Pulsatile secretion pattern of growth hormone during the luteal phase and mid-anoestrus in beagle bitches

Abstract

The pulsatile secretion pattern of growth hormone was investigated during four stages of the luteal phase and during mid-anoestrus in six cyclic beagle bitches. Plasma samples were obtained via jugular venepuncture at 10 min intervals for 12 h at 19 +/- 2 (mean +/- SEM; luteal phase 1), 38 +/- 2 (luteal phase 2), 57 +/- 2 (luteal phase 3), 78 +/- 2 (luteal phase 4) and 142 +/- 4 days (mid-anoestrus) after ovulation. During all stages, growth hormone was secreted in a pulsatile fashion. The mean basal plasma growth hormone concentration during luteal phase 1 (2.2 +/- 0.3 microgram l(-1)) was significantly higher than that during luteal phase 4 (1.5 +/- 0.1 microgram l(-1)) and mid-anoestrus (1.4 +/- 0.2 microgram l(-1)). The mean area under the curve (AUC) above zero during luteal phase 1 (27.3 +/- 2.7 microgram l(-1) in 12 h) tended to be higher than that during luteal phase 4 (20.8 +/- 1.8 microgram l(-1) in 12 h) and mid-anoestrus (19.2 +/- 2.5 microgram l(-1) in 12 h). In contrast, the mean AUCs above the baseline during luteal phase 1 (1.1 +/- 0.5 microgram l(-1) in 12 h) and luteal phase 2 (1.2 +/- 0.5 microgram l(-1) in 12 h) were significantly lower than that during luteal phase 4 (2.8 +/- 0.5 microgram l(-1) in 12 h). In conclusion, the pulsatile secretion pattern of growth hormone changes during the luteal phase in healthy cyclic bitches: basal growth hormone secretion is higher and less growth hormone is secreted in pulses during stages in which the plasma progesterone concentration is high. It is hypothesized that this change is caused by a partial suppression of pituitary growth hormone release by progesterone-induced growth hormone production in the mammary gland. The progesterone-induced production of growth hormone in the mammary gland may promote the physiological proliferation and differentiation of mammary gland tissue during the luteal phase of the bitch by local autocrine-paracrine effects. In addition, progesterone-induced mammary growth hormone production may exert endocrine effects, such as hyperplastic changes in the uterine epithelium and insulin resistance.

Effects of the dopamine agonist cabergoline on the pulsatile and TRH-induced secretion of prolactin, LH, and testosterone in male beagle dogs

“…This and the complete lack of prolactin pulses may indicate that the prolactin secretion before cabergoline treatment was really basal. In contrast to another study [6], in which occasional distinct elevations in prolactin concentration in individual male dogs of different breeds were reported in a time window of 2.5 h, we identified in beagle dogs just a smoothly oscillating baseline without significant pulses over 6 h. In beagle bitches some prolactin pulses could be identified even in anoestrus, the period of lowest basal prolactin secretion in female dogs [42].…”

Cyclic Morphological Changes in the Beagle Mammary Gland

“…In this retrospective study, serum hormones were not measured, and estrous status (day of ovulation or previous cycle) prior to the animals being euthanized was unknown. Although hormone levels and day of ovulation were unknown, from a pure pragmatic perspective to the toxicologic pathologist, the four different morphological changes may be roughly correlated with the four luteal phases based on levels of progesterone and prolactin (Kooistra, et al 2000;Kooistra and Okkens 2001;Kooistra and Okkens 2002). Beyond the morphological changes, it is important to view the canine mammary gland in diestrus as an endocrine organ (Peaker 1995;Selman et al 1994).…”

Treatment of growth hormone excess in dogs with the progesterone receptor antagonist aglépristone

“…Blood samples for determination of the plasma concentrations of GH and IGF-I were collected before MPA treatment, at days À9, À8, À7, À5, À3, À2, À1, and 0 (=immediately before aglépristone treatment and after MPA treatment for over 1 year), at days 1, 3,5,7,8,11,13,15,18,20,22, and 25 (=during aglépristone treatment), and at days 46 and 60 (=3.5 and 5.5 weeks after the last aglépristone treatment). On days of treatment (MPA or aglépristone), blood samples were collected prior to the drug administration.…”

“…In dogs, endogenous progesterone secreted during the luteal phase of the estrous cycle or exogenous progestins such as medroxyprogesterone acetate (MPA) used for estrus prevention may promote www.journals.elsevierhealth.com/periodicals/the Theriogenology 66 (2006) [797][798][799][800][801][802][803] hypersecretion of GH from foci of hyperplastic ductular epithelium of the mammary gland [2][3][4]. In contrast to the pulsatile secretion pattern of GH in healthy dogs [5][6][7], the plasma GH profile in bitches with progestininduced acromegaly is not pulsatile [8]. In addition, progestin-induced GH hypersecretion cannot be stimulated with GH-releasing hormone (GHRH) and aadrenergic agonists, nor can it be inhibited by somatostatin [8,9].…”