Down-regulation in the insulin-like growth factor (IGF) axis during hibernation in the golden-mantled ground squirrel, Spermophilus lateralis: IGF-I and the IGF-binding proteins (IGFBPs)

The golden‐mantled ground squirrel, Spermophilus lateralis, undergoes a profound winter hibernation that represents, among other changes, a prolonged period of starvation. In addition to dramatic metabolic and other physiological adaptations during hibernation which serve to reduce fuel energy expenditure, we have hypothesized that there may also be significant changes in the endocrine axis that regulates energetically‐expensive somatic growth. As compared with euthermic, non‐hibernating controls, hibernating S. lateralis were found to have 75%‐reduced serum concentrations of insulin‐like growth factor‐I (IGF‐I; from ∼625 to ∼150 ng/ml in both females and males, P < 0.05). While IGFBP‐3 was the predominant IGFBP in serum of the euthermic controls, its levels were reduced to a similar degree in serum from the hibernating animals. IGFBP‐4 was present at relatively low levels in the euthermic controls, and was reduced to undetectable levels in hibernating animals. Surprisingly, there was no IGFBP detectable in the 30 kDa range in either euthermic or hibernating S. lateralis, suggesting that IGFBP‐1 does not play a role in hibernation‐related changes in the IGF axis. In accordance with these endocrine changes, when serum from hibernating S. lateralis was added to cartilage explant cultures (at a 5% v/v concentration), it exhibited no ability to alter 35S‐proteoglycan synthetic rate, whereas serum from the euthermic squirrels significantly stimulated synthetic activity by 2‐fold. These results suggest that part of hibernation adaptation in S. lateralis includes down‐regulation in the growth‐regulatory IGF axis. J. Exp. Zool. 289:66–73, 2001. © 2001 Wiley‐Liss, Inc.