| Summary: | Adipose tissue serves a vital role in both thermoregulation and energy storage in marine mammals. Blubber may also function as an endocrine organ, producing adipokine hormones. While adipokines are the subject of intensive study in biomedical systems, their functional roles in wildlife are poorly studied. Capital breeding phocids undergo large changes in adipose reserves during natural fasts creating an ideal system to examine adipokine function. We measured serum concentrations of four adipokines, leptin, adiponectin, resistin and visfatin, in 130 adult male northern elephant seals across their breeding and molting fasts and examined associations with mass, body condition, cortisol, the immune marker interleukin-6 (IL-6), and blood glucose concentrations. Leptin concentrations were highest at the beginning of fasts and showed complex associations with IL-6. Adiponectin varied with body condition and decreased across the fast. It showed a strong negative association with blood glucose, consistent with its insulin-sensitizing role. Resistin concentrations were highest during breeding but did not vary with fasting or body condition. Resistin showed a strong positive association with IL-6, suggesting production by activated macrophages and an important role in regulating immune function. Visfatin concentrations were highest during breeding, increased with fasting, and were strongly associated with cortisol and IL-6. Expression of genes encoding these adipokines was analyzed in blubber samples from a subset of 28 males to better understand the regulation of production and release of these adipokines during fasting. Expression of gene encoding leptin, adiponectin, and resistin shared no relationship with concentrations suggesting post-transcriptional modification or clearance of circulating hormone. However, blubber expression of visfatin, together with body condition, was predictive of serum visfatin concentrations. Together these findings suggest essential regulatory roles for adipose-derived hormones that link ...
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