Functional, cellular, and biochemical adaptations to elastase-induced emphysema in hamster medial scalene

The scalene has been reported to be an accessory inspiratory muscle in the hamster. We hypothesize that with the chronic loads and/or dynamic hyperinf lation associated with emphysema (Emp), the scalene will be actively recrui ted, resulting in functional, cellular, and biochemical adaptations. Emp wa s induced in adult hamsters. Inspiratory electromyogram (EMG) activity was recorded from the medial scalene and costal diaphragm. Isometric contractil e and fatigue properties were evaluated in vitro. Muscle fibers were classi fied histochemically and immunohistochemically. Individual fiber cross-sect ional areas (CSA) and succinate dehydrogenase (SDH) activities were determi ned quantitatively. Myosin heavy chain (MHC) isoforms were identified by SD S-PAGE, and their proportions were determined by scanning densitometry. All Emp animals exhibited spontaneous scalene inspiratory EMG activity during quiet breathing, whereas the scalene muscles of controls (Ctl) were silent. There were no differences in contractile and fatigue properties of the sca lene between Ctl and Emp. In Emp, the relative amount of MHC2A was 15% high er whereas that of MHC2X was 14% lower compared with Ctl. Similarly, the pr oportion of type IIa fibers increased significantly in Emp animals with a c oncomitant decrease in IIx fibers. CSA of type IIx fibers were significantl y smaller in Emp compared with Ctl. SDH activities of all fiber types were significantly increased by 53 to 63% in Emp. We conclude that with Emp the actively recruited scalene exhibits primary-like inspiratory activity in th e hamster. Adaptations of the scalene with Emp likely relate both to increa sed loads and to factors intrinsic to muscle architecture and chest mechani cs.