Seasonal effects on follicular dynamics and hemodynamics, ovarian hormone levels, nitric oxide levels, and lactate dehydrogenase levels in native mares.

Overview

• This study investigated how seasonal changes, particularly heat stress in the summer, affect the reproductive physiology of native mares, focusing on follicular dynamics, blood flow characteristics, hormone levels, and biochemical markers across the estrous cycle.
• The research compared these parameters between cold and hot months to understand the impact of heat on mare fertility.

Background and Objective

• Heat stress during summer is known to affect the estrous cycle regularity in various animal species.
• The study aimed to evaluate the influence of season (cold vs. hot months) on:
• Follicular dynamics (growth and development of ovarian follicles)
• Hemodynamics (blood flow related parameters in reproductive organs)
• Ovarian hormone levels (estradiol and progesterone)
• Other biochemical markers such as nitric oxide (NO) and lactate dehydrogenase (LDH)
• The goal was to see how these factors change over three consecutive estrous cycles during cold (November to March) and hot (May to August) seasons in native mares.

Methodology

• Subjects: Native mares studied during two distinct periods – cold and hot seasons.
• Data Collection:
• Doppler ultrasound scans to measure ovarian and uterine structures, including:
• Dominant Follicle (DF) size and color area (indicator of blood flow)
• Follicle antrum diameter (fluid-filled cavity)
• Uterine horn dimensions
• Corpus luteum (CL) dimensions and vascularity
• Blood samples taken throughout estrous cycles to analyze:
• Hormones: Estradiol (E2) and Progesterone (P4)
• Biochemical markers: Cholesterol, Lactate Dehydrogenase (LDH), Nitric Oxide (NO)
• The estrous cycle was monitored over 20 days, from day -5 to day 14 relative to ovulation.

Key Findings

• Follicular Dynamics:
• The total number of follicles was significantly lower during hot months compared to cold months (p < 0.01).
• The diameter and color area of the dominant follicle were affected by both the day of the estrous cycle and season interaction (p < 0.001).
• The follicle antrum diameter was influenced separately by season and estrous days (p < 0.001).
• Hemodynamics and Ovarian Structures:
• Season and estrous cycle days significantly impacted uterine horn size, corpus luteum dimensions, and blood flow indicators (color area percentages) in follicles and uterine tissues (p < 0.001).
• During hot months, the area, antrum size, and circumference of dominant follicles, as well as uterine horn area, were increased compared to cold months.
• Hormonal and Biochemical Parameters:
• Progesterone (P4) and estradiol (E2) levels changed with season and estrous cycle days (p < 0.001).
• Total cholesterol, nitric oxide (NO), and lactate dehydrogenase (LDH) levels were higher during hot months, especially at certain days within the estrous cycle (p < 0.001).
• Influence of Season:
• While ovarian macrostructures and follicular growth were not disrupted by hot season, blood flow mediators, enzymes, and biochemical markers were altered markedly.
• These biochemical and hemodynamic changes may negatively affect intrafollicular environment and oocyte quality.

Conclusions and Recommendations

• Heat stress during the hot months does not substantially disturb the physical growth of follicles and ovarian structures but does alter the physiological environment critical for follicle and oocyte health.
• The disruption in blood flow-related markers and enzymes indicates compromised follicular microenvironment, potentially reducing oocyte quality and fertility.
• For optimal breeding outcomes, it is advised to mate mares during the cold season when environmental conditions are favorable and quality green forage is readily available.