Immunoradiometric quantification of tissue plasminogen activator secreted by fetal organs. Comparison with urokinase

Explants of fetal tissues were maintained in organ cultures for 2-3 weeks and the conditioned culture media analysed for the two main plasminogen activators, tissue activator and urokinase. A new immunoradiometric method was used for determining the tissue activator. The method is based on 125I-labelled antibodies to a tissue plasminogen activator which was purified from the culture medium of an established melanoma cell line. It detected tissue activator in a concentration of 1 microgram/1 or even less. Urokinase was measured with a RIA. Explants of kidney as well as thyroid gland and thymus released very substantial amounts of urokinase and smaller amounts of tissue activator. Urokinase was also detected in media conditioned by skin, spleen and pancreas. Aorta explants released only the tissue activator. The capacity of many tissues to release urokinase indicates that this is a more significant plasminogen activator in extrarenal organs that hitherto realized. The tissue activator is probably confined to vascular structures.