SPECIFIC BINDING OF 11-KETOTESTOSTERONE IN AN ANDROGEN TARGET ORGAN, THE KIDNEY OF THE MALE 3-SPINED STICKLEBACK, GASTEROSTEUS-ACULEATUS

The kidney of male three-spined stickleback, Gasterosteus aculeatus, h ypertrophies during the breeding season and produces a ''glue'' which is used in the building of the nest. This hypertrophy is androgen depe ndent, with 11-ketotestosterone (11KT) being more effective than other tested steroids in stimulating this secondary sexual character. In th e present study kidneys were excised from stickleback males that had b een castrated two days earlier. The purpose of this gonadectomy was to reduce the endogenous levels of androgens without allowing time for t he kidney to regress. Tissue fragments were incubated with tritiated 1 1KT with and without unlabelled steroids at increasing concentrations. Displaceable specific 11KT binding was found in kidney tissue fragmen ts whereas only non-specific binding was observed when liver and muscl e were investigated in a similar way. Unlabelled 11KT displaced specif ically bound, tritiated 11KT with an ED50-value (50% of displaceable b inding) of 28 nM. Similar ED50 values were found for 17 beta-hydroxy-5 alpha-androstane-3,11-dione (29 nM) and 5 alpha-dihydrotestosterone ( 20 nM), whereas higher ED50 concentrations were estimated for testoste rone (T; 203 nM) and progesterone (69 nM). No displacement of tritiate d 11KT was found for the other investigated substances tested; estradi ol, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one, flutamide or cyprotero ne acetate. No specific binding to kidney tissue fragments could be de tected when labelled T was used instead of labelled 11KT. Specific bin ding of 11KT or T was not found either in the kidney cytosol or nuclea r extracts. However, using the kidney membrane fraction a displacement of tritiated 11KT with unlabelled 11KT(10(-6) M) was observed. In con clusion there is a specific binding of 11KT in the stickleback kidney. The absence of binding in liver and muscle, the ED50 value observed a nd the displacement with some, but not all steroids are consistent wit h a receptor function. The presence of binding in membrane fractions, but not in cytosol or nuclear extracts suggests that the binding is no t related to classic steroid receptors.