A hormone secreted by bone helps to coordinate our flight-or-fight response, suggesting our skeletons are more active than we thought.
When faced with a sudden threat, our heart and breathing rate, blood pressure, circulating blood sugar and body temperature increase to prepare our muscles to fight or run away. This fight-or-flight response is known to be controlled by direct nerve pathways from the brain and hormones released by the adrenal glands.
Now, Gerard Karsenty at Columbia University and his colleagues have discovered that a hormone released by bones called osteocalcin also coordinates this response.
They found that blood levels of osteocalcin quickly rose in humans when they had to perform a stressful public speaking task. The same thing happened in mice and rats when they were restrained, given electric foot shocks, or exposed to the smell of fox urine.
Additional experiments in mice showed that this osteocalcin surge suppressed the body’s “rest-and-digest” functions in order to allow the opposite flight-or-fight mechanisms to proceed.
The results build on the group’s previous work showing that bones release osteocalcin to help the muscles burn fuel during exercise, and that osteocalcin injections in older mice make their ageing muscles more youthful.
Together, these findings suggest we need a radical re-think of the role of bones, which have previously been viewed as mostly inert structures, says Karsenty. They may have evolved to protect us from acute danger by activating the flight-or-fight response, optimising muscle function, providing the structural framework needed for our bodies to move and escape, and forming a protective cage around our organs, he says.
At this stage, it’s unclear why the body has different ways of mounting the flight-or-fight response – through bone, direct nerve pathways and the adrenal glands – but it may be so we have back-ups in place if one system fails, says Robin McAllen at the University of Melbourne.
In support of this idea, we know that people with faulty adrenal glands and mice with no adrenal glands can still mount the flight-or-fight response, he says.