Collagen is the predominant component of the extracellular matrix of the heart, where it is organized in a hierarchy of structures. To establish the cellular origin of the various collagen types, type I-procollagen α2 chain and types III and IV collagen mRNAs were examined in preparations of myocytes and non-myocyte heart cells freshly isolated from rats 1 to 6 months old. The cardiomyocytes appeared morphologically intact and functionally competent. Fibroblast-like cells predominated in the non-myocyte cell fractions but endothelial and smooth muscle cells were also present. RNA from whole ventricular tissue served as a control. Northern and dot blot analyses were used to establish the presence or absence of mRNAs.

In RNA prepared from whole ventricular tissue, the mRNAs for α-, β-, and γ-actin isotypes were detected whereas mRNA for α-actin was found in myocytes and those for β- and γ-actins were found in non-myocyte cells, confirming further the nature of the cell populations. Procollagen types I and III mRNAs were not detected in the total RNA of cardiomyocytes but mRNA for type IV collagen was present. The mRNAs for all three collagen types were present in the non-myocyte cells. These results suggest that in the rat heart the non-myocyte cells, probably fibroblasts, are responsible for interstitial collagen production. Both cell populations may engage in the formation of basement membrane collagen type IV.