Stress Streamlines Physiological Processes (lizard)

Chronic stress, energy transduction, and free-radical production in a reptile – Voituron et al. 2017

• Abstract: corticosterone plays a crucial role in adapting to stressful environments. 3 main processes of respiration, ATP production and free radical production in the mitochondria may also affect stress responses, so this study tried to dissect these sub-cellular mechanisms. The study exposed lizards to 21 days of corticosterone and found no effect on mitochondrial activity and efficiency, but it did increase liver mass and mitochondrial protein content.

• Introduction: Chronic exposure to a stressor has been known to cause negative consequences like immuno-deficiency, neural degeneration, and premature mortality. However some physiological responses may mitigate these costs when exposed to chronic stress, for example the increase activation of glucocorticoid secretion increases survival and reproductive rate. Allostatic load is the general wear and tear that the body undergoes when it is exposed to chronic stress. Mitochondrial allostatic load has not been clearly investigated, defined as the structural and functional changes that it goes through as a result of glucocorticoid levels in response to environmental stressors. Mitochondria use oxidative phosphorylation to couple fuel oxidation and oxygen reduction with the production of ATP -> the three main processes are oxygen consumption, ATP production and ROS emission and are known to be plastic with variable degrees of coupling b/t them -> in turn these influence growth, survival and reproduction. This paper focuses on the effects of increasing plasma corticosterone levels on mitochondrial functioning in the liver of lizards b/c the liver is crucial to metabolic rate, hormonal regulation, metabolism and oxidative balance. Hypothesis: increase in corticosterone shoud increase metabolic rate, in turn enhancing oxygen consumption, ATP production and ROS emission. However it should also reduce mitochondrial efficiency (total ATP produced per unit of time).

• Methods: Zootoca vivpara is a common lizard. n= 24. Duration of dosing: 3 weeks and non invasive, daily.
  1. Blood was collected on day 22.
  2. Used Standard Metabolic Rate (SMR) which is the minimum energetic cost of maintenance estimated in a fasting individual at rest at a given temperature during the time period that the animals are normally inactive.
  3. Extracted liver for functional analyses of mitochondria: oxygen consumption, ATP production, and ROS (reactive oxygen species) production.
  Check paper for specifics

• Results:
  1. plasma concentrations of corticosterone were significantly higher in treatment groups, but no difference b/t low and high treatment. Also the obtained values were within range of free living individuals which can get pretty high.
  2. Body mass b/t treatments and placebo did not significantly change. Liver mass however was significantly lower in placebo and specific liver mitochondrial proteins as well when compared to treatment groups.
  3. SMR was not significantly different b/t groups
  4. No effect was seen on ATP production as well as oxygen consumption
  5. For Basal ROS emission it decreased significantly with increasing plasma corticosterone lvl, the treatment groups were significantly less than placebo but did not differ b/t each other.

• Discussion: They showed that there is a positive treatment effect on liver mass and possibly mitochondrial protein content, but also a decrease of ROS emission with no changes in mitochondrial oxidative phosphorylation activities and efficiency. An explanation for the higher liver mass could be because of lipid storage and size of lipid droplets. Also glucocorticoids encourage mitochondrial growth by increasing expression of autosomal genes -> mitochondrial biogenesis (also shown by the higher mitochondrial protein yield).
  Possible reason for discrepancy in the results, since usually these results are also followed by a decrease in respiratory activity, maybe b/c this study was relatively long term (21 days), studies have shown that the resp rate seems to become normal with chronic exposure while showing dips in short-term (mere days).
  The treatment also showed a significantly negative effect on ROS emissions (61% less in treatment compared to control) which was explained by a decrease in the electron leak within the resp chain -> this means that electrons are better channeled to the oxygen. All in all the study’s data shows that liver of chronically stressed individuals can produce more ATP, less ROS and this better ratio would better their capacity to respond to the environment’s energetic demands -> in turn increasing survival. Most theories state that allostatic load occurs when the energy requirements of the individual are beyond the capacity of the individual to extract said energy from the environment thus increasing plasma corticosterone levels and in turn streamlining the system by taking energy from non-essentials. However this study shows that the coupling of reactions and processes are extremely variable, maybe the body is far more efficient than imagined. The lizards were capable of optimizing respiration and encouraged mitochondrial biogenesis w/o having to worry about too many drawbacks.