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Photoperiod Associated Changes in Insulin-Like Growth Factor-I in Reindeer^*

Institute of Arctic Biology, University of Alaska Fairbanks, Alaska 99775
MAFTech, Invermay Agricultural Centre Mosgiel, New Zealand
Department of Pediatrics, University of Auckland Medical School Auckland, New Zealand

Address all correspondence and requests for reprints to: Dr. James M. Suttie, MAFTech, Invermay Agricultural Cent, Private Bag, Mosgiel, New Zealand.

Abstract

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.

Footnotes

^* This study was funded by the Institute of Arctic Biology, the Ministry of Agriculture and Fisheries, New Zealand, and the Medical Research Council of New Zealand.

Received March 11, 1991.

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