Inducible isoform of HSP70 is constitutively expressed in a muscle fiber type specific pattern

The most prominent group of stress or heat-shock proteins (HSPs) has an Mr of approximately 70,000 and is collectively referred to as the HSP70 family. The extent of stress inducibility and subcellular location of the various HSP70 isoforms differ, but all appear to be involved with ATP-dependent stabilization or solubilization of proteins. One isoform, termed the inducible isoform of HSP70 (HSP72i), is normally absent in unstressed cells. In a previous study, we detected a protein corresponding in Mr and pI to HSP72i in unstressed rat muscle. Therefore, it was of interest to determine if this expression in unstressed muscle cells is general or confined to specific muscle fiber types. To answer this question we have employed various rat hindlimb muscles that differ in fiber type proportion from predominantly type I (soleus) to predominantly type IIB (white gastrocnemius). Proteins from muscle homogenates were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, blotted to a nylon membrane, probed with a monoclonal antibody for HSP72i, and visualized using an alkaline phosphatase-conjugated secondary antibody. Immunoblot analyses demonstrate the constitutive expression of HSP72i in rat muscles comprised primarily of type I muscle fibers (soleus), but not in muscles comprised primarily of type IIB fibers (white gastrocnemius). In muscles of mixed fiber type, HSP72i content is roughly proportional to the percentage of type I fibers. These results substantiate that unstressed rat muscles express the inducible HSP72 isoform and demonstrate that its constitutive expression is proportional to the type I muscle fiber composition.