Myoneural interactions affect diaphragm muscle adaptations to inactivity.

We hypothesized that inactivity effects on diaphragm muscle contractile and morphometric properties are attenuated if phrenic motoneurons are also inactive. Three models of rat diaphragm inactivity were compared: 1) spinal isolation; 2) tetrodotoxin (TTX) nerve blockade; and 3) denervation (Dnv). Motoneuron and muscle fiber inactivities were matched only in spinal isolated animals. After 2 wk, maximum tetanic force decreased in all three groups compared with control group but to a greater extent in TTX and Dnv animals. Fatigue resistance improved, and maximum unloaded shortening velocity slowed only in TTX and Dnv groups. Type IIa fiber proportions decreased in all three groups, and type IIx fiber proportions increased in TTX and Dnv animals. Type I fiber cross-sectional area increased in all three groups but to a greater extent in TTX and Dnv animals. Type IIa fibers hypertrophied, whereas type IIx and IIb fibers atrophied only in TTX and Dnv groups. These results support the hypothesis that muscle adaptations to prolonged inactivity are attenuated when muscle fiber and motoneuron inactivities are matched.