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Departments of Kinesiology and Anatomy and Physiology, Kansas State University, Manhattan, Kansas 66506-5602; and Department of Physiology, Kirksville College of Osteopathic Medicine, Kirksville, Missouri 63501
Whether the diaphragm retains a
vasodilator reserve at maximal exercise is controversial.
To address this issue, we measured respiratory and hindlimb muscle
blood flows and vascular conductances using radiolabeled microspheres
in rats running at their maximal attainable treadmill speed (96 ± 5 m/min; range 71-116 m/min) and at rest while breathing either room
air or 10% O2-8% CO2 (balance N2). All hindlimb and respiratory muscle blood flows
measured increased during exercise (P < 0.001), whereas
increases in blood flow while breathing 10% O2-8%
CO2 were restricted to the diaphragm only. During exercise,
muscle blood flow increased up to 18-fold above rest values, with the
greatest mass specific flows (in
ml · min
1 · 100 g1) found in the vastus intermedius (680 ± 44),
red vastus lateralis (536 ± 18), red gastrocnemius (565 ± 47), and
red tibialis anterior (602 ± 44). During exercise, blood flow was
higher (P < 0.05) in the costal diaphragm (395 ± 31 ml · min1 · 100 g1) than in the crural diaphragm (286 ± 17 ml · min1 · 100 g1). During hypoxia+hypercapnia, blood flows in both
the costal and crural diaphragms (550 ± 70 and 423 ± 53 ml · min1 · 100 g1, respectively) were elevated (P < 0.05)
above those found during maximal exercise. These data demonstrate that
there is a substantial functional vasodilator reserve in the rat
diaphragm at maximal exercise and that hypoxia + hypercapnia-induced
hyperpnea is necessary to elevate diaphragm blood flow to a level
commensurate with its high oxidative capacity.
costal diaphragm; crural diaphragm; vasodilator reserve; maximal exercise; hypoxia; hypercapnia
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