Summary and Conclusions

Overall, fasting elephant seal pups conformed to the general mammalian model of fuel homeostasis for fasting animals in which protein conservation is paralleled by increased mobilization and utilization of lipid. It is surprising that these same adaptations are also seen in suckling pups just prior to weaning. However, when we examine the nutritional life history of elephant seals, we find that fat is the major energy source throughout development whether individuals are consuming high fat milk, high fat fish, or body fat stores. Since elephant seals consume only a minimal amount of dietary carbohydrate, all glucose must be made from precursors derived from the diet or from tissue stores. To date, changes in fuel distribution, fuel turnover rates, and plasma metabolites confirm that lipolysis is the major source of energy—and possibly glucose—in northern elephant seals.

The insulin and glucose studies show that nursing elephant seals are preadapted for fasting. The lack of pancreatic islet response and the impaired glucose clearance (K £ 1; fig. 21.2), the tissue insensitivity to insulin (fig. 21.3), and low plasma insulin and low ratio of I:G all contributed to maximizing the release of lipid from adipose tissue and were not a function of age, body fat, or nutritional status in seals. This emphasis on lipid mobilization concurrent with minimal oxidation of glucose and protein suggests that glucose may be synthesized from glycerol while the fatty acids are oxidized to meet the total energy needs of feeding and fasting seals. The important difference between the feeding and fasting state would be the partitioning of amino acids into protein synthesis or into gluconeogenesis.

Despite the fact that insulin is considered essential to protein synthesis in muscle tissue, suckling pups can gain a kilogram of lean tissue daily, even

with plasma insulin levels of less than 12 µU/ml. We suggest that this same anabolic condition exists for young fasting seals who are actively recycling protein and glucose as they reorganize their tissues. The observation that active synthesis and reorganization of tissue protein occurs throughout the postweaning period of fasting and development is supported by the previously mentioned protein turnover studies. Although we do not understand how these seals balance anabolic and catabolic processes during the post-weaning fast, the hormonal data suggest that we reevaluate the importance of insulin and glucagon in fuel regulation in these animals in particular and in other carnivores in general.

If insulin is not important in fuel homeostasis in this species, then other factors may play a more important role in glucose homeostasis. The influence of diving hypoxia on various tissues as well as the brain may change the influence that insulin has on muscle. It has been observed that hypoxia enhances muscle tissue sensitivity to insulin, reducing the amount needed to stimulate tissue growth (King et al. 1987). Diving may also enhance plasma fuel availability because exercise induces a release in epinephrine, which then increases glucose and free fatty acid release (Hamburg, Hendler, and Sherwin 1979). Increases in epinephrine and glucocorticoid plasma levels can also inhibit glucose-stimulated insulin release (Ploug, Galbo, and Richter 1984; Porte, Smith, and Ensinck 1979).

Alternative speculations suggest that insulin may not be necessary for protein synthesis in muscle tissue because seals may have high plasma levels of growth hormone or growth factors. The cellular requirements for energy are also regulated in part by thyroid hormone and glucocorticoids. These hormones are important for phocid molting (Ashwell-Erickson et al. 1986) and could be important to overall fuel homeostasis in seals. However anabolic processes are regulated, the glucose needs of the brain and other glucose obligate tissues must first be met so that seals can survive as diving carnivores who routinely experience apnea, tissue hypoxia, extreme exercise (as in deep diving), and periodic fasting. While fuel homeostasis in fasting seals is consistent with the general mammalian model of fasting, adipose tissue may be far more important to the elephant seal for glucose homeostasis than it is in other mammals.