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In vitro effect of a thymic epithelial culture supernate or thymosin fraction 5 on rabbit platelet aggregation and intracellular cyclic AMP levels

Supernates of thymic epithelial cell culture (STEC) strongly inhibit aggregation induced by addition of adenosine diphosphate (ADP: 1 μM) or thrombin (0.5 unit per ml) to washed platelet suspensions and accelerated the restoration from ADP‐triggered aggregation. At the same time, STEC increased the level of platelet adenosine 3′,5′‐cyclic monophosphate (cyclic AMP) in a dose‐dependent manner. Depending on the concentration used, thymosin fraction 5 increased the level of intracellular cyclic AMP ranging between 5 and 100 μg per ml, as well as inhibiting ADP‐induced platelet aggregation. The activities of both STEC and thymosin fraction 5 were found to act exclusively on cyclic AMP phosphodiesterase activity in platelets. In contrast the supernates from Chang, HeLa, or HCC‐M cells did not affect platelet aggregation induced by ADP, but slightly increased the cyclic AMP level (Chang, HeLa). Within 2 min after the treatment with STEC, more than 50% of the maximum inhibitory activity on platelet aggregation and increases in intracellular cyclic AMP were observed. These activities disappeared following STEC treatment with pronase E. STEC activity was found predominantly in the 1,000–50,000‐dalton fractions. These activities were not altered when STEC was treated by adenosine deaminase. The level of prostaglandin E (PGE) derivaties in STEC was about two times that found in the control culture medium. These data suggest that the biological activity of STEC in the platelets might be attributed to thymosinlike polypeptides and PGE1.