Temporal analysis of ecdysteroidogenic activity of the prothoracic glands during the fourth larval instar of the silkworm, Bombyx mori.

The cellular mechanism underlying ecdysteroidogenesis during the fourth larval instar of the silkworm, Bombyx mori, was analyzed by determining the in vitro ecdysteroid biosynthetic activity of the prothoracic glands, cAMP accumulation of the gland cells, the in vitro release of prothoracicotropic hormone (PTTH), etc. According to the differential responsiveness of prothoracic glands to PTTH, dibutyryl cAMP (dbcAMP), and 1-methyl-3-isobutylxanthine (MIX), the following different stages were classified and changes in PTTH signal transduction were assumed. During the first stage (between days 0 and 1), the glands showed low basal and PTTH-stimulated activities in both cAMP accumulation and ecdysteroidogenesis, and PTTH release in vitro was maintained at low but detectable levels, implying that a low but sustained PTTH signal may be transduced to prothoracic gland cells. On day 1.5, when low basal ecdysteroid production of the prothoracic glands was being maintained, both the responsiveness of glands to the stimulation of PTTH and PTTH release in vitro dramatically increased, indicating greatly increased PTTH transduction. On day 3 (when the basal ecdysteroidogenesis became maximal) and afterwards, high PTTH release in vitro was maintained, but the gland showed no response to PTTH, implying that the refractoriness of gland cells to PTTH may occur at this stage. We assume that the development-specific changes in PTTH signal transduction during the penultimate larval instar may play a critical role in regulating changes in ecdysteroidogenesis of the prothoracic glands.