The effect of prolonged hyperprolactinaemia on the secretion of LH, progesterone and oestradiol, and its relationship to the maintenance of pregnancy was examined in pigs. Twelve crossbred, pregnant gilts were injected i.m. with 1.5 mg haloperidol kg−1 body weight (n = 6) or vehicle (n = 6) once a day from day 60 to day 66 of pregnancy. Blood samples were collected at 08:00, 12:00, 16:00, 20:00, 24:00 h from day 60 to day 67 and every 15 min for 4 h (08:00–12:00 h) h) on days 60, 63 and 66. Plasma concentrations of prolactin were higher (P < 0.001) in haloperidol-treated gilts than in control gilts (121.3 ± 4.3 ng ml−1 and 13.6 ± 0.4 ng ml−1, respectively). Hyperprolactinaemia completely inhibited the pulsatile secretion of LH and diminished (P < 0.001) basal peripheral concentrations of LH (hyperprolactinaemia, 0.3 ± 0.04 ng ml−1 and control, 0.6 ± 0.005 ng ml−1). Despite the inhibition of LH release in hyperprolactinaemic gilts, plasma concentrations of progesterone were higher (P < 0.001) than in the control group (20.8 ± 0.6 and 12.6 ± 0.2 ng ml−1, respectively). Oestradiol concentrations were not different between groups, although oestradiol tended to be higher in hyperprolactinaemic gilts than in the control group throughout the sampling period (29.1 ± 1.9 versus 23.7 ± 1.6 pg ml−1, respectively). Abortion did not occur in any of the gilts. These results are the first to demonstrate that induced hyperprolactinaemia during the second half of pregnancy (days 60–66) will drastically suppress the major porcine luteotrophin but not affect pregnancy maintenance in pigs. It is possible that prolactin has a more important function in the luteotrophic complex as an additional protectant associated with the regulatory mechanism of late pregnancy maintenance than was previously reported.
B. Szafranska and J. E. Tilton
L. Dusza, J. E. Tilton and R. M. Weigl
Summary. In Exp. I infusions of prolactin (0·5 mg in 2 ml sterile saline) were repeated every 2 h for 36 h on Days 12–13 of the cycle. In Exp. II infusions of prolactin were administered from Days 17 to 19 (60 h) at 2-h intervals. Control gilts were given 2 ml sterile saline at similar intervals during the same period. Basal prolactin concentrations before initiation of infusions ranged from 1·3 ± 0·1 to 5·6 ± 2·2 ng/ml in both experiments. By 5 min after a prolactin infusion, mean plasma prolactin concentration ranged from 74·9 ± 5·8 to 113·0 ± 9·5 ng/ml, but then declined to ≃ 10 ng/ml just before the next infusion of prolactin. Administration of prolactin during the luteal phase of the oestrous cycle of the gilts had no effect on basal levels of progesterone, oestradiol or LH. During the follicular phase there were no differences (P >0·05) between control and prolactin-treated gilt progesterone and LH concentrations, but oestradiol plasma values were decreased (P <0·05) on the 2nd and 3rd day of prolactin treatment. Our results would indicate that prolactin does not play a major role in the regulation of the oestrous cycle of the pig.
C. Biggs, J. E. Tilton, J. Craigon, G. R. Foxcroft, C. J. Ashworth and M. G. Hunter
Comparisons were made between characteristics of pre-ovulatory follicles recovered from prolific Chinese Meishan gilts (n = 12) and from European Large-White hybrid gilts (n = 13) in the late follicular phase preceding their fifth oestrous cycle, to determine whether there is an ovarian basis for the enhanced prolificacy in the Meishan. A total of 177 follicles per breed was classified as pre-ovulatory, based on follicular fluid oestradiol concentrations. Results obtained demonstrated high variability in all follicular characteristics in both breeds and no decrease in heterogeneity was evident in the Meishan. The Meishan follicles tended to be smaller (P < 0.06) and had less follicular fluid (P < 0.005), but total oestradiol content per follicle was similar (P > 0.1) with the result that the concentration of oestradiol in follicular fluid tended to be higher (P < 0.06) in Meishan than Large-White hybrid pigs. There were no differences between breeds in terms of testosterone concentration in follicular fluid, hCG binding to granulosa cells or total DNA content of granulosa cells. Concentrations of inhibin in follicular fluid were similar in both breeds (P > 0.1) which resulted in a trend towards less total inhibin content in Meishan than Large-White hybrid follicles (P = 0.065). Corpora lutea were recovered from both breeds (n = 12 per breed) on days 27–31 of pregnancy after mating at first, second and third oestrus:corpora lutea were smaller (P < 0.001) and contained less progesterone per corpus luteum in the Meishan (P < 0.05) than in Large White hybrid pigs. However, since ovulation rate was higher in these particular Meishan pigs (P < 0.005), total ovarian progesterone content per animal was similar in both breeds. These results demonstrate that there was no decrease in the variability in follicular characteristics from Meishan pigs. However, both follicles and corpora lutea were smaller in Meishan than in Large-White hybrid pigs, and progesterone per corpora lutea was also lower in the Meishan. In spite of their smaller size, Meishan follicular oestradiol content was similar to that of Large-White hybrid pigs, such that the oestradiol concentration in follicular fluid tended to be higher in Meishan follicles.
M. G. Hunter, C. Biggs, G. R. Foxcroft, A. S. McNeilly and J. E. Tilton
Attainment of puberty, cycle lengths, ovulation rate and endocrinology during the periovulatory period were studied in Meishan (MS) and European Large-White hybrid (LW) gilts. The mean age at onset of puberty of 115 days in MS (n = 20) gilts was younger (P < 0.001) than the 235 days in LW (n = 23). In the MS population studied, ovulation rate was not different (P > 0.1) during the third and fourth oestrous cycles, nor were there differences (P > 0.1) in the mean cycle length over the first three cycles. Overall changes in plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH) and oestradiol did not differ significantly (P > 0.1) between the breeds (MS, n = 6; LW, n = 5) during the periovulatory period, but plasma inhibin concentrations were significantly (P < 0.05) higher in the MS. The time intervals from the oestradiol peak concentration and the onset of the LH surge until the onset of behavioural oestrus were significantly different (P < 0.005) between the breeds, with oestrus occurring earlier in the MS. However, no difference (P > 0.1) was found between the groups when the intervals from the peak oestradiol concentration to the onset of the LH surge were compared. These results indicate differences between the breeds, particularly in terms of the age of attainment of puberty and the timing of the onset of behavioural oestrus relative to the oestradiol and LH surges.