The Unique Metabolism of the Heart

To understand the unique metabolism of the heart and its role in keeping the heart healthy, we first have to discuss some background information. Humans, as well as all mammals, have what is called an Autonomic Nervous System (ANS). It is the aspect of our nervous system that is monitoring our external environment to determine if we are in a safe or threatening situation. Depending on which it is, it will tell your body to have the appropriate reaction. Rest/relax or run away/defend yourself.

There are two aspects to the mammalian ANS, the sympathetic and parasympathetic, and dominant stimulation of one or the other tells your body which response to have in a given situation. The sympathetic is the “fight or flight” aspect that is used to helps us fight off or flee from a threatening situation, while the parasympathetic is the “rest and digest” aspect that takes over when everything is “safe” and we can focus on digestion, metabolism, sleeping, socializing, etc.

The information for the ANS is communicated in the nervous system through one of the cranial nerves called the vagus nerve. The part of the vagus nerve associated with the sympathetic signal is called the dorsal motor nucleus (DMN) and the part associated with the parasympathetic aspect is called the nucleus ambiguous (NA) (1). Below is a cross section of the spinal cord showing the different areas of the nerve.

But this dual vagus nerve system wasn’t always this way. Most reptiles, and anything that evolved before reptiles, had a single vagus nerve tract. The dorsal motor nucleus was the only aspect of the vagus nerve in these animals. Because the animals were very metabolically slow (think cold blooded like reptiles) this was all they needed. If they had an extreme stress response that overstimulated the vagus nerve it would cause their body to severely slow down. They would have slowed breathing and slowed heart rate (bradycardia). It was almost like a play dead defense mechanism for them.

Now, fast forward to higher evolved reptiles, like crocodiles and some turtles, and we start to see evidence of a split in the vagus nerve. This began to happen because these animals were becoming more active and metabolically demanding. If you tried to slow down bodily systems during a stress response in these animals, it would not end up well for them. This is because in these more active animals slowing down metabolism and blood flow would result in decreased oxygen to tissues, or hypoxia.

When mammals finally appear on the evolutionary scene (about 225 million years ago), we now see the complete split of the vagus nerve into the dorsal motor nucleus and the nucleus ambiguous, like we humans have today. Mammals are much more metabolically demanding than any living thing that evolved before them. This split of the vagus nerve into two pathways is what allowed mammals to become so metabolically demanding (running, quick movements, warm blooded) while retaining the ability of our Autonomic Nervous System to trigger a stress response to get away from something threatening without overstimulating metabolism and shutting down our very metabolically demanding organ systems.

If mammals had maintained the single dorsal motor nucleus pathway of the vagus nerve while evolving higher metabolically demanding organ systems, then when they had an overstimulating stress response the single tract dorsal motor nucleus would have tried to slow the metabolism of the organ systems just like it did in the reptiles and pre reptile animals. The highly metabolically active organ systems of mammals would not tolerate this, and it would likely cause shut down of organ systems and likely death.

Those older evolved species, without a split pathway, had a physiology that allowed them, in certain situations, to severely slow metabolism and blood flow (creating oxygen deprivation, or hypoxia, and metabolite deprivation) without cause harm to their tissues. In order to become more metabolically active, higher evolved reptiles, and then mammals, lost this ability in exchange for being faster, stronger, warmer, and bigger. However, there are a few species, that we know of, that seem to have held on to the ability to severely slow metabolism and not cause damage. A look into their metabolic physiology can help us understand what goes wrong with ours in chronic disease.

One animal that can do this is the common goldfish. It has been observed that during the winter, when water temperatures are very cold, goldfish can survive several days without any oxygen at all. Without oxygen, the fish are forced to use anaerobic glycolysis to produce energy. If done for too long this would usually cause an increase in lactic acid production causing damage. How does the fish get around this? While the complete mechanisms by which it does this are not completely fleshed out, scientists believe that the ability to do this comes from the ability of these fish to convert lactate into ethanol and excrete is very easily through their gills. It has also been observed that they do not always produce the acidic product of lactate during anaerobic glycolysis but instead produce other neutral substances. (2)

Another species of fish (carp), displays similar metabolic characteristics. When scientist put carp in a no oxygen environment they survived for 6 hours, and potentially longer because when they stopped the test the fish were still doing fine. It was determined that these fish also did this by easily converting lactate to ethanol and efficiently excreting it through their lungs into the water. They also made less lactate by instead making more neutral by-products of metabolism. (3)

Another example is North American freshwater turtles. In a low or no oxygen environment that requires these turtle to rely on anaerobic glycolosis, instead of being able to convert the excess lactic acid to ethanol like the fish, these turtles have mechanisms that buffer that lactic acid in the blood by releasing very alkaline calcium bicarbonate from their shells. (4)