Further characterization of growth hormone-dependent somatomedin-binding proteins in rat serum and demonstration of somatomedin-binding proteins produced by rat liver cells in culture.

The somatomedin-like peptide multiplication-stimulating activity (MSA) binds specifically to rat serum. The pattern of MSA binding is GH dependent. Specific binding of [125I]iodo-MSA in normal rat serum is primarily in the gamma-globulin region (peak II) on Sephadex G-200, while MSA binding in hypophysectomized (hypox) rat serum is near the albumin region (peak III).

Stimulation of DNA synthesis, cell multiplication, and ornithine decarboxylase in 3T3 cells by multiplication stimulating activity (MSA).

Multiplication stimulating activity (MSA) has been purified from the conditioned media of rat liver cells in culture by a modification of the procedure of Dulak and Temin. Purified MSA stimulates [3H] thymidine incorporation into DNA in subconfluent, serum starved 3T3 cells. Cell cycle analysis by the flow microfluorometer shows that the [3H] thymidine incorporation data reflects DNA synthesis.