Corticosteroid effects on diaphragm neuromuscular junctions

The effects of corticosteroid (CS) treatment (prednisolone continuously administered subcutaneously at a flow rate of 2.5 μ/h, daily dose 5.6 mg/kg, for 3 wk) on neuromuscular junction (NMJ) morphology and neuromuscular transmission in rat diaphragm muscle (Di(mus)) were compared with weight- matched (Sham) and ad libitum fed control (Ctl) groups. Fibers were classified on the basis of myosin heavy chain (MHC) isoform expression. CS treatment caused significant atrophy of fibers expressing MHC(2x) (type IIx), either alone or with MHC(2B) (type IIx/b). Fibers expressing MHC(slow) (type I) and MHC(2A) (type IIa) were unaffected by CS. The planar areas of nerve terminals and motor endplates at type IIx/b fibers were smaller in CS- treated D(imus) compared with Sham and Ctl. However, CS-induced atrophy of type IIx/b fibers exceeded changes in NMJ morphology. Thus, when normalized for fiber diameter, NMJs were relatively larger in the CS-treated group compared with Ctl. Neuromuscular 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 morphology after CS treatment are dependent on fiber type and may contribute to improved neuromuscular transmission.