A groundbreaking study has unraveled the mystery behind the shrew’s remarkable resting heart rate, which can reach a staggering 1,020 beats per minute. Researchers from Aarhus University and the University of Manitoba have discovered that evolutionary changes in the heart protein ‘cardiac troponin I’ have enabled these small mammals to achieve such an extreme heart rate. The findings shed light on how these tiny creatures can outpace even the swiftest human heartbeats, providing valuable insights that may one day be translated into biomedicine.
The Heart Under Strain
The resting heart rate of a shrew can be as high as 17 per second, equivalent to around 1,020 beats per minute. By way of comparison, the average human resting heart rate is between 60 to 100 beats per minute — so the shrew has their pace beat by a factor of an alarming 10-17 times.
To date, how shrews manage to perform that remarkable feat has been a mystery — but the new research reveals the evolutionary modifications responsible for allowing them to reach such an incredibly high rate. At the heart of the discovery is a protein found in the heart, cardiac troponin I, essential for the organ to bind calcium ions when contracting and relaxing.
Taking the Brakes Off Heart Relaxation
In shrews and the almost as closely related moles, this crucial part of a protein in the heart that controls when relaxation time is missing. This evolutionary modification has locked the brakes that stop heart relaxation in the ‘off’ position, allowing their hearts to beat much more quickly.
In other mammals, this protein has two particular serine amino-acid residues that are transiently modified when the heart is activated by hormones such as adrenaline. This alteration makes the heart’s muscle fibers release calcium ions faster after contraction which helps the heart muscle relax quickly so that it gets more time between beats to fill with blood.
In shrews, however, the DNA region that encodes these two series has been turned off – and as a result, the protein is always active, almost as if it were continually being zapped by adrenaline even when at rest.” This is the adaptation within shrews that allows them to reach such high heart rates.
Adapting Nature’s Playbook for Biomedicine
The post-translation modification of the highly conserved mammalian heart protein was also investigated in bats, another mammal with a heart that can accelerate to over 1,000 beats per minute. This helped inform how the trait for high heart rates came to be.
They discovered that the protein used by some bat species can be produced without the fragment of the gene for those two serine amino acids. But ancient shrews and moles likely had the same capability, according to the researchers, who suggest that evolution further selected their troponin I proteins to entirely obliterate this region for them to evolve even faster heart rates.
The new hurdle for research teams is to figure out how this can be applied to biomedicine. The researchers believe that mimicking the splicing of troponin I that bats facilitate, could recapitulate its beneficial effects in model systems and possibly even eventually human hearts to open up new areas for exploration within cardiovascular health.
