GH is secreted in a pulsatile manner, the pattern of which plays an important role in the regulation of growth and metabolism. Sex steroids are also known to participate in metabolic regulation. The present study was undertaken to elucidate the relationship between changes in GH pulsatility and metabolic transition during the estrous cycle in goats. From ovariectomized (OVX) and intact females in the early luteal, late luteal, and follicular phases, blood samples were taken every 15 min for 24 h, and plasma GH was measured by RIA. In the early luteal phase, GH was secreted in a distinct pulsatile manner, the pattern of which was similar to that in OVX goats, whereas the GH pulse frequency, amplitude, and area under the curve (AUC) were decreased in the late luteal phase. In the follicular phase, the GH pulse frequency, amplitude, and AUC were significantly larger than those in the late luteal phase. The regularity of GH pulsatility was highest and lowest in the early and late luteal phases, respectively. Both IGF-I and free fatty acid levels in the plasma were higher in the follicular than the luteal phase. Subcutaneous injection of estradiol to OVX goats increased the GH pulse amplitude and AUC, whereas the implantation of progesterone for 5 d decreased those parameters. These results suggest that the pulsatile pattern of GH secretion in goats varies with sex steroid levels and thereby affects IGF-I secretion and lipolysis during the estrous cycle.

The present study was conducted in order to characterize the secretory pattern of GH in the Shiba goat, a native Japanese miniature goat, and to examine the effects of castration and KP102, a GH secretagogue, on this pattern. Blood samples were taken from an indwelling jugular catheter every 15 min for 24 h, and plasma GH was measured by radioimmunoassay. In intact males, GH was secreted in a pulsatile manner with very regular 5-h periodicity, which consisted of a distinctive GH pulse and a trough of virtually no GH secretion. Postpubertal castration increased the height and decreased the width of GH pulses, though it did not affect the interpulse interval and area under the curve. Modification of the shape of each GH pulse by testicular androgen might play a role in the expression of GH action in the male. KP102 (10 microg/kg, i.v.) immediately induced a robust GH pulse, which was followed by a spontaneous GH pulse of normal characteristics at regular intervals, suggesting that the clock generating GH pulses was reset by KP102. From these observations, we concluded that the Shiba goat is a very suitable experimental model for elucidating the mechanisms underlying GH pulse generation, and in particular, the involvement of androgen and GH secretagogues.

Background/Aims: Growth hormone (GH) is secreted in pulsatile fashion, but the involvement of neuropeptides in the generation of GH pulses is not fully understood. The present study was conducted to elucidate the relationship between GH pulses and neuropeptide levels in the cerebrospinal fluid (CSF) of the third ventricle in ovariectomized goats. Methods: CSF and plasma samples were collected every 15 min. Levels of plasma GH and profiles of GH-releasing hormone (GHRH), somatostatin (SRIH) and neuropeptide Y (NPY) in the CSF were measured by radioimmunoassay. Pulse/trough characteristics and correlations were assessed by the ULTRA algorithm, cross-correlation analysis and approximate entropy test. Results: The periodicity of GH pulses was 2.20 h. Although most GH pulses were associated with GHRH peaks, there was no correlation between GH and GHRH or GH and SRIH. NPY levels in the CSF fluctuated episodically at 2.03-hour intervals. GH pulse peaks occurred 0–30 min after NPY troughs, and there was a significant negative cross-correlation and negative synchronicity between GH and NPY profiles. In addition, intracerobroventricular infusion of NPY suppressed GH secretagogue (KP102)-induced GH release. Conclusion: The periodic decrease in NPY may be involved in the generation of GH pulsatility in goats.