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Comparison of the “mammal machine” and the “reptile machine”: energy use and thyroid activity

Comparison of the “mammal machine” and the “reptile machine”: energy use and thyroid activity. Summary of Journal Article. Experimenters:

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Comparison of the “mammal machine” and the “reptile machine”: energy use and thyroid activity

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  1. Comparison of the “mammal machine” and the “reptile machine”: energy use and thyroid activity

  2. Summary of Journal Article • Experimenters: • Measured oxygen consumption of Amphibolurus nuchalis (Lizard) and Mus musculus (Mouse) by the amount of energy used at the cellular level while in oxygenated Ringer solution. • Inhibited the Na+-K+-ATPase of each animal’s tissues with ouabain and then compared the results to oxygenated ringer solution results (exp 1 and exp 2) • Assessed the thyroid activity of each animal by measuring the release of a radioiodine. • Measured growth rates by measuring snout-vent length.

  3. Oxygen Consumption: Experimental Procedure and Results Materials and Methods • Experimenters sacrificed the animals by decapitation and within 30 minutes extracted the liver, kidney and brain of each animal. • Oxygen consumption was then measured every 30 minutes for 2 hours at 37 degrees Celsius. • Weighed wet values and dry values of each animal’s tissues. • Calculated total organ metabolism values from combined respective oxygen consumption values.

  4. Results • In vitro metabolism dry weight : • Kidney and liver oxygen consumption was 5X greater in the mouse tissue. • Brain oxygen consumption was 2X greater in the mouse.

  5. DIAGRAMMATIC REPRESENTATION OF ENERGY CYCLE SHOWING PARAMETERS MEASURED

  6. Sodium Transport Metabolism: Results • The mouse liver had 9X the metabolic rate of sodium-potassium transfer than the lizard. • The mouse kidney had 3.5X the metabolic rate of sodium-potassium transfer than the lizard. • The mouse brain had 7X the metabolic rate of sodium-potassium transfer than the lizard.

  7. Thyroid Gland Activity: Experimental Procedure and Results Materials and Methods • Experimenters injected radioiodine into the thyroid tissue of each animal because the radioiodine would be used by the tissue to secrete the growth hormone thyroxine. • After the animals were sacrificed blood was collected and centrifuged in order to separate out the blood serum. The serum was then stored in liquid nitrogen for 4 months. The amount of thyroxine in the blood serum was then tested using a thyroxine radioimmunoassay kit. Results • Results were inconclusive due to the fact that the experimenters had no way of knowing how much iodine was taken up into the thyroid in order to make growth hormones.

  8. Growth Rates: Experimental Procedure and Results Materials and Methods • Measurements from the snout to vent of baby mice were taken daily and captured juvenile lizards were measured weekly under constant temperature of 25 degrees Celsius. • The final part of the growth experiment had the lizards kept at a increased temperature of 37 degrees Celsius. Results • The juvenile lizard’s growth rates were influenced by their body temperature but they’re growth rate was still significantly less than that of the mice.

  9. Theories and Discrepancies • The results obtained for the three tissues represents the oxygen consumption of the cellular processes needed to sustain itself. In the living animals oxygen consumption can be many times higher when they are performing functions for each organ. • Ideally metabolic rate studies should be performed on mature animals and not juveniles which was not the case in this study.

  10. The results gathered from the study of growth between ectotherms and endotherms suggest an increased ability for rapid growth that exists due to the higher metabolic rate of endotherms. • Experimenters suggest that increased temperatures lead to increased thyroid gland activity and as a result leads to an overall increase in metabolic rate as well as growth rate. There is much debate as too how the thyroid hormone actually does this.

  11. Theories explaining high energy consumption of Na+-K+-ATPase in mammals i) mammalian sodium pump may be less efficient than that of the lizard ii) the mammalian tissue may need to transport significantly higher amounts of sodium and potassium across it’s cellular membranes. If this is the case then: a) the amount of cellular membrane per unit volume of tissue may be greater than that of the lizard b) the passive permeability of the mammal membrane may be much higher than that of the lizard c) the concentration gradients of sodium and potassium maintained across the membrane may be greater than that of the lizard d) any combination of these could be possible Very little research has been done in this area.

  12. Evolutionary Implications of Endothermy • The switch from the reptilian metabolic “machinery” to the mammalian metabolic “machinery” was an evolution of the metabolic ability to achieve endothermy. • This benefited the transition animals in many ways: • It allowed animals to live in colder environments • It allowed much higher levels of sustained activity • It allowed for faster growth and maturity • Another intriguing possibility is that it also may have increased the rate of evolution • It also has one big disadvantage in that these animals require much more food than other organisms of similar size.

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