A novel experimental model was developed in cattle to investigate the requirement for FSH and LH during ovarian follicle growth and development. On Day 5 of the estrous cycle, 7 heifers were each implanted with an osmotic minipump containing a GnRH agonist (GnRHa), Buserelin (release rate, 2.5 micrograms/h). Another 7 heifers served as controls. Each minipump was replaced 28 days later with a second pump, which was left in place for a further 20 days. Blood samples were collected daily throughout the experimental period, and frequent samples were also collected on both days of minipump insertion and at 10 days after insertion of the second pump. The ovaries of all heifers were scanned daily by real-time ultrasonography to monitor follicular dynamics. All controls displayed 2 or 3 waves of FSH and follicular development per estrous cycle during the experiment. Insertion of the first minipump produced a large LH and FSH surge and induced ovulation in all 7 animals. Within 8 days of the start of treatment, serum LH concentrations fell to basal levels; they then remained constant at this level throughout the infusion period, only beginning to recover 4-5 days after the termination of infusion. After the initial increase, FSH returned to basal levels before showing a normal wave that was coincident with the emergence, growth, and regression of a dominant follicle. However, despite the peak levels of FSH, dominant follicles from the next wave failed to grow beyond 7-9 mm; they remained at this size for 3 wk until 3-4 days after insertion of the second minipump, when FSH fell precipitously to reach low levels that were maintained throughout the remainder of the infusion. After this fall in FSH concentrations, these follicles regressed rapidly, and no antral follicles > 4 mm were detected until after the termination of treatment. Thereafter, FSH concentrations increased significantly; the increase was accompanied by the emergence of a follicular wave and development of a dominant follicle, with estrus observed 8-11 days later. In conclusion, this study has demonstrated clearly that in cattle the early stages of follicle development (< or = 4 mm) are not dependent on acute support by gonadotropins. However, FSH is required for further growth of follicles up to 9 mm, while LH pulses are indispensable for follicle development beyond 9 mm in diameter. The model developed in this study should be valuable for studying the control of ovarian follicle development and atresia in vivo.

No abstract

The effect of chronic treatment with a gonadotrophin-releasing hormone agonist (GnRHa) on ovarian function in cattle was investigated by injecting heifers i.m. twice a day with saline, 5 micrograms GnRHa (Buserelin) or 10 micrograms GnRHa (n = 7) for 21 days. Blood samples were taken twice a day during the treatment period, and then three times a day for 7 days and once daily for a further 4 days. Frequent samples were also collected on day 1, day 10 and day 21 of treatment. The ovaries of all heifers were examined daily using real-time ultrasonography throughout the experimental period. No significant differences in the response were observed between two doses of GnRHa. The first GnRHa injection produced a large LH and FSH surge and this acute response was still present by day 21 of treatment, but both the magnitude and duration of response were significantly attenuated (P < 0.01). After an initial increase, LH returned to the basal concentration, which was maintained until the termination of treatment, when concentrations increased significantly, with a preovulatory surge occurring approximately 6 days later. Peripheral FSH concentrations during the oestrous cycle in control animals displayed a pattern of three waves, each of which closely preceded a wave of follicular development. Concentrations of FSH in GnRHa-treated heifers showed a normal pattern for the first wave after the start of treatment. During the next wave, concentrations increased and remained at the peak values until about 4 days after the end of treatment. An additional ovulation was induced in 11 of 14 GnRHa-treated heifers within 2-3 days of the start of treatment, and a significant (P < 0.05) increase in serum progesterone concentrations was detected 2 days later. All GnRHa-treated heifers then showed a normal follicular wave, with the development and regression of a dominant follicle. The dominant follicles from the next wave grew to only 7-9 mm in diameter and remained at this size until the end of treatment, when they resumed growth, ovulated approximately 7 days later and formed corpora lutea. We conclude that chronic treatment of heifers with GnRHa for 3 weeks suppresses pulsatile secretion of LH and blocks the development of dominant follicles beyond 9 mm in diameter, preventing the preovulatory LH surge and ovulation. However, GnRHa did not suppress the secretion of FSH within the 3 week treatment period. The maintenance of the dominant follicles for an extended period should provide an ideal model to study the control of follicular atresia in cattle in vivo.

Mastitis-induced ovarian abnormalities were studied in a field trial. At 1-3 day after calving, > or = 2 parity cows not affected with chronic recurrent mastitis and yielding < 400,000/ml somatic cell count (SCC) individual milk in the previous lactation, were enrolled in the study. Thereafter milk samples were collected three times weekly for 95-100 day for progesterone (P4) assay. Individual P4 profiles were used to monitor ovarian cyclicity. When mastitis was diagnosed in the first 80 day post-partum (pp), clinical signs were recorded and scored, and aseptic milk samples were taken to identify the mastitis pathogens. Depending on the isolated pathogens the cows were blocked into one of the three sub-groups affected by either Gram-positive (GP), or Gram-negative (GN) bacteria, or of those with no detected pathogens (NDP). Cows suffering from any type of mastitis between days 15 and 28 (n = 27) showed a delay in the onset of ovarian cyclicity, and estrus was postponed compared to cows affected during the first 14 day pp (n = 59) and controls (n = 175) (38.6 +/- 2.3 vs 33.4 +/- 2.1 and 32.0 +/- 1.0 day, respectively, for onset of ovarian cyclicity and 90.7 +/- 2.5 vs 80.2 +/- 2.8 and 83.9 +/- 2.1 day, respectively, for estrus; both p < 0.05). The percentage of cows ovulating by day 28 was lower in those affected by mastitis between days 14 and 28 compared to cows between days 1 and 14 and controls (22.2% vs 47.5 and 50.3%, respectively; p < 0.05). A significantly higher rate of premature luteolysis was observed in GN + NDP compared to GP mastitis and healthy cows (46.7% vs 8.3 and 2.0%, respectively; p < 0.001). If the mastitis outbreak occurred during the follicular phase, the duration of this cycle segment was lengthened in GN + NDP mastitis compared to GP mastitis and healthy cows (10.8 +/- 0.9 vs 7.9 +/- 0.1 and 7.2 +/- 0.1, respectively; p < 0.001). The results indicate that mastitis can affect the resumption of ovarian activity in pp dairy cows. Mastitis may also impair reproduction also in cyclic cows: this effect can be the consequence of premature luteolysis or a prolonged follicular phase.

The galactopoietic effect of daily injections, for five day periods, of growth hormone was examined by measuring milk yield, mammary blood flow and arteriovenous differences of glucose and amino acids on 12 occasions in four goats. In 10 periods there were marked increases (mean 18.1%) in mammary blood flow (8 statistically significant) and less-marked increases (mean 8.0%) in milk yield (6 statistically significant). On 8 of the occasions on which it was measured the maximum blood plasma growth hormone concentration was increased to more than 8 ng/ml. There were no statistically significant changes in mammary arteriovenous concentration differences of glucose or amino acids in response to growth hormone injections. It is suggested that, contrary to the usual situation in which the rate of mammary blood flow appears to be regulated by the metabolic activity of the gland, the galactopoietic response to growth hormone may be a consequence of elevated blood flow, which increases the supply to the gland of rate-limiting metabolic substrates.