Читать книгу Bovine Reproduction - Группа авторов - Страница 38

Hormone action depends on the release of hormones from the appropriate endocrine gland and transportation via the vascular circulatory system to the target tissue where the hormone binds to cellular receptors, thus inducing a physiological response. In some cases these receptors are very hormone‐specific. The response at the target tissue may depend on the level of receptor expression and concentration of hormone. Some hormones regulate their own receptors, others may require synergism between two hormones, and others still may have their receptors regulated by other hormones [34]. The general characteristics of neuroendocrine regulation of the mammalian testis by the hypothalamic–pituitary axis are well established [35], but in some species this may be seasonally regulated. Domestic cattle are considered to be continuous or non‐seasonal breeding species [36]. However, there is information in the literature to indicate that domestic beef and dairy bulls have a functional hypothalamic–pituitary–gonadal axis that is seasonally regulated and thus may influence levels of gonadal steroidal and germ cell production. In a study using composite breeds of mature bulls, Stumpf et al. [37] observed that season of the year influenced the profile of gonadotropins in the circulation of both gonadectomized and intact males, and that the greatest secretion of gonadotropins occurred at the spring equinox. In addition, these authors observed that season of the year also influenced secretion of testosterone in intact males, but that more testosterone was released in response to LH during the time of the summer solstice. Others have reported similar effects, where location affected reproductive traits of bulls including semen quality and blood concentrations of LH and testosterone [38, 39]. In addition, sensitivity in responsiveness to exogenous gonadotropin administration and testosterone secretion in bulls was observed to be seasonally influenced [40].

The regulation of testicular function by hormonal mechanisms depends on the integrated actions of gonadotropins, such as LH, FSH, and prolactin, and steroids (androgens and estrogens) on the Leydig cell [41]. Gonadotropin‐releasing hormone is the primary hypothalamic hormone governing regulation of the synthesis and release of the gonadotropins LH and FSH by the anterior portion of the pituitary gland. LH is primarily responsible for testosterone production by the Leydig cells, while FSH facilitates Sertoli cell proliferation and support of the germinal cells. Although it has been well established that gonadotropins stimulate testicular function, it was during the 1970s that the basic mechanism and site of action in the testis were identified. LH has been shown to be the gonadotropin essential for the maintenance of testicular testosterone production [41, 42]. Catt and Dufau [43] have reviewed the mechanisms of action of the gonadotropins and concluded that they all elicit target cell response by similar mechanisms. In fact, LH provides the most important physiological regulation of the production of androgens by the Leydig cells of the testis [44, 45].

Leydig cells contain plasma membrane‐bound receptors that specifically bind LH. The binding of LH stimulates adenyl cyclase to produce cyclic adenosine monophosphate (cyclic AMP), a second messenger in the cell cytoplasm, which in turn activates cyclic AMP‐dependent protein kinase, thereby increasing the conversion of cholesterol to pregnenolone. The action of LH can be readily demonstrated in hypophysectomized and intact males. Removal of the pituitary gland is followed by rapid cessation of testosterone production, loss of enzymes involved in steroidogenesis, and testicular atrophy [46]. There is little direct evidence to support a role of FSH in Leydig cell steroidogenesis, but there is some evidence indicating that this gonadotropin may play a function in the conversion of androgens to estrogens in the Sertoli cells [47]. Bartke et al. [42] have suggested that FSH may act on the aromatizing enzyme system of testosterone biosynthesis, but little evidence exists for action on other steps of the steroidogenesis pathway in the testis. In bulls, testosterone secretion is not tonic, but is characterized by episodic pulses dictated by the release of luteinizing hormone releasing hormone (LHRH) from the hypothalamus and LH from the anterior pituitary gland [48]. Schanbacher [48] reports that a temporal relationship exists between concentrations of LH and testosterone in the blood, and that there is evidence that episodic secretion depends on discrete episodes of LHRH discharge from the hypothalamus.