Date of Award

12-2008

Level of Access

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology and Environmental Sciences

Advisor

Frederick Servello

Second Committee Member

Robert Causey

Third Committee Member

Clifford Rosen

Abstract

Mechanical strain is an essential anabolic stimulus for bone. Skeletal unloading causes rapid, marked bone loss. Hibernating bears uniquely avoid net bone loss, and maintain eucalcemia, during approximately six months of immobility and anuria. This study's first goal was to elucidate the mechanism by which the hibernating black bear (Ursus americanus) maintains eucalcemia, by developing a biochemical and radiographic picture of bone metabolism in active and hibernating, free-ranging, female black bears. Four alternative hypotheses were evaluated: High Turnover, Resorption Balance, Bone Metabolic Suppression, and Mechanostat Suppression. The second goal was to compare lactating and non-lactating, hibernating bears to understand how skeletal demands of milk production, effects of unloading, and the need to maintain eucalcemia, are balanced. Bears were sampled from three areas in Maine, during spring and autumn active seasons and winter hibernation, 2005 through 2007. During spring and winter, serum markers of bone turnover and bone metabolic hormones were measured (16 to 50 bears per season), including: calcium corrected for albumin, phosphate, creatinine, the bone formation marker bone specific alkaline phosphatase (BSAP), bone resorption markers Cterminal telopeptide of type I collagen (CTX) and tartrate-resistant acid phosphatase (TRAcP), parathyroid hormone (PTH), insulin-like growth factor- I (IGF-I), leptin, 25-hydroxy vitamin D [25(OH)D], and calcitriol. Metacarpal cortical thickness, measured by digital x-ray radiogrammetry, was compared between autumn (60 bears) and winter (79 bears). During hibernation, bone turnover was persistent, but suppressed. Cortical thickness was greater in autumn than winter in younger bears, but older bears showed no difference between seasons. The weight of evidence supports Mechanostat Suppression: the hibernating ursine skeleton may perceive that it is loaded, when it is unloaded. Leptin positively correlated with BSAP in non-lactating bears, and with CTX in lactating bears. Sympathetic nervous system suppression associated with hibernation may prevent unloading-induced bone loss by counteracting leptin's catabolic effect in non-lactating bears. In lactating bears, bone resorption was greater, and PTH suppressed, compared to non-lactating bears. Calcitriol was produced despite anuria. Leptin and calcitriol may facilitate milk production. Correlations of 25(OH)D and calcitriol with bone turnover markers suggest that vitamin D may suppress bone resorption during hibernation.

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