UC Santa Cruz

Animals that fast depend on mobilizing lipid stores to power metabolism. Northern elephant seals (Mirounga angustirostris) incorporate extended fasting into several life history stages: post weaning development, molting, breeding and lactation. The mobilization and subsequent utilization of lipid stores could have consequences for the current and future survival and reproduction. Despite the importance of lipid metabolism to elephant seals little is known about the regulation of lipid mobilization and the physiological drivers of milk lipid content. This thesis focused on three aspects of lipid reserve rationing in adult female elephant seals. Chapter 1 investigated the mobilization of specific fatty acids from blubber and the partitioning of fatty acids between maternal metabolism and milk synthesis in northern elephant seals. In Chapter 2 I examined the hormonal regulation of lipid mobilization and milk lipid content and in Chapter 3 I investigated the activity of lipolytic enzymes and their relationship to milk lipid content. To examine these topics, fatty acid signatures were compared between the blubber and milk in early and late lactation. Additionally, lipid metabolites and milk lipid content were assessed in relationship to circulating levels of growth hormone, cortisol and insulin. Milk lipid content was also assessed relative to the activity of two lipolytic enzymes in theblubber, hormone sensitive lipase and adipose triglyceride lipase. Results indicate that elephant seals mobilize specific fatty acids in a similar fashion to many other mammals and birds. Additionally, the milk is enriched in long chain monounsaturated fatty acids in late lactation. Mass and adiposity were significant (p<0.05) predictors of milk lipid content. Other important predictors of milk lipid content were circulating non-esterified fatty acid and triglyceride levels, cortisol and insulin. Unexpectedly I found that growth hormone was uncoupled from both circulating lipid metabolite concentrations and milk lipid content. Hormone sensitive lipase was found to be negligible in northern elephant seal blubber, another unexpected result. Adipose triglyceride lipase protein content was higher in late lactation relative to early lactation, but activity levels were stable across lactation and unrelated to milk lipid content.

Molting seals regulate lipolysis differently than lactating seals, with insulin mediating an even greater release of lipid stores in lactating females, to facilitate availability for milk production. There was a more positive relationship between triglycerides and milk lipid content than expected, but there was strong evidence supporting the hypothesis that milk lipid content is regulated at the level of mobilization of milk precursors from lipid stores. Contrary to previous hypotheses, hormone sensitive lipase was not the primary lipolytic enzyme and adipose triglyceride lipase assumed a more prominent role in lipid mobilization. Additionally, the specific regulation of these enzymes may differ among age classes and species. Despite their phenomenal lipid mobilization ability, mechanisms of thelipid mobilization of specific fatty acids are conserved among northern elephant seals and other mammals. Conversely, the hormonal and enzymatic regulation of lipid mobilization exhibit differences between elephant seals and other mammals.