[Permeability characteristics of the endocardium and the myocardial capillaries to horseradish peroxidase in fetal and postnatal rats].

After intravenous or intracardial application of horseradish peroxidase (HRP) at the myocardial capillaries of fetal, juvenile and adult rats a fast transport of the tracer into the interstite can be observed. In the fetal cases the transport takes place especially through the interendothelial junctions, in the postnatal cases a considerably amount of the peroxidase traversed also by plasmalemmal vesicles. In adults the T-tubules of the myocytes are labelled already after 15 min of circulation. From fetal stages up to adults the junctions of the endocardium are well permeable to HRP. After a short circulation time we found large deposits of reaction product in the subendocardial space. The reasons for the different permeability of fetal heart and skeletal muscle capillaries were discussed.