Insect moulting is regulated by the steroid hormone, 20-hydroxyecdysone (20-HE), the immediate precursor of which, ecdysone, is secreted by the prothoracic glands (PG) in Lepidopterous larvae. The PG are influenced by a trophic hormone, which originates in the neurosecretory cells in the insects' brain and is known as prothoracicotrophic hormone (PTTH). A third hormone, juvenile hormone (JH), directs which type of cuticle (larval, pupal or adult) is produced at each moult. Prothoracic glands of the tobacco hornworm, Manduca sexta (Sphingidae; Lepidoptera), were maintained in short term tissue culture, and secreted ecdysone measured by radioimmunoassay. The in vitro secretory rate was calculated for glands isolated from animals at daily intervals during the last larval instar, and compared with whole animal ecdysteroid titre and with total protein content of the gland cells. The effect of a number of humoral factors on ecdysone secretion was assessed. Whilst ecdysone itself had no effect, 20-HE was found to inhibit secretion when used at physiological levels, thus forming the basis of a possible feedback mechanism regulating PG activity. The responsiveness of PG to PTTH activity in extracts of Manduca larval brains was investigated at various times during the last larval instar. It was found that the PG would respond to these extracts only at times when PTTH is thought to be released in vivo. The effect of JH on PG from both normal and head-ligated larvae was investigated. No influence on secretory activity of PG could be demonstrated, even at JH levels well above the in vivo titre. The results are discussed in relation to control of in vivo PG activity, and control of whole animal titre of 20-HE.
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