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

Follicular activity and daily circulating concentrations of follicle s timulating hormone (FSH), luteinizing hormone (LH), and progesterone w ere 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 encompas sed three consecutive ovulatory periods for each mare; the interovulat ory interval after the second ovulation was shortened by the administr ation of prostaglandin F2alpha (PGF2alpha). Follicular activity was as sessed by daily transrectal ultrasonic imaging from 5 days prior to th e first ovulation to 3 days after the second ovulation. The diameters of the largest and second-largest follicles and the numbers of follicl es in different diameter categories (3-5, 6-10, 11-15, 16-20, and 20 o r more mm) were recorded. On Day 8 after the second ovulation, 5 mg of PGF2alpha were administered, and ultrasonic imaging was used to detec t 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 i nteractions for the diameter of the largest follicle (P<0.0001) and nu mber of 11-15 mm follicles (P<0.04). Follicular activity was greater d uring the late-luteal phase in Group 2 than in Groups 1 and 3 as indic ated by the following: (1) greater (P<0.05) diameter of the largest fo llicle 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 follicul ar 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 Gr oups 1 and 3. The oldest mares (20 or more years of age) had a prolong ed (P<0.05) interovulatory interval owing to a longer (P<0.05) follicu lar phase and a longer (P<0.05) interval from PGF2alpha treatment to o vulation than mares in Groups 1 and 2. These findings corresponded wit h a smaller (P<0.05) follicle for mares in Group 3 than for mares in G roup 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 da ys prior to ovulation was different (P<0.0003) among groups; young mar es had a faster rate of follicular growth than older mares (15 or more years of age). Age effects on progesterone concentrations were not co nsistent, although mares in Group 2 had lower progesterone levels on D ays 1 and 2 after the second ovulation than younger or older mares. Th e findings demonstrated profound effects of age on follicular activity and hormonal profiles.