A hormonally controlled serum factor stimulating the thymidine uptake into lectin-activated lymphocytes.

The in vitro stimulation of [3H]thymidine uptake into lectin-activated lymphocytes in the presence of various sera was studied. The mean precision of the assay is 5%, and the study of the confidence intervals shows variations from 4% to 12%. Compared to a normal reference serum (fixed as 1 U/ml), the serum thymidine uptake stimulating activity (mean +/- SEM) was 1.04 +/- 0.07 U/ml in normal adult males, 2.63 +/- 0.48 U/ml in acromegalic patients, 1.51 +/- 0.13 U/ml in constitutional dwarfism and 0.37 +/- 0.04 U/ml in untreated hypopituitary dwarfism with a significant difference between the groups (P less than 0.001). In patients with hypopituitarism a single im hGH dose (6 mg/m2 increased the thymidine uptake stimulating activity of serum within 24 to 48 h following injection. The effects of directly adding hGH, insulin and T3 to the assay, have been studied: pharmacological concentrations are required to produce only a slight effect. Physiological concentration of a purified preparation of somatomedin A stimulated thymidine uptake and its effect is increased in the presence of serum. These data demonstrate that [3H]thymidine uptake into lectin-activated lymphocytes is stimulated by a GH-dependent serum factor. The data suggest that this method should be proposed for an accurate and sensitive biological evaluation of serum thymidine uptake stimulating activity.