AGING EFFECTS ON FOLLICULAR ACTIVITY AND CONCENTRATIONS OF FSH, LH, AND PROGESTERONE IN MARES

Follicular activity and daily circulating concentrations of follicle stimulating hormone (FSH), luteinizing hormone (LH), and progesterone were compared between young and old mares. Mares were divided into the following age groups: Group 1, 5-7 years (n = 9); Group 2, 15-19 years (n = 11); Group 3, 20 or more years (n = 11). The experiment encompassed three consecutive ovulatory periods for each mare; the interovulatory interval after the second ovulation was shortened by the administration of prostaglandin F2alpha (PGF2alpha). Follicular activity was assessed by daily transrectal ultrasonic imaging from 5 days prior to the first ovulation to 3 days after the second ovulation. The diameters of the largest and second-largest follicles and the numbers of follicles in different diameter categories (3-5, 6-10, 11-15, 16-20, and 20 or more mm) were recorded. On Day 8 after the second ovulation, 5 mg of PGF2alpha were administered, and ultrasonic imaging was used to detect the day of the third ovulation. Day of ovulation was defined as Day 0. Significant effects of age on follicular activity included a group effect (P<0.02) for the number of 16-20 mm follicles and day X group interactions for the diameter of the largest follicle (P<0.0001) and number of 11-15 mm follicles (P<0.04). Follicular activity was greater during the late-luteal phase in Group 2 than in Groups 1 and 3 as indicated by the following: (1) greater (P<0.05) diameter of the largest follicle on Days 13 and 14; (2) more (P<0.05) 11-15 mm follicles on Day 13; (3) more (P<0.05) 16-20 mm follicles on Day 13. Increased follicular activity in Group 2 was preceded by an earlier mid-luteal elevation of FSH and higher (P<0.05) FSH levels on Days 7, 9, and 11 than in Groups 1 and 3. The oldest mares (20 or more years of age) had a prolonged (P<0.05) interovulatory interval owing to a longer (P<0.05) follicular phase and a longer (P<0.05) interval from PGF2alpha treatment to ovulation than mares in Groups 1 and 2. These findings corresponded with a smaller (P<0.05) follicle for mares in Group 3 than for mares in Group 2 at the time of natural or induced luteolysis and lower (P<0.05) maximal concentrations of LH for the second ovulation in Group 3 than in Groups 1 and 2. The growth rate of the ovulatory follicle for 5 days prior to ovulation was different (P<0.0003) among groups; young mares had a faster rate of follicular growth than older mares (15 or more years of age). Age effects on progesterone concentrations were not consistent, although mares in Group 2 had lower progesterone levels on Days 1 and 2 after the second ovulation than younger or older mares. The findings demonstrated profound effects of age on follicular activity and hormonal profiles.