Distinct myosin heavy chain isoform transitions in developing slow and fast cat hindlimb muscles

The expression of myosin heavy chain (MHC) isoforms leading to adult fiber phenotypes in the tibialis anterior (TA) and soleus muscles of the cat were investigated from embryonic day 35 to 1 year after birth. Electrophoresis and immunoblotting of myofibrils demonstrated the expression of 5 different MHC isoforms, i.e. I, IIa, IIx, embryonic, and neonatal, during developmen t. Based on electrophoresis, the adult-like MHC composition of the soleus a nd TA were not. observed until postnatal day 40 (P40) and 120(P120), respec tively. In contrast, immunohistochemical analyses revealed that the adult-l ike fiber phenotype composition was attained much later (P120) in the soleu s. The existence of multiple MHC isoforms in individual fibers suggested th at transitions occurred until P120 in both muscles. Adult type I fibers wer e first observed at P1. Adult IIA fibers were first observed at P30 in the TA and P40 in the soleus. IIX fibers were not identified until P40 in the T A. The transition to the predominantly slow phenotype of the soleus involve d a gradual loss Of embryonic and fast isoforms accompanied by an accumulat ion of slow MHC. In contrast, the expression of slow and fast MHC in the fa st TA muscle was relatively unchanged throughout development. These results show that the establishment of a given MHC-based fiber phenotype varies si gnificantly between slow and fast muscles in the kitten. Copyright (C) 2000 S. Karger AG, Basel.