You can’t stop the (heart) beat!

By Jonathan Huie

Temperature has a strong effect on how well our body functions, particularly the heart. As mammals, we have the ability to generate heat and regulate our internal body temperature (endothermy). In extreme hot and cold scenarios, our hearts beat faster and work harder to meet metabolic demands. However, there are many animals such as modern-day alligators that cannot regulate their body temperature and must conform to ambient thermal conditions (ectothermy).

Figure 1. Example photos of the European rabbit, Oryctolagus cuniculus, (left) and American alligator, Alligator mississippiensis, (right) used in this study. Rabbit photo by JJ Harrison and alligator photo by Ianaré Sévi.

Effective cardiac function relies on steady heartbeats, produced by the coordination of electrical and chemical signals within the body. Major interruptions may introduce ventricular fibrillation (a type of arrhythmia, or irregular heartbeats) and potentially cause heart failure. Luckily, our hearts have a built-in safety function (long action potential wavelengths) to help reduce the risk of ventricular fibrillation. Unfortunately, increased heart rates reduce action potential wavelengths and increase the risk of arrhythmia. For mammals that often have high heart rates under extreme temperatures, this is not so good. For alligators and other ectotherms that operate under a wide range of temperatures, it is unclear how their hearts react and continue to function.

A team of scientists led by Conner Herndon, a Georgia Institute of Technology graduate student, investigated this question in a recent study. The team electrically stimulated hearts from recently deceased rabbits and alligators at 23 ̊C and 38 ̊C under various heart rates and recorded different heart electrical wave parameters (action potential duration and conduction velocity). Compared to rabbits, alligator hearts were less sensitive to thermal fluctuations and displayed minimal changes in their ability to function. Furthermore, the fastest stimulated heart rate (that could elicit a response) in alligators was still substantially slower than that of the rabbits, keeping alligators well away from the risk of temperature induced arrhythmia. Notably, the authors were able to induce ventricular fibrillation in the rabbit hearts but not the alligators.

Figure 2 (Figure 3 from Herndon et al. 2021). Heart electrical wave parameters in rabbits and alligators at 38 ̊C (red) and 23 ̊C (blue) across basic cycle length (inversely proportional to heart rate). At different temperatures, alligators retained similar relationships between each wave parameter and basic cycle length while the rabbits did not. Action potential wavelength decreased enough in rabbit hearts to cause ventricular fibrillation but did not in alligator hearts.

These results suggest an important trade-off in heart function. While mammals have the ability to regulate their body temperature, it comes at the cost of high heart rates and high risk of heart failure under extreme temperature changes. In contrast, endotherms like alligators can withstand a wide range of heart temperatures with a low risk of ventricular fibrillation but are restricted to slower heart rates. However, not all ectotherms are as insensitive to thermal fluctuations (e.g., zebrafish, frogs, and turtles) and some ectothermic mammals are fairly good at resisting ventricular fibrillation (e.g. sloths). The authors suggest that more studies on the heart physiology of other species are needed to tease apart the evolutionary history of arrhythmic resistance.

Jonathan Huie is a PhD student at George Washington University studying salamander limb biomechanics and the water to land transition. He has also studied fish feeding morphology and anole ecomorphology. Find out more about his work at www.jonathanhuie.com, follow him on twitter @jmhuiee, or email him at jonathanmhuie@gmail.com.

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