The kidney of male three-spined stickleback,Gasterosteus aculeatus,
hypertrophies during the breeding season and produces a "glue" which is used in the building of the nest. This
hypertrophy is
androgen dependent, with
11-ketotestosterone (11KT) being more effective than other tested
steroids in stimulating this secondary sexual character. In the present study kidneys were excised from stickleback males that had been castrated two days earlier. The purpose of this
gonadectomy was to reduce the endogenous levels of
androgens without allowing time for the kidney to regress. Tissue fragments were incubated with tritiated 11KT with and without unlabelled
steroids at increasing concentrations. Displaceable specific 11KT binding was found in kidney tissue fragments whereas only non-specific binding was observed when liver and muscle were investigated in a similar way. Unlabelled 11KT displaced specifically bound, tritiated 11KT with an ED50-value (50% of displaceable binding) of 28 nM. Similar ED50 values were found for 17\-hydroxy-5α-
androstane-3,11-dione (29 nM) and 5α-dihydrotestosterone (20 nM), whereas higher ED50 concentrations were estimated for
testosterone (T; 203 nM) and
progesterone (69 nM). No displacement of tritiated 11KT was found for the other investigated substances tested;
estradiol, 17α,20β-dihydroxy-4-pregnen-3-one,
flutamide or
cyproterone acetate. No specific binding to kidney tissue fragments could be detected when labelled T was used instead of labelled 11KT. Specific binding of 11KT or T was not found either in the kidney cytosol or nuclear extracts. However, using the kidney membrane fraction a displacement of tritiated 11KT with unlabelled 11KT (10(-6)M) was observed. In conclusion there is a specific binding of 11KT in the stickleback kidney. The absence of binding in liver and muscle, the ED50 value observed and the displacement with some, but not all
steroids are consistent with a receptor function. The presence of binding in membrane fractions, but not in cytosol or nuclear extracts suggests that the binding is not related to classic
steroid receptors.