Karl-H. TOMASCHKO. Roland GUCKLER, and Juraj HARMATHA
Universität Ulm, Abteilung Aligemeine Zoologie, 89069 Ulm, Germany
The classical function of ecdysteroids (ES) in arthropods is the hormonal control of molting, growth and gametogenesis by acting on gene expression (1). Little is known about extranuclear activities of ES. From the few reports available (2), it appears that membrane associated ecdysteroid receptors differ significantly from the classical nuclear receptor with respect to their sensitivity and structure specificity.
The recent finding of an allelochemical function of ES in marine arthropods (3) rendered the possibility to investigate the structure specificity of a membrane associated ES receptor. The common shore crab Carcinus maenas rejects food containing overcritical amounts of ES. Applications of ES solutions to the anterior oesophagus lead to an immediate reflex-like dilatation of the oesophagus. Based on this reaction, a bioassay was developed in order to investigate the structure specificity of the involved receptors (4). A series of chemical compounds were tested as to their biological activities in this bioassay, including different zoo- and phytoecdysteroids, vertebrate steroids, brassinolids, and synthetic ES-antagonists.
The most effective compound was makisterone A, showing an EC50 of 2.2 x 10-6 M. The EC50 of ecdysone and 20-hydroxyecdysone were 4.1 x 10-6M and 9.6 x 10-6 M, respectively. Ponasterone A, which has a high affinity to the nuclear receptor, is almost 30 times less effective than ecdysone in our bioassay. Vertebrate steroids, brassinolids, and the ecdysteroidanaloga RH 5849 and RH 5992 are 10 to 1000 times less effective in our bioassay or showed no effect at all.
The results suggest that the membrane associated ES receptors at the anterior oesophagus of decapod crustaceans is to a high degree ES specific. However, the structure specificity significantly differs from those of all known nuclear ES receptors.
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