BIOLOGICAL-ACTIVITY AND RECEPTOR-BINDING OF ECDYSTEROIDS IN IMAGINAL DISKS OF CALLIPHORA-VICINA - A COMPARISON

The biological potency of an ecdysteroid is assumed to indicate its re ceptor-binding affinity, and vice versa, although it is not yet apprec iated 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 biolog ical potency of an ecdysteroid in terms of its ability to induce evagi nation of isolated imaginal discs; the other assay enables us to measu re the receptor-binding affinity of ecdysteroids in individual imagina l discs. The ED50 for 20-hydroxyecdysone to elicit imaginal disc evagi nation is 80 nM, and the estimated K(d) of its receptor complex in ima ginal discs is 37 nM. The K(d) of ecdysteroids for their receptor is e ither of the same order (ponasterone A, muristerone A) as, or lower (2 0-hydroxyecdysone, makisterone A, inokosterone) than the concentration to elicit a biological response. The biological response appears to b e a switch turned on by the hormone, whereas receptor-binding follows Michaelis-Menten kinetics. Comparison with dose-response data for poly tene chromosome puffing indicates that the switching is a post-recepto r event, occurring between transcription of the early and late genes.