Biological activity and receptor-binding of ecdysteroids in imaginal discs of Calliphora vicina: a comparison
Terentiou, P. | Blattmann, M. | Bradbrook, D. | Kauser, G. | Koolman, J.
The biological potency of an ecdysteroid is assumed to indicate its receptor-binding affinity, and vice versa, although it is not yet appreciated how closely these two parameters are correlated. To investigate this, we have developed two assays using imaginal discs of 3rd instar larvae of the blowfly Calliphora vicina: one assay measures the biological potency of an ecdysteroid in terms of its ability to induce evagination of isolated imaginal discs; the other assay enables us to measure the receptor-binding affinity of ecdysteroids in individual imaginal discs. The ED50 for 20-hydroxyecdysone to elicit imaginal disc evagination is 80 nM, and the estimated Kd of its receptor complex in imaginal discs is 37 nM. The Kd of ecdysteroids for their receptor is either of the same order (ponasterone A, muristerone A) as, or lower (20-hydroxyecdysone, makisterone A, inokosterone) than the concentration to elicit a biological response. The biological response appears to be a switch turned on by the hormone, whereas receptor-binding follows Michaelis-Menten kinetics. Comparison with dose-response data for polytene chromosome puffing indicates that the switching is a post-receptor event, occurring between transcription of the early and late genes.
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