Identification and pattern of transitions of some developmental and adult isoforms of fast troponin T in some human and rat skeletal muscles.

Using a monoclonal antibody (F24) in an immunoblotting procedure, the composition of fast troponin T in several adult and developing skeletal muscles of rat and human was studied. With the exception of diaphragm, four isoforms of fast troponin T (HF1-HF4) were detected in all the adult human skeletal muscles investigated. Another isoform of fast troponin T undetectable in the adult human skeletal muscles, designated the fetal isoform (HFF1), was found to be present in all fetal skeletal muscles at 20 weeks of gestation except the diaphragm. Unlike isoform HF4 that was undetectable in all the fetal skeletal muscles, isoforms HF1-HF3 were present in all the human fetal skeletal muscles including the diaphragm. At least five isoforms of fast troponin T (AF1-AF5) could be detected in adult rat skeletal muscles. An additional isoform designated (D) appeared to be present in the rat diaphragm. In some muscles one of the isoforms, AF1, could be further resolved into two to three variants. The proportions and the level of expression of AF1-AF5 isoforms varied not only in different muscles but in some cases also in different parts of the same muscle. In addition to the adult isoforms, four other developmental isoforms termed fetal (FF1 and FF2) and neonatal (NF1 and NF2), were detected during the early development in the rat skeletal muscles. Their presence was first detected during the late fetal to early neonatal period and these isoforms were generally undetectable in a majority of the muscles after 1-2 months of age although their low level of expression persisted in a small number of muscles.