Protein study reveals how tiny shrew achieves resting heart rate of 1,020 beats per minute

The shrew’s resting heart rate can reach up to 17 beats per second, which equates to about 1,020 beats per minute. In comparison, the average human resting heart rate is about 60 to 100 beats per minute, making the shrew’s resting heart rate about 10 to 17 times higher than that of humans.

Until now, it remained a mystery how these small mammals achieve such extreme resting heart rates, but a new study, just published in the journal Sciencesheds light on the mystery.

An international research team led by postdoc William Joyce, during his time at Aarhus University (AU), and Professor Kevin Campbell, previously affiliated with AU and now at the University of Manitoba (Canada), studied how evolutionary changes in the heart protein “cardiac troponin I” allowed shrews to achieve their unusually high resting heart rate.

“We discovered that a crucial part of the cardiac protein that regulates cardiac relaxation time is absent in shrews and closely related moles. This evolutionary loss permanently removes the brakes on cardiac relaxation, allowing their hearts to beat a lot faster,” says Joyce, who now works at the Centro Nacional de Investigaciones Cardiovasculares Carlos III (CNIC) in Spain.

A missing DNA region

The protein plays a critical role in the heart’s ability to bind calcium ions during contraction. Calcium ions are necessary for the contraction of the heart muscle and are therefore essential for heart function.

In other mammals, this protein contains two specific serine amino acids that are temporarily changed when the heart is stimulated by hormones such as adrenaline, released during stress or physical activity. This change helps the muscle fibers of the heart release calcium ions more quickly after contraction, allowing the heart muscle to relax more quickly and giving the heart more time to fill with blood between beats.

However, an evolutionary change has occurred in shrews.

“In an early ancestor of shrews, the DNA region encoding the two serines became inactivated. This means that the protein still functions as if activated by adrenaline, even when the animal is at rest, allowing for shrews to reach their extreme heart rates.” Campbell explains.

Bats chart the path of evolution

The researchers also studied the heart proteins of bats, which, like shrews, can reach heart rates above 1,000 beats per minute. This allowed them to better understand how the ability to achieve a high heart rate evolved.

“Our analysis shows that some bat species can ignore the part of the gene that codes for the two serine amino acids when forming the protein. Ancient shrews and moles likely had the same ability, and evolution gradually favored their troponin I proteins to completely lose this region, which allowed them to evolve with even higher heart rates,” says Campbell.

The research team’s next goal is to explore how the findings can be translated into biomedicine. “This means replicating troponin I splicing, as observed in bats, in model organisms and potentially, ultimately, in the human heart to mimic the beneficial effects,” says Joyce.