Respiratory muscle blood flows during physiological and chemical hyperpneain the rat

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 microsphere s in rats running at their maximal attainable treadmill speed (96 +/- 5 m/m in; range 71-116 m/min) and at rest while breathing either room air or 10% O-2-8% CO2 (balance N-2). All hindlimb and respiratory muscle blood flows m easured increased during exercise (P < 0.001), whereas increases in blood f low while breathing 10% O-2-8% CO2 were restricted to the diaphragm only. D uring exercise, muscle blood flow increased up to 18-fold above rest values , with the greatest mass specific flows tin ml min(-1) 100 g(-1) found in t he 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 min(-1) 100 g(-1)) than in the crural diaphragm (286 +/- 17 ml min(- 1) 100 g(-1)). During hypoxia+hypercapnia, blood flows in both the costal a nd crural diaphragms (550 +/- 70 and 423 +/- 53 mi min(-1) 100 g(-1), respe ctively) 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 + hyperc apnia-induced hyperpnea is necessary to elevate diaphragm blood flow to a l evel commensurate with its high oxidative capacity.