A kinetic analysis of the action of the insect prothoracicotropic hormone.

A modified in vitro assay was used to assess the kinetics of activation and the decay of activation of the prothoracic glands (PG) by the prothoracicotropic hormone (PTTH) in Manduca sexta. Time-courses of ecdysone synthesis by PTTH-activated day 3 larval and day 0 pupal PG were comparable both quantitatively and temporally, but dose-responses of PTTH activation revealed that larval glands were 1.8 times more sensitive to the neurohormone. The exposure time necessary for maximal activation of the PG by PTTH was the same for both glands, with half-maximal activation in approximately 0.5 min. Once PTTH was removed the rate of ecdysone synthesis by larval and pupal PG remained constant for about 2 h, after which the activated response for both glands decayed rapidly, reaching the unactivated basal synthesis rate within 45 min. These kinetics data suggest that PG activation by PTTH in vitro occurs in a manner indicative of activation in situ and, thus, that this in vitro system is suitable for probing the molecular mechanism by which PTTH activates the PG.