Insulin acts on the fibroblast to inhibit glucocorticoid stimulation of lung maturation.

The effects of insulin and cortisol on saturated phosphatidylcholine synthesis are examined in fetal type II cell cultures and in mixed cell cultures containing type II cells and fibroblasts. In 19-day fetal rat lung type II cell cultures, 100 nM cortisol and 2 nM insulin have no significant effect. Fibroblast-pneumonocyte factor results in enhanced saturated phosphatidylcholine synthesis by fetal type II cells. The significant stimulatory effect of cortisol in mixed-cell cultures is abolished in the presence of insulin or of monoclonal antibodies to fibroblast-pneumonocyte factor. Incubation of type II cells with conditioned media from fibroblasts exposed to cortisol results in increased saturated phosphatidylcholine synthesis. This process is not stimulated when type II cells are incubated with conditioned media from fibroblasts exposed to insulin and cortisol (or to insulin alone). These observations demonstrate that insulin inhibits cortisol induction of lung maturation and suggest that this antagonism results from an inhibitory effect of insulin on the elaboration of fibroblast-pneumonocyte factor by fetal lung fibroblasts.