A new study has found that during exercise, the vagus nerve, essential to the body’s ‘rest and digest’ response, has a more important role in heart function than we thought. The findings could be useful for treating diseases like heart failure, which can cause exercise intolerance.
It’s traditionally been assumed that, during exercise, the sympathetic (‘fight or flight’) nervous system predominantly controls heart function, while the parasympathetic (‘rest and digest’) nervous system’s only role is to keep the heart rate in check.
However, a new study by researchers at the University of Auckland in New Zealand has proven this assumption wrong, finding that the parasympathetic nervous system needs to be given more credit.
“Our study finds the activity in these ‘rest and digest’ vagal nerves actually increases during exercise,” said Rohit Ramchandra, a corresponding author of the study. “Our group has used ‘tour de force’ electrical recording techniques to directly monitor vagal nerve activity in exercising sheep and has found the activity in these vagal nerves going to the heart increases during exercise.”
Sympathetic and parasympathetic nerves travel from the brain to the heart, modulating the heart’s ability to pump blood. These nerves are called ‘autonomic’ because they work without conscious thought and include the vagal nerves (or vagus nerve), which regulate the parasympathetic nervous system. Cardiac vagal nerves innervate the whole heart.
Using sheep, the researchers measured the effect of exercise on cardiovascular variables, including cardiac vagal nerve activity, heart rate, coronary artery blood flow and cardiac output (the amount of blood the heart pumps in one minute). A subset of the sheep had their left cardiac vagal nerve branch removed.
They found that cardiac vagal nerve activity increased as soon as exercise started, and plateaued as exercise intensity increased. Removal of the vagal nerve significantly impacted cardiac function during exercise in terms of the sheep’s ability to elicit maximum heart rate, cardiac output and coronary artery blood flow.
From these findings, the researchers proposed a novel hypothesis: there is an increase in both vagal (parasympathetic) and sympathetic nerve activity during exercise, which has a synergistic effect on maintaining cardiac function. The researchers then looked at the chemicals underlying this vagal nerve response.
“The cardiac vagus nerve releases multiple mediators, and previous research has focused on a neurotransmitter, acetylcholine, which has no impact on our ability to exercise,” Ramchandra said.
Because the researchers were interested in the effects of the vagus nerve during exercise, they looked at the effect of another mediator: vasoactive intestinal peptide (VIP). Despite having ‘intestinal’ in its name, VIP is also found in the heart, where it causes dilation of the coronary blood vessels, improves contractility and increases heart rate.
“Our study focused on a different mediator, vasoactive intestinal peptide (VIP), and it shows that the vagus nerve releases this peptide during exercise, which helps the coronary vessels dilate, allowing more blood to pump through the heart,” said Ramchandra.
The researchers say their findings could have implications in disease treatment, including heart failure, where those with the condition can’t tolerate exercise.
“One potential reason why exercise tolerance is reduced is that the diseased heart simply does not receive enough blood,” said Ramchandra. “Our follow-up study will try to see whether we can use this important role of cardiac vagal nerves to improve exercise tolerance in heart failure.”
The study was published in the journal Circulation Research.
Source: University of Auckland