Variation in plasma leptin levels in response to
fasting in Antarctic fur seals (Artocephalus
gazella)
Figure 1. Antarctic fur seal mother and pup (http://www.timgreyhavens.com/images/large/TG076004.jpg)
Arnould, J.P.Y, Morris, M.J., Rawlins, D.R., Boyd, I.L. (2002). Variation in plasma leptin levels in response to fasting in Antarctic fur seals (Artocephalus gazella). J Comp Physiol B. 172:27-34.
Aim of study: Determine plasma levels and how they vary
during fasting in Antarctic fur seals, (Arctocephalus
gazella) an otariid seal.
Summary of experiment:
Pinnipeds are known to have one
of the highest body lipid reserves. This is important for adult females and pup
otariids as they have to undergo fasting period. Adult females fast while they
are nursing their pups onshore (1-2 days) and pups must fast while mothers are
away foraging (up to 8 days).
Study was
conducted on Bird Island on a large Antarctic fur seal breeding colony during their
4-month lactation period. For adult females blood samples were collected from
venipuncture of inter-digital vein in hindflipper. Samples were taken 24h and
72h post start of fasting and then 6h after last feed. For pups blood sample
was taken at start of fast, and at the end of fast (5 days). Leptin
concentrations in the plasma were determined using radioimmunoassay, with an
antibody (anti-human leptin antibody with broad cross-reactivity to leptin
molecules of many species). Body mass was also measured for adult and pups to
observe change in weight with fasting. Total triglyceride levels were used from
a previous study in adults and by intravenous injection of tritiated water
followed by a blood sample later to determine body composition. Statistical analysis of results were done
using Systat statistical software.
Summary of results:
- Body mass and total body lipid percentage decreased during fasting and slightly increased when fed for female adults. (Figure 1)
- Plasma leptin levels increased during first 24h of fasting but only decreased slightly afterwards with the exception of seal #3 (Figure 2, adult females)
- Plasma triglyceride decreased sharply for the first 24h of fasting but varied for the rest of the experiment. Levels were not significantly related to body mass, total body lipids or plasma leptin levels. (Figure 2, adult females)
- Male pups significantly heavier then females but daily loss was consistent between the two sexes (Figure 3)
- Plasma leptin levels were significantly higher at the beginning of the fast than at the end (Figure 3, male and female pups)
- Plasma triglyceride levels decreased significantly throughout the fast (Figure 3)
In the study they associated
decrease in mass of pups with decrease in plasma leptin levels. This was an expected outcome. Adult female response was not as expected as leptin levels
increased in the first 24h. They think it could potentially be due to breakdown
of lipids in adipose tissue from meal before arriving at the colony (Arnould et al., 2002).
The low leptin levels at the
beginning of the fast may also be due to higher energy expenditure of adults as
they often have to swim continuously for up to 20h to return to breading
grounds (Arnould et al., 2002). A supporting study showed plasma leptin levels to be decreased post
exercise in humans.
Another factor they suggest could
explain the elevation in leptin levels is patterns for production of milk (Arnould et al., 2002). In a human study they found that prolactin
stimulates leptin secretion, with elevated leptin levels during lactation. The sucking stimulus could account for changes
in leptin levels (increasing when first returning and decreased between 24-72h
of fast). This correlates with information that pups gain 90% of their mass
within the first 24h (sucking would be greatest).
Lactating animals such as humans
and rodents have been shown to have decrease in leptin levels following parturition.
Role of leptin would be to increase foraging and food consumption to support
energy demand to produce milk. Results however in this experiment had weak
trend for decrease in leptin levels even though body mass decreases
significantly during fasting in lactating females. They think this may be
because they are required to fast for extended periods of time to provide milk
for their pups with having to leave their pups for longer periods of time to
forage (Arnould et al., 2002). If leptin levels decrease too fast this may stimulate hunger resulting
with them leaving their young prematurely to forage.
Findings from this study shows
that body lipid reserves and plasma leptin levels trends found for the Antarctic
fur seals is quite different from most mammalian species. Study suggests that
primary role of leptin in fur seals and maybe most carnivores is not
necessarily an indicator of body energy reserves and that levels are not solely
determined by fat reserves (Arnould et al., 2002).
Critique:
- The paper was well written and concise. The graphs were straight forward and explained well within the results section. All the results were thoroughly explained in the discussion with other studies used to back up or support findings. At the end of the paper the reader was not left with any questions on what was the meaning of a particular trend.
- I like how paper was written in a way that majority of readers do not require a lot knowledge about the topic to understand what they did and what their findings meant. I honestly do not have any complaints! Article was interesting and easy to read and understand.
Future experiments:
- They suggest that prolactin levels may influence leptin levels by stimulating leptin release. In a future experiment they could measure prolactin levels in lactating females. This would be to answer the question do prolactin levels follow similar trend to leptin in the Antarctic fur seal? This would then be used to answer the question does prolactin help control/regulate leptin levels? Experiment could be conducted just like this one, except from extracted blood sample they would need to use an antibody specific for prolactin to quantify this hormone using the radioimmunoassay method. I would expect that there would be an increase in prolactin levels at the beginning of fasting (when they are nursing pups) due to sucking stimulus. When sucking stimulus decreases prolactin should decrease as well as leptin levels. Initially when adult females first return prolactin and leptin levels should be at its lowest.
- They also suggest in the experiment that the study may not accurately reflect absolute levels of circulating leptin because although the antibody chosen had broad cross reactivity I ad only 3% specificity to canine leptin. A future experiment could be to use another method to quantify amount of leptin hormone present. They could maybe use Real-time quantitative PRC to quantify gene expression using a fluorescent probe that binds to DNA and fluoresce to different amount depending on the amount of DNA present. This would answer the equation, are the levels of circulating leptin obtained in the experiment accurate? I would expect this would show levels to be accurate, as there was a supporting study that found low levels of leptin in dogs using another antibody so it is possible that a characteristic of carnivores is low leptin levels.
Figure 2. Antarctic fur seal pup
(http://www.leatherwoodonline.com/ee/images/uploads/antarctica-c-09.jpg)
