise is yet to be determined.
Quail
A
A
Discipline of Human Physiology, School of Biomedical Sciences, Faculty of Health, University of Newcastle, Callaghan, 2308 NSW, Australia.
McIlveen
S
S
Bishop
R
R
McLeod
D
D
Gunther
R
R
Davis
J
J
Talken
L
L
Cottee
D
D
Parsons
G
G
White
S
S
D000109:Acetylcholine / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001341:Autonomic Nervous System / Q000502:physiology*; D001794:Blood Pressure / Q000187:drug effects; D001980:Bronchi / Q000033:anatomy & histology / Q000098:blood supply* / Q000294:innervation; D018678:Cholinergic Agents / Q000494:pharmacology; D005260:Female; D006339:Heart Rate / Q000187:drug effects; D017078:Laser-Doppler Flowmetry / Q000379:methods; D019832:N-Methylscopolamine / Q000494:pharmacology; D010276:Parasympatholytics / Q000494:pharmacology; D010646:Phentolamine / Q000494:pharmacology; D010805:Physical Conditioning, Animal / Q000379:methods / Q000502:physiology*; D011652:Pulmonary Circulation / Q000187:drug effects / Q000502:physiology*; D012039:Regional Blood Flow / Q000187:drug effects / Q000502:physiology; D012756:Sheep
D000317:Adrenergic alpha-Antagonists; D018678:Cholinergic Agents; D010276:Parasympatholytics; D000109:Acetylcholine; D019832:N-Methylscopolamine; D010646:Phentolamine