Can the reduced GH, IGF-1, and ovarian steroids concentrations be considered as suspected biomarkers of age-associated functional deficit in mares?

In humans' and experimental animals' components of the somatotropic axis, such as growth hormone (GH) and insulin-like growth factor 1 (IGF-1) concentrations, decrease with advancing age. Although there is evidence regarding IGF-1, the effect of age on GH in mares, as well as the relationships between both parameters, have not yet been elucidated. On the other hand, although GH and IGF-1 are related to follicular development, it is unknown if they could be correlated with the circulating concentrations of ovarian steroids in mares, as occurs in other species. The hypothesis of this study was that both GH and IGF-1 could experience physiological changes with advancing age also in mares, and that both GH/IGF-1 could be correlated with oestradiol-17ß (E2) and progesterone (P4), as recorded for other species. Hence, the objective of this study was to evaluate the concentrations of GH, IGF-1, E2, and P4 in mares, according to the different ages. Blood samples were drawn from 56 healthy cyclic Spanish Purebred mares belonging to four different age groups: 6-9 years, 10-13 years, 14-16 years and >16 years. Mares aged 6-9 years and 10-13 years showed higher GH concentrations (P < 0.05) than mares of 14-16 and >16 years; and mares aged 14-16 showed higher GH concentrations (P < 0.05) than >16 years (P < 0.05). Mares aged >16 years showed lower IGF-1 concentrations (P < 0.05) than mares of 6-9, 10-13 and 14-16 years (P < 0.05). The concentrations of E2 and P4 showed no significant differences among different age groups. Both GH and IGF-1 were not correlated with each other or with E2 and P4. The concentrations of E2 and P4 did not change with age. Advancing age leads to a decrease in the activity of the somatotropic axis in physiological cyclic mares, represented by a significant GH reduction, which, however, was ascribed for IGF-1 exclusively to mares over 16 years of age, without alterations in steroid hormone patterns.

Effect of age on androgens pattern in cyclic mares.

Androgens are produced in both sexes. In females produced by the adrenal gland and the ovaries they play a crucial role in regulating ovarian function, estrogen synthesis and follicular growth. Age leads to a reduction in androgen concentrations, although, at present, these mechanisms are not elucidated in mares. The objective of this study was to evaluate the concentrations of testosterone (T), androstenedione (A4) and dehydroepiandrosterone (DHEA) in mares of different ages. Blood samples were drawn from seventy cyclic Spanish Purebred mares belonging to five age groups: 3-5 years, 6-9 years, 10-13 years, 14-16 years and > 16 years. The concentrations of T, A4 and DHEA were determined by EIA, validated specifically for horses. Mares aged 3-5, 6-9 and 10-13 years had higher T concentrations (P < 0.05) than mares aged >16 years, and mares aged 6-9 years had also higher concentrations than those 14-16 years old (P < 0.05). A4 concentrations were lower (P < 0.05) in mares >16 years old when compared with those of other age groups. DHEA concentrations were lower (P < 0.05) in mares 14-16 years and > 16 years old when compared with those of other age groups. DHEA was positively correlated with T (r = 0.61; P < 0.05) and A4 (r = 0.51; P < 0.05). Age induces reduction in androgens' synthesis in physiologically cyclic Spanish Purebred mares. These physiological variations must be duly considered for a correct and objective interpretation of the analytical data.