A receptor protein that selectively binds oestrogens has been demonstrated in the cytosol of the canine prostate gland. The steroid--receptor complex was found to have a sedimentation coefficient of 4--5 S with respect to bovine serum albumin after sucrose density-gradient centrifugation. The high affinity and low capacity of the protein for oestrogens was indicated by displacement studies, which gave a value of 3.8 +/- 1.53 (S.D.) X 10(-10) mol/l for the dissociation constant. A metastasizing prostatic tumour was also shown to possess this receptor, with binding properties similar to those exhibited by the receptor in normal prostatic cytosol. The implications of these findings are discussed with regard to normal prostatic function in the dog and the virtually inevitable advent of prostatic hyperplasia with age in this species.

The stromal and epithelial elements of the canine prostate gland have been mechanically separated for the individual measurement of the presence of oestrogen receptor. The purity of each fraction was assessed by light and electron microscopy and by certain biochemical parameters and was found to be good. The oestrogen receptor was demonstrated to be predominantly located in the stroma at a concentration of 181.5 +/- 49.3 fmoles/mg cytosol protein compared with that in the epithelium of 44.5 +/- 22.4 fmoles/mg cytosol protein. A high affinity for the tritiated ligand was also shown (kd = 2.81 +/- 0.8 (X 10(10)). A significance of these findings is discussed, especially with regard to the development of prostatic hyperplasia.

High-affinity and low-capacity molecules that bind 5 alpha-dihydrotestosterone and oestradiol-17 beta have been demonstrated in the canine epididymis. The molecules were shown to be proteinaceous in nature, thermolabile and sedimented in a sucrose density gradient with a coefficient of 8S with respect to bovine serum albumin. Only minor differences were found for dissociation constants for oestradio-17 beta (3.5 +/- 1.74 S 10(-10) M) and 5 alpha-dihydrotestosterone (4.2 +/- 10.75 X 10(-10) M) (means +/- s.d.) in the three anatomically distinct regions of the epididymis.

An anal adenoma from a dog has been examined for cytosol binding proteins for androgens and oestrogens. Sucrose‐density gradient analysis revealed low capacity binding moieties for testosterone with sedimentation coefficients of 2S and 8S, for 5α‐dihydrotestosterone (8S) and for oestradiol‐17β (4S and 8S). In each case it was shown that the steroids could be translocated to the nuclei and were found there to be protein‐bound. The dissociation constants for 5α‐dihydrotestosterone and oestradiol‐17β were 5×1 × 10‐10M and 2×6 × 10‐10M respectively and the corresponding receptor site concentrations were 9 and 38 fmol/mg cytosol protein. The possible significance of these findings with regard to the induction and maintenance of such tumours is discussed.