The roles of photoperiod and nutrition in the seasonal increases in growthand insulin-like growth factor-1 secretion in male red deer

Young male red deer follow a seasonal growth pattern that can be shed by al tering the photoperiod they experience. An increase in photoperiod to 16 h of light per day (16L: 8D) during winter advances the onset of rapid growth and high food intake that normally commences in spring. These changes are associated with increased growth hormone (GH) and insulin-like growth facto r-1 (IGF-1) secretion. The GH/IGF-1 axis is acutely sensitive to the level of nutrition and the relative roles of photoperiod and nutrition in determi ning the spring IGF-1 rise is unknown. The present experiment set out to ex amine this by exposing two groups of deer (no. = 8 per group) to a photoper iod shift during their 1st year of life (16L: 8D from 2 June), designed to cause accelerated growth and increased food intake after approximately 7 we eks. However, after 6 weeks the food intake (pellets containing 11 MJ metab olizable energy and 160 g crude protein per kg dry matter (DM)) of one grou p (LDRES) was clamped, thereby preventing the intake component of the respo nse. The intake of the other group (LDAL) remained ad libitum for a further 12 weeks until 6 October, when the experiment concluded. During the first 6 weeks of 16L: SD, growth rate (118 (s.e.15.4) g/day) and food intake (1.37 (s.e. 0.031) kg DM per head per day) did not differ betw een the groups. Food intake following the clamp in LDRES averaged 1.40 (s.e . 0.015) kg per head per day. The intake of LDAL increased 2 weeks after th e clamp and thereafter was higher than LDRES (P < 0.001). Food intake of LD AL averaged 2.13 (s.e. 0.051) kg during the nutritional clamp period. Growt h rates increased in both groups during the first 3 weeks of the clamp, ave raging 237 (s.e. 25.0) g/day, then growth slowed in LDRES and live weights diverged. Growth rates until the end of the experiment (147 (s.e.23.0) g/da y v. 299 (s.e.12.5) g/day, P < 0.001) and mean live weight over the last 5 weeks of the experiment were lower (P < 0.05) in LDRES than LDAL, weights r eaching 88.3 (s.e.1.86) kg and 97.9 (s.e. 2.74) kg respectively on the fina l sampling date. Metatarsal bone length grew more in LDAL than in LDRES (3. 1 v. 2.2 cm, s.e.d. = 0.23, P < 0.01). Prior to the nutritional clamp, mean plasma prolactin and IGF-1 concentrations increased at 3 and 6 weeks after 16L: SD respectively, in both groups. Prolactin concentrations were lower in LDRES than LDAL on two occasions, at weeks 3 and 7 after the onset of th e nutritional clamp, and IGF-1 concentrations were lower in LDRES than LDAL (676 v. 872 ng/ml, s.e.d. = 73.8, P < 0.05) over the last 7 weeks of sampl ing. In summary, a photoperiodically driven increase in IGF-1 occurred even when the usual associated increase in food intake was prevented. This indicates that the seasonal IGF-1 rise in red deer is not a consequence of the incre ased food intake, although the latter appears necessary to maintain elevate d IGF-1 concentrations. The rise in IGF-1 may therefore be considered as a component of the photoperiodically entrained seasonal drive to grow, and th e increase in food intake a response to satisfy the increased energy demand .