With Valentine’s Day just around the corner, heart-themed items seem to be everywhere. It is no surprise that I started thinking more and more about how humans are unique from other animals when it comes to heart disease. In my last entry, we talked about a few genetic variants that protect some humans from heart disease as well as the observation that many carnivores do not develop atherosclerosis, even when they have high cholesterol.
According to the Centers for Disease Control, cardiovascular disease is the leading cause of death for adults and every year approximately 805,000 people have a heart attack in the United States alone. What’s more, in 1 out of 5 cases, the individual is not even aware they had a heart attack.
Did you know that heart attacks are rarely observed in non-human species, including chimpanzees and other primates? That is not to say other species do not develop heart diseases, it is just that they are rarely known to have a heart attack. Dr. Philip Gordts at the University of California at San Diego was quoted in Live Science as saying, “In general, animals don’t die naturally from the typical heart attack you see where you clog up the coronary arteries in humans.” It is also possible that we just have not studied enough species to know for sure whether humans are unique in this regard as necropsies are rarely done to evaluate the cause of death.
Animals have specific blood vessels responsible for supplying nutrients and oxygen to the heart muscle. Heart attacks happen when these coronary arteries become blocked. Without sufficient blood flow, the heart muscle dies and can no longer contract or send electrical signals to other muscle cells in the heart, ultimately causing the heart to stop. When that happens, CPR is necessary to preserve blood flow in the body by manually compressing the heart to move the blood.
So why do humans have heart attacks? In our last post we talked about gene mutations and variants that make humans more prone to atherosclerosis, which could contribute to our risk for heart attacks. Although, while including these mutations in mice results in atherosclerosis, they are still resistant to heart attacks. Non-mammalian vertebrates may resist heart attacks as the heart muscle is not only supplied with blood from arteries, but also with blood that comes directly from the heart chambers owing to the spongy nature of their heart muscle. So even if an artery is blocked, the muscle can still receive oxygenated blood. In fact, experiments of alligators showed that their hearts worked normally with blocked coronary arteries, even during exercise.
About 10 years ago, researchers at the University of Pennsylvania speculated that mammalian cardiovascular disease may have originated as a way to protect mammals from injury. As it turns out, platelets are unique to mammals. They are important for our ability to form blood clots necessary to stop blood loss and survive a traumatic injury. The problem is that these same cells can also form the clots that cause strokes and heart attacks in an individual with atherosclerosis. To fight these clots, patients often take medications such as aspirin, which inhibit platelets. In contrast to mammals, non-mammalian vertebrates have thrombocytes which are bigger than a platelet. While thrombocytes have similar proteins as mammalian platelets, they have lower levels of the fibrinogen receptor and are lacking the adenosine diphosphate receptor. These two differences are thought to be key as they are necessary to form 3-dimensional blood clots that block blood flow in arteries.
All in all, there appear to be multiple factors that contribute to our unique ability to form 3-dimensional blood clots and suffer from heart attacks. Hopefully, with increased understanding of genetic variants that protect some humans from heart attacks, new therapies can be developed to help protect us all. If only we could be more like those alligators…I could get used to basking in the sun and eating more fish…not so sure about muskrats or whole deer though.
AA Schmaier, TJ Stalker, JJ Runge, D Lee, C Nagaswami, P Mericko, M Chen, S Cliché, C Gariepy, LF Brass, DA Hammer, JW Weisel, K Rosenthal, ML Kahn. Occlusive thrombi arise in mammals but not birds in response to arterial injury: Evolutionary insight into human cardiovascular disease. Blood. 118(13): 3661-3669, 2011.