RAd-IGF-I increases release of IGF-I by astrocytes. Levels of IGF-I in the culture medium of astrocytes incubated for 24 h with medium alone (uninfected cells), RAd-TK/GFP or RAd-IGF-I. Total IGF-I was assayed in supernatants and the peptide concentration referred to the original volume of medium per well. Bars on columns represent mean ± SEM, (n = 4). ***, Significant difference (P < 0.001) versus the values of the other two experimental groups.