Scientific Evidences Supporting the Activation of the Renin-Angiotensin-Aldosterone System during Estral Cycle and Pregnancy in Mares

Abstract

In women and laboratory animals, local and circulating components of the renin-angiotensin-aldosterone system (RAAS) are related to specific reproductive functions that occur during the estrous cycle, such as folliculogenesis, ovulation, corpus luteum development, and steroidogenesis. Also, in pregnant females of these species, maternal cardiovascular and renal systems undergo intense modifications, with the aim of matching the increased energy requirements of the fetus and fetoplacental unit. Some of these changes can be the origin, and others the consequence of a new endocrine environment. The fetus and the placenta induce endocrine changes, with modifications in the protein, lipid, carbohydrate, and mineral metabolism, together with simultaneous cardiovascular changes derived from the uterine growth and its content. The participation of RAAS during this period is of vital importance to regulate these cardiovascular, hemodynamic, hematological, and metabolic adjustments imposed by pregnancy because they will have a direct influence on the correct development and viability of the fetus. In mares, our research team has been investigating the changes of RAAS in mares during the estral cycle and during pregnancy, and these results are presented in the current chapter, comparing with the data previously reported for women and laboratory animals.

Endocrine and Electrolyte Balances during Periovulatory Period in Cycling Mares

“…Two recent studies conducted by the same authors in Spanish Purebred mares showed the highest concentrations of E2 at the time of ovulation [ 15 ] and positive correlations between follicular diameter and E2 [ 16 ]. Although in mares it is not well known how ovarian steroid hormones affect the HPA axis, the study of adrenal sex hormone receptors is of interest, since their presence or absence indicates whether the ovarian hormone fluctuations occurring during the estrous cycle.…”

“…Carey et al [ 45 ] proposed that P4 may have different stimulatory and inhibitory effects on the HPA axis in the rat, by means of different mechanisms. In Spanish purebred mares, correlations between CORT and P4 suggested certain stimulation of CORT from P4 [ 16 ], since P4 is a precursor to CORT, both hormones can be released when adrenal gland is stimulated.…”

“…Intra- and inter-assays CVs were 4.7–6.4% and 8.5–9.6%, respectively. These methods were validated for horse and have been used in previous research in the same species [ 3 , 13 , 14 , 15 , 16 , 23 , 24 ].…”

“…On the other hand, the activation of renin angiotensin aldosterone system (RAAS) during periovulatory period in mare [ 14 ] and its progressive increases, as soon as the significant positive correlations between estrogens 17-β (E2) and ALD [ 15 ] and between aldosterone (ALD) and follicle diameter, have been documented [ 16 ]. However, the possible relationships among ACTH, CORT, and ALD and electrolytes (sodium—Na + , potassium—K + and chloride—Cl − ) in the mare in ovulatory period have not been clearly established.…”

“…ALD exerts differential effects in relation to estrous cycle, since it is related to folliculogenesis, oocyte maturation, ovulation, development of the corpus luteum (CL), and steroidogenesis [ 17 ]. On the one hand, ALD increases during luteal period in women with high but not low sodium balance [ 18 ] and in mares [ 14 , 16 ] according to the P4 secretion [ 18 ]. On the other hand, the ovulatory peak of ALD in women is related to the stimulatory effect of estrogen on the synthesis of angiotensinogen (AOGEN) [ 19 ], with modifications of Na + concentrations at the level of the dense macula, and alterations in local sympathetic activity [ 20 ].…”