A method was established for measuring molecule numbers of three different myosin heavy chain (MHC) mRNA isoforms in total RNA preparations. The quantification was based on a combination of primer-directed reverse transcriptase and polymerase chain reactions with 5'-digoxigenin-labeled oligonucleotides, using external standards. The sensitivity of the method allowed the quantitation of mRNA amounts down to the range of 1,000 molecules (detection limit 50 molecules). The numbers determined for eight different rabbit muscles are in the range of 103–109/μg total RNA. In soleus muscle, the value of 1.11 × 109 MHCI mRNA molecules corresponds to approximately 8% of the total mRNA. With reference to myonuclei, this amount corresponds to 1–2 × 104 molecules/nucleus. A quantitative comparison of the two fast MHC mRNA isoforms with the distribution of different MHC isoforms at the protein level indicates that one of these two fast sequences is specific to MHCIIb and the other to MHCIId. However, our data point to the existence of additional MHCIId mRNA subtypes.
