Abstract
Adipose tissue is fundamental to energy balance, which underpins fitness and survival. Knowledge of adipose regulation in animals that undergo rapid fat deposition and mobilisation aids understanding of their energetic responses to rapid environmental change. Tissue explants can be used to investigate adipose regulation in wildlife species with large fat reserves, when opportunities for organismal experimental work are limited. We investigated glucose removal, lactate, glycerol and NEFA accumulation in media, and metabolic gene expression in blubber explants from wild grey seals. Glycolysis was higher in explants incubated in 25 mM glucose (HG) for 24 h compared to controls (C: 5.5 mM glucose). Adipose-derived lactate likely contributes to high endogenous glucose production in seals. Lipolysis was not stimulated by HG or high hydrocortisone (HC: 500 nM hydrocortisone) and was lower in heavier animals. HC caused NEFA accumulation in media to decrease by ~30% relative to C in females, indicative of increased lipogenesis. Lipolysis was higher in males than females in C and HG conditions. Lower relative abundance of 11-β-hydroxysteroid dehydrogenase 1 mRNA in HG explants suggests glucose involvement in blubber cortisol sensitivity. Our findings can help predict energy balance responses to stress and nutritional state in seals, and highlight the use of explants to study fat tissue function in wildlife.
Highlights
Knowledge of energy balance regulation is central to understanding how animals survive in changing environments
We investigated whether rates of glucose removal and accumulation of lactate, non-esterified fatty acid (NEFA) and glycerol were correlated within each treatment (Table 2)
Subcutaneous fat is a significant source of whole body lactate production in humans and rodents, contributing 10–30% of glucose metabolism[53]
Summary
Knowledge of energy balance regulation is central to understanding how animals survive in changing environments. Manipulations via injection, implants, osmotic pumps, food or drink have been used in vertebrates to investigate behavioural, metabolic, hormonal, gene expression and fitness consequences of exposure to hormones involved in stress and energy balance[8,9,10,11,12,13,14] Such studies may require captivity or lethal sampling, or reduce survivorship or reproductive output[15, 16]. Grey seals (Halichoerus grypus) experience substantial fluctuations in fat reserves, from as low as 12% in juveniles and adults after moulting, to 33% in breeding adults, and 45% in weaned pups[27,28,29,30] They rely heavily on fat as a metabolic fuel[29, 31] and for insulation[32]. The conflicting evidence to date concerning the role of glucose and GCs in fat regulation in phocids highlights the need for novel experimental approaches to explore underlying physiology in seals and other wild animals that have unusual fat regulation
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