COSTAL DIAPHRAGM BLOOD-FLOW HETEROGENEITY AT REST AND DURING EXERCISE

To gain insight into diaphragm functional heterogeneity, blood flow (e xpressed as ml . min-1 . 100 g(-1))was measured using radiolabeled mic rospheres in the ventral, medial, and dorsal regions of the costal dia phragm and in the crural diaphragm of sedentary control (S) and exerci se trained (ET) female Wistar-Kyoto rats at rest and during treadmill exercise. ET animals had performed moderate intensity exercise trainin g on a motorized treadmill (22 m/min, 10% grade, 60 min/d) for 12 mont hs, while S were cage-confined. The efficacy of exercise training was demonstrated by a 12% increase (P < 0.05) in ventricular weight-to-bod y weight ratio and increases (P < 0.05) in citrate synthase activity i n hindlimb skeletal muscles of ET. At rest, blood flow in the ventral costal diaphragm (16 +/- 1) averaged similar to 61% of that in the med ial (26 +/- 3) and dorsal (25 +/- 2) costal regions (P = 0.035), and c rural diaphragm flow was 23 +/- 3. During treadmill exercise (5 min at 22 m/min, 10% incline), blood flow increased an average of 5-fold (P < 0.001) throughout the diaphragm, but the heterogeneous now pattern p ersisted; i.e., blood flow remained lower (P = 0.003) in the ventral r egion (77 +/- 7) than either the medial (135 +/- 15) or dorsal (127 +/ - 11) costal regions. Flow in the crural diaphragm during exercise was intermediate (105 +/- 9). Exercise training did not alter either the magnitude of blood flows or the flow distribution pattern within the d iaphragm. Citrate synthase activity was two-fold that of the plantaris muscle and was uniform across the ventral, medial, and dorsal costal and the crural diaphragm of a second group of age-matched rats (P = 0. 57). These data demonstrate that, although oxidative capacity is unifo rm throughout the diaphragm, there is a significant regional heterogen eity of blood flow within the rat diaphragm both at rest and during lo comotory exercise. The greater flow in the medial and dorsal regions o f the costal diaphragm suggests that these regions sustain a greater p ortion of the inspiratory work load at rest and during exercise compar ed to the ventral region.