Jänkälä, Heidi, Veli-Pekka Harjola, Niels Erik Petersen, and Matti Härkönen. Myosin heavy chain mRNA transform to faster isoforms in immobilized skeletal muscle: a quantitative PCR study. J. Appl. Physiol. 82(3): 977–982, 1997.—A quantitative polymerase chain reaction (PCR) method was used to measure the quantities of type I, IIa, IIx, and IIb myosin heavy chain (MHC) mRNA in total RNA preparations of the soleus, gastrocnemius, and plantaris muscles of normal and hindlimb-immobilized rats. Type IIx and even type IIb MHC mRNA were demonstrated at extremely low levels in normal soleus, 2.1 ± 0.4 × 10⁵and 5.0 ± 0.2 × 10⁵molecules of mRNA per microgram total RNA, respectively. Immobilization for 1 wk significantly altered the gene expression of MHC isoforms. In soleus, both type IIx and IIb MHC genes became significantly upregulated, 24-fold (P < 0.005) and 2.6-fold (P < 0.05), respectively. In gastrocnemius, the level of type IIa MHC mRNA decreased by 51% (P < 0.01) and the level of type IIx MHC mRNA increased by 140% (P < 0.05). In plantaris, the level of type IIa MHC mRNA decreased by 58% (P < 0.005). In conclusion, immobilization changed the MHC mRNA profile in three different types of skeletal muscle toward faster isoforms. The quantitative results permit reliable evaluation of changes in mRNA levels.