A new study finds that the vagus nerve, key to the body's "rest and digest" response, plays a more important role in heart function during exercise than we thought. The findings may help treat conditions such as heart failure, which can lead to exercise intolerance. Conventional wisdom holds that during exercise, the sympathetic nervous system ("fight or flight") primarily controls heart function, while the parasympathetic nervous system ("rest and digest")'s sole role is to control heart rate.
However, a new study from researchers at the University of Auckland in New Zealand proves this assumption wrong, finding that the parasympathetic nervous system needs to be given more credit.
"Our study found that activity in these 'rest and digest' vagus nerves actually increases during exercise," said Rohit Ramchandra, corresponding author of the study. "Our research team used 'touring' electrical recording technology to directly monitor vagus nerve activity in exercising sheep and found that activity in these vagus nerves leading to the heart increases during exercise."
Sympathetic and parasympathetic nerves lead from the brain to the heart and regulate the heart's ability to pump blood. These nerves are called "autonomic nerves" because their work does not require conscious thought, and include the vagus nerve (or vagus nerve), which regulates the parasympathetic nervous system. The vagus nerve innervates the entire heart.
Using sheep, the researchers measured the effects of exercise on cardiovascular variables, including cardiac vagal activity, heart rate, coronary blood flow and cardiac output (the amount of blood pumped by the heart in one minute). The left cardiac vagus nerve branches were removed in some sheep.
They found that cardiac vagus nerve activity increases as soon as exercise begins and levels off as exercise intensity increases. After vagus nerve removal, the ability of sheep to stimulate maximum heart rate, cardiac output, and coronary blood flow during exercise was significantly affected by cardiac function.
Based on these findings, the researchers proposed a new hypothesis: Both vagal (parasympathetic) and sympathetic nerve activity increase during exercise, which has a synergistic effect on maintaining heart function. The researchers then studied the chemicals behind this vagal response.
"The vagus nerve in the heart releases a variety of mediators, and previous research has focused on one neurotransmitter, acetylcholine, which has no effect on our ability to exercise," Ramchandra said.
Because researchers were interested in the effects of the vagus nerve during exercise, they examined the effects of another mediator: vasoactive intestinal peptide (VIP). Although VIP has the word "intestine" in its name, it is also found in the heart, where it causes coronary vasodilation, improves contractility, and increases heart rate.
"Our study focused on a different mediator, vasoactive intestinal peptide (VIP), and showed that the vagus nerve releases this peptide during exercise, which helps the coronary blood vessels dilate, allowing more blood to be pumped to the heart," Ramchandra said.
The researchers say their findings could have implications for the treatment of conditions including heart failure, where patients cannot tolerate exercise.
One potential cause of reduced exercise tolerance is that the diseased heart simply cannot get enough blood. Follow-up studies will attempt to see whether this important role of the cardiac vagus nerve can be exploited to improve exercise tolerance in patients with heart failure.
The research was published in the journal Circulation Research.