Follicular activity and concentrations of FSH and LH associated with senescence in mares

Abstract

The nature of reproductive senescence was studied during the expected ovulatory season in pony mares by transrectal ultrasonic monitoring of the ovaries and assay of circulating concentrations of follicle stimulating hormone (FSH) and luteinizing hormone (LH). Old mares (⩾20 years) were less likely ( P<0.005) to ovulate at regular intervals than younger mares as indicated by the number of mares with two consecutive interovulatory intervals (IOI; 5–7 years, 9/9; 15–19 years, 10/10; ⩾20 years, 8/16). In addition, a central area of nonechogenic fluid (unknown source) was detected at the ovulatory site in more ( P<0.005) ovulations in old mares than in younger mares (5–7 years, 1/91; 15–19 years, 4/78; ⩾20 years, 10/68). Old mares were grouped as follows: (1) normal length of the follicular phase (interval from luteolysis to ovulation) of the IOI ( n=9); (2) elongated follicular phase (>2 SD from the mean of all mares with an IOI) ( n=2); (3) one ovulation in 80 days ( n=2); and (4) no ovulations or follicles ⩾5 mm in 60 days ( n=3). Mares with an IOI but with an elongated, compared with a normal length of the follicular phase, had fewer follicles per ovulatory wave (1.5 ± 0.5, 3.1 ± 0.3, P< 0.05), a longer IOI (30.0 ± 1.0, 25.8 ± 0.6 days, P < 0.01), a longer interval from administration of a luteolytic dose of PGF 2α to ovulation (29.5 ±8.5, 15.8 ± 0.9 days, P < 0.02), and higher mean concentrations of FSH (13.0 ± 3.4, 3.3 ± 0.6 ng ml −1, P < 0.001) and LH (18.5 ± 5.3, 7.6 ± 1.5 ng ml −1, P < 0.02) during the follicular phase. Mean concentrations of FSH, LH and progesterone during the luteal phase were not different among ovulating mares; however, during the follicular phase, concentrations of FSH and LH were significantly different among groups with the lowest mean concentrations occurring in mares with a follicular phase of normal length. Mean concentrations of FSH for mares that did not ovulate were higher ( P<0.05) than for the 3 days encompassing ovulation in the mares with an IOI and tended to be higher ( P<0.08) than during the 3 days prior to luteolysis (period of maximal FSH concentrations). Results indicated that ovarian senescence in these mares involved the following progression: lengthening of the follicular phase, sporadically occurring ovulations, and a final state of persistent follicular inactivity (follicles ≤5 mm). Ovarian senescence was accompanied by elevated circulating concentrations of gonadotropins.