EFFECTS OF SEASON AND NUTRITION ON GROWTH-HORMONE AND INSULIN-LIKE GROWTH-FACTOR-I IN MALE RED DEER

GH and insulin-like growth factor (IGF)-I are important components of the growth axis. We undertook to determine how plasma levels of these hormones altered with different seasonal and nutritional states in you ng male red deer to provide an insight into how the growth axis change s under these conditions. Growth rate alters dramatically with season in male red deer, providing an opportunity to sample the same animals at two different growth rates within a short period of time. GH was me asured every 15 min for 24 h in the fed state and after a 48-h fast, d uring slow growth in winter (23 June to 16 July). and during rapid gro wth in spring (8 September to 2 October). At the end of each sampling period, the animals were treated with N-methyl-D, L-aspartic acid (NMD A) (5 mg/kg live weight) and sampled for a further 1 h, 45 min. Glucos e and IGF-I were measured hourly during each sampling period. Live wei ght was measured at weekly intervals. GH was secreted in a characteris tic pattern in which pulses tended to occur in rapid succession, terme d a volley, that was separated from the subsequent volley by a period of baseline GH levels, termed a latent period. There were more GH puls es/24 h in the fasted state than in the fed state in winter (12.4 vs. 7.8, standard error of the difference [SED] = 1.07, P < 0.001) and in spring (11.5 vs. 8.8, SED = 1.04, P < 0.05). The increased number of G H pulses in the fasted state could be attributed to a higher number of pulses per volley (winter = 3.7 vs. 2.5, SED = 0.16, P < 0.001; sprin g = 3.1 vs. 2.8, SED = 0.19). Consequently, the volleys were wider in the fasted state than the fed state (winter = 197 min vs. 122 min, SED = 25, P < 0.05; spring = 173 min vs. 154 min, SED = 24, P > 0.05), an d the latent periods between volleys were shorter in the fasted state than the fed state (winter = 175 min vs. 280 min, SED = 14, P < 0.001; spring = 183 min vs. 262 min, SED = 11, P < 0.001). The main differen ces between seasons in the fed state were larger amplitude pulses (12. 4 vs. 8.3 ng/ml, SED = 1.57, P < 0.05) and higher mean GH concentratio ns (4.1 vs. 2.3 ng/ml, SED = 0.44, P < 0.01) in spring than in winter. The number of volleys and the intravolley pulse interval did not chan ge significantly with nutritional state or season. NMDA administration was followed by an increase in GH with higher GH levels found in the fed state than in the fasted state in both seasons. Fed animals also h ad a larger initial increase in GH (until 60 min post NMDA) than faste d animals in spring (P < 0.01). Plasma IGF-I was higher in the fed sta te than the fasted state in both winter (315 vs. 221 ng/ml, SED = 21.0 , P < 0.001) and spring (651 vs. 494 ng/ml, SED = 37.5, P < 0.001) and in the fed state was higher in spring than in winter (SED = 29.1, P < 0.001). Blood glucose was higher in the fed state than fasted state i n winter (6.1 vs. 5.5 mmol/l, SED = 0.07, P < 0.001) and there was a s trong trend toward this same effect in spring although it did not reac h statistical significance (6.0 vs, 5.7 mmol/l, SED = 0.26, P > 0.05). Growth rate in winter at 117 g/day was less than that in spring when 220 g/day was recorded (SED = 36.8, P < 0.05). These results demonstra te that the secretory pattern of GH and plasma IGF-I levels alter in r esponse to changes in season and nutrition. The alterations in respons e to a 48-h fast show that the control of GH and IGF-I secretion may b e rapid and is probably a response to maintain energy balance, whereas alterations with season reflect long term control that underlies the seasonal growth pattern of the animal.