FOLLICULAR ACTIVITY AND CONCENTRATIONS OF FSH AND LH ASSOCIATED WITH SENESCENCE IN MARES

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 stimu lating hormone (FSH) and luteinizing hormone (LH). Old mares (greater- than-or-equal-to 20 years) were less likely (P< 0.005) to ovulate at r egular intervals than younger mares as indicated by the number of mare s with two consecutive interovulatory intervals (IOI; 5-7 years, 9/9; 15-19 years, 10/10; greater-than-or-equal-to 20 years, 8/16). In addit ion, a central area of nonechogenic fluid (unknown source) was detecte d at the ovulatory site in more (P< 0.005) ovulations in old mares tha n in younger mares (5-7 years, 1/91; 15-19 years, 4/78; greater-than-o r-equal-to 20 years, 10/68). Old mares were grouped as follows: (1) no rmal length of the follicular phase (interval from luteolysis to ovula tion) of the IOI (n = 9); (2 ) elongated follicular phase ( > 2 SD fro m the mean of all mares with an IOI) (n = 2); (3 ) one ovulation in 80 days (n = 2); and (4) no ovulations or follicles greater-than-or-equa l-to 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 foll icles per ovulatory wave ( 1.5 +/- 0.5, 3.1 +/- 0.3, P< 0.05), a longe r IOI (30.0 +/- 1.0, 25.8 +/- 0.6 days, P< 0.01), a longer interval fr om administration of a luteolytic dose of PGF2alpha 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 ( 1 8.5 /- 5.3, 7.6 +/- 1.5 ng ml-1, P < 0.02 ) during the follicular phase. M ean concentrations of FSH, LH and progesterone during the luteal phase were not different among ovulating mares; however, during the follicu lar phase, concentrations of FSH and LH were significantly different a mong groups with the lowest mean concentrations occurring in mares wit h 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 en compassing 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 maxim al FSH concentrations). Results indicated that ovarian senescence in t hese mares involved the following progression: lengthening of the foll icular phase, sporadically occurring ovulations, and a final state of persistent follicular inactivity (follicles less-than-or-equal-to 5 mm ). Ovarian senescence was accompanied by elevated circulating concentr ations of gonadotropins.