Adrenal axis and hypogonadotropism in the growth-restricted female lamb

Growth retardation induced by dietary restriction in the lamb results in a decrease in LH pulse frequency and therefore in delayed puberty. Increased circulating cortisol levels have been associated with nutritional restriction in a variety of species. The current study tested the hypothesis that hyperactivity of the adrenal axis sustains hypogonadotropism in the growth-restricted lamb. Our approach was to compare the patterns and levels of circulating cortisol and LH in ovariectomized, growth-restricted (n = 8) and ad libitum-fed (n = 6) lambs. At 37 wk of age, after the growth-restricted lambs had been on the reduced diet for 31 wk, basal cortisol levels were determined hourly for 31 h. In addition, during this period, pulsatile LH and cortisol release was determined during a 4-h period (samples every 12 min). Finally, the cortisol response to a physiologic ACTH stimulus and an audiovisual stimulus (barking dog for 2 min) was determined in frequent samples collected during the last 5 h of the 31-h period. As expected, growth-restricted lambs exhibited a low LH pulse frequency (0.25 +/- 0.10 pulses/h) compared with ad libitum-fed lambs (1.37 +/- 0.07 pulses/h). No diurnal cortisol rhythm was observed in either group, and a similar cortisol pulse frequency occurred in the two groups (1.00 +/- 0.07 pulses/h in growth-restricted lambs and 1.05 +/- 0.10 pulses/h in ad libitum-fed lambs). There was no significant difference between the groups in cortisol pulse amplitude. ACTH administration (i.v.) induced a similar cortisol pulse in 4 of 8 growth-restricted lambs and in 5 of 6 ad libitum-fed lambs. An endogenous cortisol pulse occurred just before ACTH administration in the other lambs. Finally, neither group of lambs showed an obvious cortisol rise in response to the barking dog. These results indicate that hypogonadotropism is not associated with adrenal gland hyperactivity in the growth-restricted lamb