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

The pulsatile secretion pattern of growth hormone was investigated during f our stages of the luteal phase and during mid-anoestrus in six cyclic beagl e 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 (lu teal phase 2), 57 +/- 2 (luteal phase 3), 78 +/- 2 (luteal phase 4) and 142 +/- 4 days (mid-anoestrus) after ovulation. During all stages, growth horm one was secreted in a pulsatile fashion. The mean basal plasma growth hormo ne concentration during luteal phase 1 (2.2 +/- 0.3 mu g l(-1)) was signifi cantly higher than that during luteal phase 4 (1.5 +/- 0.1 mu gl(-1)) and m id-anoestrus (1.4 +/- 0.2 mu g l(-1)). The mean area under the curve (AUC) above zero during luteal phase 1 (27.3 +/- 2.7 mu g l(-1) in 12 h) tended t o be higher than that during luteal phase 4 (20.8 +/- 1.8 mu g l(-1) in 12 h) and mid-anoestrus (19.2 +/- 2.5 mu g l(-1) in 12 h). In contrast, the me an AUCs above the baseline during luteal phase 1 (1.1 +/- 0.5 mu g l(-1) in 12 h) and luteal phase 2 (1.2 +/- 0.5 mu g l(-1) in 12 h) were significant ly lower than that during luteal phase 4 (2.8 +/- 0.5 mu g l(-1) in 12 h). In conclusion, the pulsatile secretion pattern of growth hormone changes du ring the luteal phase in healthy cyclic bitches: basal growth hormone secre tion 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 hor mone release by progesterone-induced growth hormone production in the mamma ry gland. The progesterone-induced production of growth hormone in the mamm ary gland may promote the physiological proliferation and differentiation o f mammary gland tissue during the luteal phase of the bitch by local autocr ine-paracrine effects. In addition, progesterone-induced mammary growth hor mone production may exert endocrine effects, such as hyperplastic changes i n the uterine epithelium and insulin resistance.