Western blot analysis of neuregulins in cell lysates and culture media. A shows that astrocytes produced two neuregulin forms in vitro: a 52 kDa membrane-spanning precursor in cell lysates (lane 2) and some soluble isoforms in culture media (lane 4). The small arrowheads in A show neuregulin immunopositive bands with apparent molecular sizes of 35, 32, 30, and 25 kDa, respectively. B, Forskolin and cAMP analogs enhanced the production of the 52 kDa neuregulin precursor by cultured astrocytes for 3 hr (lane 1, control; lane 2, 30 μm forskolin; lane 3, 1 mm 8Br-cAMP; lane 4, 1 mmdb-cAMP). The positions of molecular mass markers (lanes 1 and 3, in kilodaltons) are shown to theleft of each panel.