Aspects of energy metabolism during lactation in Weddell seals (Leptonychotes weddelli): A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Science with Honours at Lincoln University

Abstract

During the lactation period, Weddell seals (Leptonychotes weddellil) maintain low plasma levels of ketone bodies (<0.2 mmol.l⁻¹ ) in spite of substantial mobilisation of adipose tissue. Two different hypotheses have been proposed to explain this phenomenon: 1) high rates of ketone body utilisation , and 2) effective gluconeogenesis from glycerol and suppression of ketogenesis. In order to test these hypotheses in Weddell seals, carnitine, acetylcarnitine, glycerol and free fatty acids were measured in plasma samples from non-lactating females ('singles', n=9) and lactating females ('mothers', n=15), as well as their pups (n=10). Plasma carnitine levels (9.8 ± 0.6 µmol.l⁻¹ , n=34) were low by human standards, with no significant differences be­tween groups, consistent with prolonged fasting in the adult animals. Plasma acetylcarnitine concentration was low also but was significantly higher in lactating animals (3.1 ± 0.6 µmol.l ⁻¹, P< 0.05) than non-lactating females (1.7 ± 0.3 µmol.l⁻¹ ) and pups (1.4 ± 0.4 µmol.l⁻¹ ). The acetylcarnitine data indicate absence of acetyl-CoA accumulation in mitochondria in all groups, consistent with an inability to generate ketone bodies as a glucose substitute. Plasma glycerol concentration was higher in lactating (350 ± 25 µmol.l⁻¹ ) versus non-lactating (230 ± 25 µmol.l⁻¹ ) females, but lower than in pups (620 ± 100 µmol.l⁻¹ ). Free fatty acids were highest in the lactating group (1300 ± 130 µmol.l⁻¹ ) followed by pups (750 ± 35 µmol.l⁻¹ ) and non-lactating animals (500 ± 60 µmol.l⁻¹ ). Mean plasma concentrations of glycerol and free fatty acids are consistent with the utilisation of glycerol for energy metabolism in lactating females and, to a lesser degree, in non-lactating females. The results presented here sup­port the hypothesis proposed by Castellini et al. (1987) that gluconeogenesis from glycerol is a major source of glucose and energy in Weddell seals during the lactation period.