Respiratory muscle reserve in rats during heavy exercise

The extent to which the respiratory pump muscles limit maximal aerobic capacity in quadrupeds is not entirely clear. To examine the effect of reduced respiratory muscle reserve on aerobic capacity, whole body peak oxygen consumption (V̇O₂) was measured in healthy Sprague-Dawley rats before and after Sham, unilateral, or bilateral hemidiaphragm denervation (Dnv) surgery. V̇O₂(peak) was determined by using a graded treadmill running test. Hemidiaphragm paralysis was verified apter testing by recording the absence of electromyographic activity during inspiration. Before surgery, V̇O₂ averaged 86, 87, and 92 ml · kg^-1 · min^-1 for the Sham, unilateral, and bilateral Dnv groups, respectively. Two weeks after surgery, there was no significant change in V̇O₂ for either the Sham or unilateral Dnv group. However, V̇O₂ decreased ~19% in the bilateral Dnv group 2 wk after surgery. These findings strongly suggest that the pulmonary system in rats is designed such that during heavy exercise, the remaining respiratory pump muscles are able to compensate for the loss of one hemidiaphragm, but not of both.