Little is known about the mechanisms that allow capital breeders to rapidly mobilize large amounts of body reserves. Northern elephant seals Mirounga angustirostris) utilize fat reserves for maternal metabolism and to create high fat milk for the pup. Hormone-sensitive lipase (HSL) has been hypothesized to be an important lipolytic enzyme in fasting seals, but the activity of HSL and adipose triglyceride lipase (ATGL) has not been quantified in fasting adult seals, nor has their relationship to milk lipid content been assessed. Blubber and milk samples were obtained from 18 early lactation and 19 late lactation females, as well as blubber from five early and five late molting female seals. Blubber lipolytic activity was assessed with radiometric assays. HSL activity was negligible in seal blubber at all fasting stages. Total triglyceride lipase activity was stable among early and late lactation and early molt but increased in late molting seals. Relative abundance of ATGL protein increased across fasting, but neither activity nor relative protein levels were related to circulating nonesterified fatty acids or milk lipid content, suggesting the possibility of other regulatory pathways between lipolytic activity and milk lipid content. These results demonstrate that HSL is not the primary lipolytic enzyme in fasting adult female seals and that ATGL contributes more to lipolysis than HSL.

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