Effects of serum collected from rats of different ages on in vitro cell proliferation

It has been reported by Carrel and his co-workers that serum from old hens inhibits cell growth in culture. However, as we had previously demonstrated contradictory results using serum from old rabbits, we examined whether serum from old rats would also show strong induction of cell proliferation. Sera from young and adult rats of either sex strongly stimulated the growth of rat fetal skin fibroblasts and human fetal lung fibroblasts (TIG-1). Sera of old female and male rats (24-29 months old) produced much greater fluctuations in growth-stimulatory activity than sera from young animals. Most samples of serum from old rats stimulated the growth of TIG-1 cells, as did fetal bovine serum and samples from younger rats, even when a higher concentration of serum (up to 50%) was used. On the other hand, a small proportion of samples repressed the growth of the cells. A study on the effects of serial mixtures of both different types of serum samples from old rats on cell growth suggested that this minor proportion of serum samples contain a large amount of inhibitory factor(s). The cell growth-stimulatory activity of serum did not correlate with the total protein and albumin concentrations, albumin/globulin ratio, and the levels of lipid peroxide in the sample. These results therefore seemed to imply that serum induced a striking increase in the heterogeneity of cell growth stimulatory activity with age, although most samples of serum from old rats of either sex stimulated cell proliferation as effectively as samples from younger rats. The biological significance of the small proportion of serum samples from old rats which do inhibit cell proliferation was discussed.