Discrepancy between biological activity of thymosin and its cyclic nucleotide-modulating activity.

We have studied cyclic adenosine 3':5'-monophosphate (cAMP) and cyclic 3':5'-guanosine monophosphate (cGMP) concentration in cord blood lymphocytes (CBL) and in adult peripheral blood lymphocytes (PBL) after stimulation with thymosin V-1 and V-2. At concentrations of 100 micrograms/ml, thymosin V-1 produced an 85% increase of cAMP in adult PBL, alterations being demonstrated within 1 min, whereas it did not affect the cAMP level in CBL which have a nine times higher basal level than PBL. On the other hand, thymosin V-1 did not have any effect on cGMP in CBL nor in adult PBL. Thymosin V-2 which inhibits E-rosette formation also increased cAMP in adult PBL like thymosin V-1. The increasing effect of thymosin V-1 on cAMP was observed even when cAMP phosphodiesterase was completely inhibited with isobutyl methylxanthine. From these data it was assumed that thymosin probably acts through the stimulation of adenylate cyclase.