It could be hypothesized that the adiponectin system is regu

It could be hypothesized that the adiponectin system is regulated by gonadal steroid hormones. The lowest concentrations of adiponectin, AdipoR1 and AdipoR2 proteins in the endometrium and myometrium during the mid-luteal and late-luteal phases suggest that progesterone suppresses the expression of adiponectin and both receptor proteins in the uterus. The highest contents of adiponectin and its receptor proteins were detected in the myometrium during the follicular phase, therefore, it cannot be ruled out that the expression of the adiponectin system in the myometrium was promoted by oestradiol. The possible impact of the oestrogens on the production of adiponectin in the body is controversial. According to Chu et al. (2006), the administration of exogenous oestradiol increases adiponectin levels in women. Other authors observed a drop in serum adiponectin levels under the influence of oestrogens (Im et al., 2006), whereas in some studies oestradiol produced no visible effects (Chalvatzas et al., 2009, Sumino et al., 2004). The results of our previous study on pigs (Maleszka et al., 2014) further contribute to the evidence of oestradiol's inhibitory effect on plasma adiponectin concentrations. Similar adiponectin levels were observed during the early-luteal, mid-luteal and late-luteal phases of the oestrous cycle, and a significant decrease in adiponectin plasma levels was noted during the follicular phase. In healthy women, serum adiponectin levels remain stable throughout the menstrual 3230 (Asimakopoulos et al., 2009, Dafopoulos et al., 2009, Hall et al., 2009, Kleiblova et al., 2006), which suggest that the relationship between circulating adiponectin levels and hormonal status linked to the phase cycle of reproductive cycle is species-dependent. Circulating levels of adiponectin are approximately two- to three-fold higher in females than in males, and it has been suggested that ovarian steroids could regulate adiponectin secretion (Bottner et al., 2004, Combs et al., 2003).
Conclusions
Acknowledgment This research was supported by National Science Centre (projects no: 2011/03/B/NZ9/04187).
Introduction Increased livestock productivity has become a major research topic in response to increases in the population and consumption [1]. In this context, understanding the reproductive physiology of these animals is key to improving reproductive techniques and, consequently, increasing productivity. In ruminants, reproductive function can be influenced at different levels of the hypothalamic-pituitary-gonadal axis by energetic alterations related to diet [2], with a well-known relationship between energetic balance and reproduction [3]. Adipose tissue, once recognized only for its role as an energy reservoir, has been identified as an important modulator of this relationship [4], [5]. Now recognized as an endocrine organ, adipose tissue releases a wide variety of protein factors called adipokines [6]. These adipokines have important effects on various physiological processes, including reproduction [7]. Adiponectin, a 30 kDa, 244 amino acid protein, has an N-terminal signal sequence, a species-dependent variable sequence, a collagen domain, and a C-terminal globular domain [8], [9]. Its secretion is inversely related to adipose tissue levels [10], and its circulating levels are two to three times higher in females than those in males [11]. Adiponectin acts through two specific membrane receptors, AdipoR1 and AdipoR2 [12]. These receptors are formed by seven transmembrane domains, with an intracellular N-terminus and an extracellular C-terminus, and have an opposite orientation to that of classical G-proteins-coupled receptors [13], [14]. The adiponectin system was identified in different organs of the female reproductive system in various species. Adiponectin, AdipoR1 and AdipoR2 were observed in ovarian cells of cattle [15], chickens [16], mice [17] and swine [18]. AdipoR1 and AdipoR2 were also identified in ovarian fish cells [19] and humans [20], [21]. Adiponectin protein was also visualized in ovine follicular cells [22].