65 ASSOCIATION BETWEEN ALDOSTERONE AND CORTISOL LEVELS DURING THE OVULATORY PERIOD IN SPANISH PUREBRED MARES

Abstract

Preovulatory increased aldosterone (ALD) has been described as one of the physiological events that characterise the oestrous cycle in relation to the dynamics of the renin angiotensin aldosterone system. Within the mechanisms associated with this response are the stimulating effect of oestrogen on the synthesis of angiotensinogen, haemodynamic variations of renal blood flow, changes in sodium concentration, changes in the local sympathetic activity, and the effect of cortisol (CORT). In fact, CORT via adrenocorticotropic hormone contributes primarily to preovulatory ALD, and it is the primary regulator factor synthesis ALD (Chidambaram et al. 2002; Pechère-Bertschi et al. 2002; Szmuilowicz et al. 2006; Sealy and Laragh 2011). This mechanism has been described in several animal species, but remains unknown in the mare. The aim of this study was to investigate the association of endogenous CORT increases, with the changes in the synthesis of ALD during the ovulatory period in the mare. This research was approved by the Animal Ethics Committee of the CEU-Cardenal Herrera University. The study was carried out on 25 healthy Spanish purebred between 4 and 17 years old. Venous blood samples were taken daily from Day –2 to 2 of the oestrous cycle, considering the time 0, the day of ovulation identified by transrectal ultrasound. Serum CORT and ALD concentrations were determined to by competitive immunoassay. To analyse the effect of cycle day on concentrations of CORT and ALD, ANOVA was used. The interrelations between both parameters were expressed by Pearson correlation coefficient. A significant rise of CORT and ALD were observed on the day of ovulation (CORT: 87.68 ± 10.71 ng mL^–1; ALD: 535.3 ± 71.49 pg mL^–1) with respect to previous 2 days (CORT: 67.13 ± 3.165 ng mL^–1; ALD: 380.3 ± 36.20 pg mL^–1), decreasing to 2 days postovulation (CORT: 61.61 ± 1.766 ng mL^–1; ALD: 495.2 ± 14.75 pg mL^–1; P < 0.05). Significant correlations between CORT and ALD (r = 0.53) were founded. The results of this study show that, as has been reported in cyclic females of other species, in Spanish purebred mares, the ALD and CORT concentrations increase significantly during the period of ovulation. Similarly, although CORT is involved in the synthesis of ALD, its contribution is not important enough to induce activation of the renin angiotensin aldosterone system by itself, so other CORT-independent factors must also be involved in the synthesis of ALD.