Identification and pattern of expression of a developmental isoform of troponin I in chicken and rat cardiac muscle.

A monoclonal antibody that reacts with all known isoforms of troponin I detected a single isoform of cardiac troponin I in both atrial and ventricular chambers of adult chicken and rat hearts in an immunoblotting analysis. Another isoform of troponin I in addition to the adult cardiac form, however, was present in all chambers of the heart during early development in both species. This developmental isoform appeared to have the same electrophoretic mobility on SDS tris glycine polyacrylamide gels as that observed for the adult slow skeletal muscle isoform. In the rat, only the developmental isoform of troponin I was present in the early foetal heart and small amounts of the adult cardiac isoform were not apparent until late in gestation, whereas the developmental and adult isoforms were expressed in approximately equal amounts throughout embryonic development in the chicken. The level of developmental isoform of troponin I in both the chicken and the rat hearts gradually decreased so that only small amounts of this variant were detectable two weeks after birth or post hatching.