PHOTOPERIOD ASSOCIATED CHANGES IN INSULIN-LIKE GROWTH FACTOR-I IN REINDEER

Insulin-like growth factor-I (IGF-I) concentrations were measured in the plasma of reindeer calves exposed to a manipulated photoperiod, indoors, of either 16 h light followed by 8 h dark each day (16L:8D) (n = 3) or 8L:16D (n = 3) from about the autumnal to the vernal equinox, to determine whether the seasonal growth spurt normally seen in spring is associated with a photoperiod-induced rise in IGF-I. A high quality concentrate diet was available ad libitum, and individual food intake was recorded daily. The animals were weighed, bled, and the diameters of their testes were measured every 2 weeks. Plasma samples were assayed for IGF-I by RIA. Six to 8 weeks after the start of the study those calves exposed to 16L:8D showed a significant increase in plasma IGF-I concentration, which was maintained until the close 24 weeks after the start. In contrast, IGF-I plasma concentrations in those calves exposed to a day length of 8L:16D did not significantly alter during the study. The elevated IGF-I in the 16L:8D group was associated with rapid weight gain, higher food intake, and decreased testes size compared with the 8L:16D group. We have shown that the seasonal growth spurt is preceded by an elevation in plasma IGF-I concentration. Further, this elevation in IGF-I is day length dependent. This is the first account of any growth factor secretion being regulated by photoperiod.