Serum from pneumonectomized rabbits stimulates alveolar type II cell proliferation in vitro.

Serum samples collected from pneumonectomized rabbits 9 and 21 days after surgery significantly enhanced the incorporation of [3H]thymidine into DNA by alveolar Type II cells in vitro, but they had no effect on lung or skin fibroblasts. Activity was absent from serums collected 140 days after surgery. Active serums showed a dose-response effect at concentrations between 1% and 10% in the alveolar Type II cell culture system and the activity was heat stable. The active factor eluted from columns of Sephadex G75 at the void volume at neutral pH, but after acid-ethanol extraction and chromatography under acidic conditions, it eluted with a Kd of approximately 0.65. We propose that the active substance is a somatomedin or somatomedin-like peptide.