Adult male northern elephant seals maintain high rates of glucose production during extended breeding fasts

Abstract

Many species undergo natural fasts as part of their life histories. Extended fasting is associated with increased β-oxidation of fatty acids and reduced oxidation of glucose to minimize commitment of body protein to gluconeogenesis. However, the metabolic strategies used to sustain extended fasts simultaneous with high rates of energy expenditure are not well understood. Studies in fasting adult female and weanling northern elephant seals (NES) have revealed high rates of endogenous glucose production (EGP) under constraints of high nutrient demand for lactation or development but relatively low rates of metabolism. These studies revealed low rates of glucose oxidation and high rates of glucose recycling through the Cori cycle. We measured rates of glucose flux in fasting adult male NES to assess how significantly longer fasting durations, higher metabolic rates, and greater rates of muscular activity affect glucose kinetics. We measured glucose turnover in 18 adult males using the clearance of [6-H] glucose during breeding and molting. Adult male NES maintain high rates of EGP across extended fasts. EGP greatly exceeded estimated needs for glucose-dependent tissues, varied directly with plasma insulin and lactate concentrations, and was inversely related to plasma ketoacid concentrations. Together, these findings suggest that high rates of glucose production and recycling during breeding maintain high blood glucose levels to support glucose-dependent tissues while minimizing production of ketoacids and commitment of protein stores to glucose production.

In vitro Lipolysis and Leptin Production of Elephant Seal Blubber Using Precision-Cut Adipose Tissue Slices

“…Insulin circulating levels are generally low in NES and insulin sensitivity declines over the fast, allowing seals to sustain longterm lipolysis as well as glucose and ketone body availabilities to specific tissues with limited metabolism of lipids (Houser et al, 2007;Viscarra et al, 2012;Crocker et al, 2014). NES exhibit high circulating glucose concentrations during the fast (Houser et al, 2013;Crocker et al, 2017), the highest concentrations being encountered in fasting pups (around 9 mM or 1.62 g/L). The final glucose concentrations in our LHG and LLG conditions were respectively 2.6 times higher and 1.8 times lower than the glucose concentrations encountered in vivo in NES pups (Houser et al, 2013).…”

Response of metabolic hormones and blood metabolites to realimentation in rehabilitated harbor seal (Phoca vitulina) pups

“…While glucose concentrations at admit were highly variable, no changes were observed throughout realimentation. Stable glucose concentrations throughout rehabilitation are not surprising for marine mammals, which tightly regulate blood glucose concentrations with efficient gluconeogenesis, even during fasting (Champagne et al 2012;Crocker et al 2012Crocker et al , 2017. Total cholesterol increased throughout the realimentation period, positively associated with concentrations of ghrelin and negatively associated with cortisol.…”