Production and Action Mechanism of Sex Hormones in Male Dogs

Abstract

The present study was undertaken to assess the production and the action mechanism of sex hormones in male dogs. The following results were obtained. 1) Plasma concentrations of LH, androstenedione, testosterone, 5 α-dihydrotestosterone (DHT) and estradio1-17 β were measured by the radioimmunoassay method. The level of LH increased with days of age and stayed constant from 8 years through 13 years of age in male dogs. The level of testosterone increased with days of age and stayed constant from 1 year through 10 years of age. Thereafter, the level of testosterone declined gradually. In contrast, there was no age-related increase in the level of estradiol-17β in mature dogs. 2) By measuring androstenedione, testosterone and DHT simultaneously in peripheral and spermatic vein plasma, and determining the temporal relationship between the two vein plasma levels of these hormones, it was suggested that plasma testosterone and DHT were derived mainly from the testis, while plasma androstenedione was derived from the testis and other organs like the adrenal. 3) Plasma concentrations of LH, androstenedione, testosterone and DHT in mature dogs fluctuated episodically, but neither diurnal rhythm nor annual variation of LH and testosterone concentrations was observed. 4) It was suggested that the pathway for activation of steroidogenesis in canine testes by LH was through cAMP formation. 5) In canine testes, major C19-metabolites of pregnenolone were dehydroepiandrosterone, androstenedione and testosterone. Following the addition of a 17α-hydroxylase inhibitor to the testis caused a significant increase in pregnenolone production, indicating that the formation of testosterone from pregnenolone by the Δ 5-pathwaywas dominant in androgen biosynthesis of the testis. 6) Hexestrol dicaprylate (H8), a long-acting estrogen, was used in an attempt to induce temporary sterilization of male dogs. H8 treatment caused significant decreases in the gonadotrophic activities of the pituitary gland and testosterone formation by the testis with the inhibition of spermatogenesis in the dog. It was suggested that H8 interfered with testosterone production by blocking the pituitary function, and also directly inhibited the testicular steroidogenesis. 7) In canine prostates, major metabolites of testosterone were DHT and 5α-androstanediols. DHT had a high binding affinity for androgen receptor in the prostate, while 5α-androstanediols had a low affinity for the receptor. The prostate was shown to contain estrogen receptor. Following injection of estrogen to dogs, it bounded to the estrogen receptor in the prostate gland and caused the enlargement of the gland.