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.
Suttie
J M
JM
Institute of Arctic Biology, University of Alaska, Fairbanks 99775.
White
R G
RG
Breier
B H
BH
Gluckman
P D
PD
D000818:Animals; D004435:Eating; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008027:Light; D008297:Male; D010507:Periodicity; D012053:Reindeer / Q000033:anatomy & histology / Q000097:blood*; D012621:Seasons; D013737:Testis / Q000033:anatomy & histology; D015430:Weight Gain
D007334:Insulin-Like Growth Factor I