Corticosteroid effects on diaphragm neuromuscular junctions

The effects of corticosteroid (CS) treatment (prednisolone continuously adm inistered subcutaneously at a flow rate of 2.5 mu l/h, daily dose 5.6 mg/kg , for 3 wk) on neuromuscular junction (NMJ) morphology and neuromuscular tr ansmission in rat diaphragm muscle (Di(mus)) were compared with weight-matc hed (Sham) and ad libitum fed control(Ctl) groups. Fibers were classified o n the basis of myosin heavy chain (MHC) isoform expression. CS treatment ca used significant atrophy of fibers expressing MHC2X (type IIx), either alon e or with MHC2B (type IIx/b). Fibers expressing MHCslow (type I) and MHC2A (type IIa) were unaffected by CS. The planar areas of nerve terminals and m otor endplates at type IIx/b fibers were smaller in CS-treated Di(mus) comp ared with Sham and Ctl. However, CS-induced atrophy of type IIx/b fibers ex ceeded changes in NMJ morphology. Thus, when normalized for fiber diameter, NMJs were relatively larger in the CS-treated group compared with Ctl. Neu romuscular transmission failure, assessed in vitro by comparing force loss during repetitive (40 Hz) nerve vs. direct muscle stimulation, was less in CS-treated Di(mus). These results indicate that alterations in NMJ morpholo gy after CS treatment are dependent on fiber type and may contribute to imp roved neuromuscular transmission.