Thin filament changes during in vivo rat heart development.

Developmental differences in myocardial performance are known to exist. It is likely that the profile of protein isoforms present on the developing thin filament contributes to these observed differences. We have prepared thin filaments from developing and mature rat hearts by using an immunoprecipitation procedure developed in our laboratory. Analysis of these isolated thin filaments by Western immunoblots and two-dimensional gel electrophoresis demonstrates troponin I and troponin T isoform switching on the developing thin filament. Troponin I isoform switching begins by embryonic d 18 and is complete before the 3rd postnatal wk. Troponin T isoform switching begins between embryonic d 18 and birth and is complete between the 2nd and 3rd postnatal wk. The degree of phosphorylation of tropomyosin in thin filaments appears to be developmentally regulated, decreasing with advancing age. Nonmuscle isoforms of tropomyosin are also detectable in thin filaments from developing and mature rat hearts. These phenomena (troponin isoform switching, the degree of phosphorylation of tropomyosin, and the presence of nonmuscle isoforms of tropomyosin on cardiac thin filaments) likely play a role in the function of immature thin filaments and in the assembly of mature thin filaments.