Biology of gonadotrophin secretion in adult and prepubertal female dogs.

Studies in the female domestic dog demonstrate that luteinizing hormone (LH) and follicle-stimulating hormone (FSH) have secretion patterns that are pulsatile, are inhibited by oestradiol during pro-oestrus and surge to maximal values before ovulation. Studies in ovariectomized bitches suggest that the periovulatory surge is triggered by a preovulatory decline at late pro-oestrus in the oestrogen:progesterone ratio. During the 3-8-month non-seasonal anoestrus, FSH concentrations usually are 50-100% of those of the periovulatory peak, whereas LH concentrations are only 10-20% of peak values. In ovariectomized bitches FSH concentrations are often 5-10 times preovulatory peak values, whereas LH concentrations are only the same as, or double, peak values. Increased LH concentration and pulse frequency are associated with the termination of anoestrus. Treatment with gonadotrophin-releasing hormone (GnRH) pulses or infusions of GnRH agonists can induce fertile oestrus in early anoestrous bitches, as can treatment with a dopamine agonist, presumably by suppression of prolactin secretion. Between 4 months of age and pubertal pro-oestrus at 8-12 months of age, serum concentrations of FSH and LH are similar to those in adult anoestrus, and are suppressed during chronic infusion of GnRH agonist. The latter resulted in a reversible inhibition of puberty during 1 year of treatment. Studies in vivo have shown that LH and prolactin are luteotrophic throughout most of the luteal phase. LH stimulated progesterone synthesis by bitch luteal cells in vitro in the presence or absence of stimulant factors or steroid precursors present in serum.

Biology and endocrinology of ovulation, pregnancy and parturition in the dog.

There is considerable variation among bitches in commonly encountered intervals between cycles (5-12 months), durations of anoestrus (1-8 months), durations of follicular phase pro-oestrus (3-21 days) and periovulatory oestrous behavior (3-21 days), intervals from preovulatory LH surge to oestrus onset (-2 to 5 days), and intervals from fertile mating to parturition (57-68 days). The extent of variation within bitches ranges from slight to great. However, there appears to be very little variation in the intervals from LH surge to ovulation (2 days), to post-ovulatory oocyte maturation (approximately 4 days), to implantation (approximately 18 days), to selected developmental stages of pregnancy, or to parturition (64-66 days). There are no tests diagnostic of early pregnancy. The onset times of persistent pregnancy-specific changes have been estimated, including radio-opaque fetal details (Day 46), elevated blood prolactin values (Day 35), elevated blood relaxin values (Day 25), echogenic heart beats (Day 24) and embryonic vesicles (Day 19), and potentially palpable uterine enlargements (Day 21). As in humans, there is an anaemia of pregnancy involving a 30% reduction in haematocrit and an increased incidence of insulin resistance during the second half of gestation. Ovarian progesterone is required throughout pregnancy. LH and prolactin are luteotrophic in the pregnant bitch as well as during the 2-month luteal phase of the non-pregnant bitch. Parturition follows a luteolysis which occurs during an increase in prostaglandin F-2 alpha that begins 36 h pre partum. Factors regulating the duration of anoestrus are not known but termination of anoestrus is associated with increased pulsatile secretion of LH.

Influence of hypophysectomy on the lifespan of the corpus luteum in the cyclic dog.

Five

dogs were hypophysectomized on Day 4 and 9 on Day 18. Prolactin and LH stimulation tests showed that hypophysectomy was complete in 6 dogs only. In these dogs, the progesterone concentration was measured in the peripheral blood; it decreased sharply immediately after surgery. It regained normal values in 3 of the 4 dogs hypophysectomized on Day 4, and remained low in the 2 dogs hypophysectomized on Day 18. This indicates that, in the dog, luteal function is autonomous during a certain period. The luteal period of the 3 dogs hypophysectomized on Day 4 was shorter than that of control animals, although the time of onset of luteal regression appeared to be similar. This indicates that pituitary luteotrophic support is required during the second part of the oestrous cycle of the dog.

Effects of hypophysectomy and of LH administration on luteal phase plasma progesterone levels in the beagle bitch.

Luteal-phase progesterone levels in 5 beagle bitches fell to 0·3–0·4 ng/ml within 3–17 days after hypophysectomies on Day 10–50 of the cycle. Plasma progesterone in 6 bitches increased (P < 0·01) within 3 h after LH (100 µg/kg) injection between Days 35 and 55 of the cycle. Normal luteal function in the bitch appears to require continuous pituitary trophic support perhaps involving LH.