The aortic tunica media of the developing rat. II. Incorporation by medial cells 3-H-proline into collagen and elastin: autoradiographic and chemical studies.

Four-week-old rats received intraperitoneal injections of 3-H-proline, and their thoracic aortas were removed at various times after injection. Autoradiography at the electron microscopic level was carried out on this tissue, together with isolation and chemical analysis of tritiated proline and hydroxyproline from procollagen, and elastin fractions. Autoradiograpic results were expressed in terms of tissue component volumes as calculated by morphometry. Autoradiographic grains were concentrated over medial cells 15 minutes after injection of label. By 30 minutes after injection some extracellular label was present as well; at 3 hours postinjection, large numbers of grains were found overlying cell surfaces, as well as over collagen and elastic fibers. Most grains were present over collagen and elastic tissue 10 hours after injection, although some intracellular labeling persisted. On a grain per unit volume basis, intracellular label steadily decreased over the time interval studied. Collagen showed maximal labeling at 3 hours, but elastic tissue labeling increased up to 10 hours postinjection. Chemical isolation of 3-H-proline and 3-H-hydroxyproline from the three fractions showed that incorporation of label into collagen and elastin is, on a time basis, consistent with autoradiographic labeling and that the labeling products are, in fact, these proteins, to the exclusion of nonspecific labeling.