Unlocking Shrew's High Resting Heart Rate Through Protein Study

The resting heart rate of a shrew can reach an astonishing 17 beats per second, or approximately 1,020 beats per minute. In contrast, the average human heart at rest typically beats between 60 and 100 times per minute. This makes the shrew's resting heart rate about ten to seventeen times higher than that of humans. 
 
Until recently, how these tiny mammals manage such extreme cardiac rates remained a mystery. A new study published in the journal Science has now revealed insights into this enigma. 
 
An international team of researchers led by postdoc William Joyce from Aarhus University (AU), along with Professor Kevin Campbell, who was previously affiliated with AU and is currently at the University of Manitoba in Canada, explored evolutionary changes in "cardiac troponin I,"  a key protein within the shrew’s heart that enables its extraordinary resting pulse rate. 
 
Joyce explained their findings: “We discovered that a crucial part of this particular protein that regulates relaxation time after each heartbeat is absent in both shrews and moles closely related to them.” He added that this loss essentially removes constraints on cardiac relaxation, allowing for much faster beating hearts. 
 
Troponin I plays an integral role as it binds calcium ions during contraction—essential for muscle contraction and thus critical for proper functioning of the heart. In other mammals, including humans, two specific amino acids known as serines are temporarily modified when hormones like adrenaline stimulate the heart during stress or physical activity. Such modification aids quicker release of calcium ions post-contraction, letting muscles relax swiftly, giving more time for blood refill before the next beat. 
 
However, evolution appears to have taken a different path with shrews, rendering them capable of maintaining high-speed pulses even while resting due to permanent activation akin to adrenaline-induced stimulation without actual presence thereof. 
 
Campbell elucidated: "In an early ancestor species common to contemporary shrews and moles, the DNA region encoding those two serine residues became inactive. Hence, their protein operates as if it is under adrenaline’s influence even during rest, allowing shrews to reach extreme heart rates.” 
 
The researchers also studied bats, another species capable of reaching over 1,000 beats per minute like the shrews. This comparative study provided insight into how high pulse rate ability evolved. 
 
Campbell explained: “Our analysis reveals that some bat species can bypass the gene segment coding for serine amino acids while forming proteins.” He added that ancient ancestors of today's shrews and moles probably had similar capabilities, leading them to gradually lose this region from troponin I proteins through evolution, thereby enabling higher cardiac rates. 
 
Following these findings, the research team plans on exploring potential biomedical applications such as replicating observed splicing in model organisms and potentially human hearts to mimic beneficial effects. Joyce concluded by stating their next goal would be translating these findings towards practical application in biomedicine.