Effects of photoperiod on secretory patterns of growthhormone in adult male goats

摘要

The aim of the present study was to clarify the effect of photoperiod on secretory patterns of growth hormone (GH) in malegoats. Adult male goats were kept at 20°C with an 8-h or 16-h light photoperiod, and secretory patterns of GH secretionwere compared. In addition, plasma profiles of prolactin (PRL), insulin-like growth factor-I (IGF-I) and testosterone (T) werealso examined to characterize GH secretion. GH was secreted in a pulsatile manner. There was no significant difference inpulse frequency between the 8-h and 16-h photoperiods. However, GH pulse amplitude tended to be greater in the groupwith the 16-h photoperiod (P = 0.1), and mean GH concentrations were significantly greater in the 16-h photoperiod(P < 0.05). The GH-releasing response to GH releasing hormone was greater in the 16-h than 8-h photoperiod (P < 0.05).Plasma PRL and IGF-I levels were higher in the 16-h than 8-h photoperiod (P < 0.05). In contrast, plasma T levels were lowerin the 16-h photoperiod (P < 0.05). These results show that a long light photoperiod enhances the secretion of GH as wellas PRL and IGF-I, but reduces plasma T concentrations in male goats.