Reproductive endocrinology of the dog : effects of medical and surgical intervention

Abstract

Relationships between different reproductive hormones were studied in the dog 1) around the time of ovulation, 2) during spontaneous and aglepristone-induced parturition, 3) before and after gonadectomy in both males and females, and 4) before and after chemical castration with the GnRH-agonist deslorelin in males. Additionally, the effect of trilostane, a competitive inhibitor of 3β-hydroxysteroid dehydrogenase, on progesterone production was studied. The most important conclusions from these studies were: - the plasma concentrations of LH and FSH are differentially regulated; - oestradiol-17β probably exerts a positive feedback effect on preovulatory LH release, as in other species; - the start of the preovulatory LH surge is associated with a slight increase in plasma progesterone concentration; - shortly after the LH surge the plasma progesterone concentration remained stable for a variable period before rising sharply. This could reflect variation in the interval between the preovulatory LH surge and ovulation, suggesting that measurement of the preovulatory LH surge is less reliable than progesterone for determining the exact time of ovulation; - aglepristone is an effective and safe drug for the induction of parturition in dogs that are 58 days pregnant and carry 3 or more pups. Parturition occurred 41 h (mean; range 32 h – 56 h), after the first dose of aglepristone. The progress of whelping and the survival and growth of pups were similar in the control and induced group. More research is necessary to determine the potential use of aglepristone in prolonged one- and two-pup pregnancies. - aglepristone-induced parturition is associated with incomplete luteolysis, an altered plasma PGFM profile, and elevated postpartum plasma cortisol concentrations; - GnRH administration in intact male and female anoestrous dogs induces increased plasma concentrations of LH, FSH, oestradiol and testosterone; - gonadectomy results in increased plasma concentrations of LH and FSH; - the ranges of the plasma oestradiol concentrations in anoestrous and ovariectomized bitches overlap; - the following parameters appear to be appropriate to verify the presence or absence of functional gonadal tissue - the basal plasma FSH concentration in bitches and male dogs; - the basal plasma testosterone concentration in male dogs; - the plasma oestradiol concentration 120 min after GnRH administration in bitches; - chemical castration of male dogs, caused by use of a deslorelin implant, induces decreased plasma concentrations of LH and FSH and, as a consequence, lowers plasma oestradiol and testosterone concentrations to values similar to those after surgical castration; - administration of the GnRH analogue buserelin in chemically castrated male dogs, at 4.5 months after administration of a deslorelin implant, induced a significant increase of the plasma LH concentration, indicating that the pituitary gonadotrophs were not completely desensitized in all dogs; - trilostane is effective in decreasing plasma progesterone concentration during the luteal phase, but the treatment regimen used in this study resulted in less clear-cut inhibition of ovarian steroidogenesis than is probably needed for abortion. Increasing the dose of trilostane is probably not a good option, for the adrenal reserve capacity had decreased more than what is considered to be safe in dogs.