Ecdysteroidogenic action of Bombyx prothoracicotropic hormone and bombyxin on the prothoracic glands of Rhodnius prolixus in vitro.

An in-vitro assay for ecdysteroid synthesis by the prothoracic glands (PGs) of fifth instar Rhodnius prolixus has been employed to evaluate the actions of prothoracicotropic neuropeptides from the silkmoth, Bombyx mori. Crude prothoracicotropic hormone (PTTH) extracts of recently emerged adult brain complexes of Bombyx induced a dose-dependent stimulation of ecdysteroid synthesis by Rhodnius PGs, which was similar to that obtained using crude Rhodnius PTTH. In both cases, maximum stimulation was obtained with one brain equivalent. Rhodnius PGs were then challenged with incremental doses of recombinant Bombyx PTTH and synthetic bombyxin-II. Dose-response curves for the action of both peptides on Rhodnius PGs were very similar to those obtained for their action on the pupal PGs of Bombyx in vitro. Bombyx PTTH stimulated the PGs of Rhodnius at concentrations comparable to those effective on Bombyx. The curve for Bombyx PTTH showed a steep ascending region from 3 to 8ng/ml and a sharp peak. For bombyxin, concentrations 40-fold higher were required to elicit the same amount of stimulation as obtained using Bombyx PTTH. Therefore, Rhodnius PGs possess recognition sites for both Bombyx PTTH and bombyxin. This is the first study of the ecdysteroidogenic properties of the Bombyx peptides on a heterologous species. It is suggested that the function and conformation of PTTH may be conserved between distantly related insect groups.