C - Michigan Technological University

Chapter Seven — ConclusionThe long-term goal of this study is to identify the biological mechanism throughwhich bears maintain bone despite the prolonged mechanical unloading of hibernation.This mechanism could bring insight into new signaling pathway targets for drugdevelopment in osteoporosis prevention. This study was broken down into three shorttermAims. The first Aim was to determine whether serum markers of bone turnoverconfirmed previously published histomorphometric data [18, 20] to suggest thatosteoclast and osteoblast activities decrease dramatically during hibernation. The boneformation marker OCN and the resorption marker ICTP both increase during hibernation[34], in apparent conflict with findings in histological studies. However, these turnovermarkers accumulate in patients with reduced renal function, and hibernating bears canbe compared to renal patients because they do not urinate during hibernation.Therefore, serum levels of BSALP, an indicator of osteoblast activity, and TRACP, anindicator of osteoclast number, were quantified in active and hibernating American blackbears. Neither of these serum markers is affected by renal function. The hypothesis wasthat levels of BSALP and TRACP in serum from hibernating bears would be lowercompared to levels in active bear serum.In support of the first hypothesis, this study found that serum BSALP and TRACPlevels were significantly decreased during hibernation. These data are mentioned inChapters 2 and 3. In Chapter 2, positive correlations between the energy homeostasishormone adiponectin and serum turnover markers were discussed. Such correlationssuggest that reduced levels of adiponectin during hibernation may be partiallyresponsible for suppressing bone metabolism at this time of low energy expenditure.This finding supports previous studies which demonstrate a positive relationshipbetween adiponectin and the activities of osteoblasts and osteoclasts [202, 203, 206].Chapter 3 discusses an inverse correlation between serum levels of the energyhomeostasis hormone NPY and bone turnover markers. These relationships suggestthat raised levels of NPY may be partially responsible for suppressing bone metabolismduring hibernation in bears. These data support a previous study which demonstrates apossible relationship between high serum NPY and adynamic bone disease [248], a132