Muscle LIM protein (MLP) is constitutively expressed in slow, but undetectable in fast, muscles of the rat. Here we show that MLP was upregulated at both the mRNA and protein levels under experimental conditions leading to transitions from fast to slower phenotypes. Chronic low-frequency stimulation and mechanical overloading by synergist removal both induced fast-to-slow shifts in myosin heavy chain (MHC) isoforms and expression of MLP in fast muscles. High amounts of MLP mRNA and protein were also present in fast muscles of the myotonic, hyperactive ADR mouse. Hypothyroidism evoked shifts in myosin composition toward slower isoforms and increased the MLP protein content of soleus (SOL) muscle but failed to induce MLP in fast muscles. Unweighting by hindlimb suspension elicited slow-to-fast transitions in MHC expression without altering MLP levels in SOL muscle. Hyperthyroidism shifted the MHC pattern toward faster isoforms but did not affect MLP content in SOL muscle. We conclude that alterations in MLP expression are associated with transitions from fast to slower phenotypes but not with slow-to-fast muscle fiber transitions.
