1. Investigación

Many physiological events occurring during the estrous cycle, including folliculogenesis, oocyte maturation, ovulation, follicular atresia, corpus luteum development, luteolysis, steroidogenesis, and angiogenesis are associated with an increased expression of the renin–angiotensin–aldosterone system (RAAS) in uterus and ovarian follicles. This enhanced expression leads to a rise in plasma concentrations of these hormones in women and laboratory animals, and the same might happen in the mare. This study aims to assess if an activation of the RAAS occurs in mares around ovulation and if this activation is related to the diameter of the predominant follicle (DPF), packed cell volume (PCV), and electrolyte concentrations. Twenty-five healthy Spanish Purebred mares were sampled during the five days before ovulation, the day of ovulation, and the first five days after ovulation. Renin (REN) concentrations increased progressively during the five days before ovulation, achieving the highest values on the day of ovulation. Angiotensin (ANG) concentrations showed a sharp decrease after ovulation, with the lowest values the fifth day after it. Aldosterone (ALD) concentrations increased progressively, from the fifth day before ovulation until the fifth day after ovulation. The highest PCV values were found on the day of ovulation. There were no relevant correlations between the RAAS components and electrolytes concentrations. Before ovulation, there was a positive relationship between REN and ALD (r = 0.760) and after ovulation, a negative correlation between ANG and ALD (r = –0.660). The DPF was correlated with REN (r = 0.740) and with ALD (r = 0.800) concentrations. Ovulation in the mare is associated with high plasma REN concentrations, and therefore, with an activation of the RAAS. In addition, after ovulation, the sharp decrease in plasma REN and ANG concentrations might be a reflex of the modulation of the previously activated RAAS, although plasma ALD concentrations increased during this period.

The hypothesis of this study was to investigate if the presence of corpus luteum (CL) in one ovary could modify the hormonal content of follicular fluid (FF) in the follicles. Sixty ovaries were taken after the slaughter of 30 clinically healthy mares. In relation to the sizes, the follicles were classified into three di erent categories, as small (20–30 mm), medium (31–40 mm) and large ( 41 mm). Blood samples were collected from the jugular vein of mares before their slaughter, and then the FF samplings were extracted from each single follicle. The ovaries that were collected were classified into two groups, according to the presence (CL-bearing) or absence (non-CL-bearing) of CL. The serum and FF samples were analysed for progesterone (P4), oestradiol-17 (E2), testosterone (T), androstenedione (A4) and dehydroepiandrosterone (DHEA). Intrafollicular P4 concentrations in large follicles of CL-bearing groups were lower than for non-CL-bearing ones. Intrafollicular E2 concentrations increased with the increase of the follicle diameter in both groups, CL-bearing and non-CL-bearing. However, in the FF with a large and medium follicle size, E2 concentrations were significantly higher in non-CL-bearing groups than in CL-bearing groups. T and A4 significantly increased in the large and medium follicle sizes when compared to the small follicle sizes in both groups, but higher concentrations in the non-CL-bearing group were obtained. Intrafollicular DHEA significantly decreased with the increase of the follicular diameter in both groups. Steroid hormones in FF dynamically changed, according to the presence or not of CL in the ovary. This study brings new knowledge on the role of the CL in the follicular hormonal composition in mares.

In some species, catecholamines in follicular fluid (FF) are related to local physiological events responsible for the regulation of ovarian functions and oocyte maturation. The aim of the present study was to determine and compare intrafollicular and systemic concentrations of dopamine (DA), noradrenaline (NA) and adrenaline (AD) in cycling mares. Sixty ovaries were collected during breeding season from 30 mares raised for slaughterhouse meat production, with clinically normal reproductive tracts, were evaluated. Blood samples were collected prior to slaughter. Follicles were classified into three categories in relation to size: small (20–30 mm; n = 20), medium ( 31–40 mm; n = 20) and large ( 41 mm; n = 20). Follicular fluid (FF) samples were extracted from each follicle. Intrafollicular DA, NA and AD concentrations were significantly higher than the systemic concentrations (p < 0.05). Intrafollicular DA concentrations were higher in medium than small and large follicles (p < 0.05). Intrafollicular NA concentrations were higher in small than medium and large follicles (p < 0.05). Intrafollicular AD concentrations were higher in large than small and medium follicles (p < 0.05). Follicle diameter was significantly and negatively correlated with NA and AD (p < 0.05). A significant correlation of the same hormone concentration in FF and in systemic fluid was observed (p < 0.05). In summary, the FF can serve as an intraovarian catecholamine-storing compartment, with the ability to release neurotransmitters in a regulated way. These results provide novel insights into the neuronal nature of the follicle, suggesting the involvement of catecholamines in normal ovarian functions in mares.