Does glucocorticoid antagonize a transient anabolic factor?
A possible explanation for the transient (0 7 h) enhancement of acid-induced PD by DEX (Figure 5B) and the transient blunting of it by RU38486 (Figure 5C) is that glucocorticoid acts by antagonizing the effect of an anabolic factor which suppresses PD only in the first 7 h after fresh medium is added to the cultures. Serum factors known to be antagonized by glucocorticoid are insulin and the IGFs. IGF-I seemed a promising candidate (see Discussion) as it is gradually inactivated when added to muscle cell cultures by IGF-binding proteins (IGFBPs) secreted by the cells [9]. In L6-G8C5 cultures, large IGF-I supplements (10 100 nmol/l) abolished the acid-induced increase in PD (Figure 6). A more physiological IGF-I supplement (1 nmol/l) transiently prevented acid-induced PD during the first 7 h (Figure 7) but not in longer incubations (Figure 6). The re-emergence of acid-induced PD after 7 h with 1 nmol/l IGF-I was almost prevented (B in Figure 8) if fresh medium with 1 nmol/l IGF-I was ad
