Vasopressin helps us to manage our body's water, but it is more complicated than we thought.
Water is present in every single cell of the human body. In fact, by weight, we are 50 to 75 percent water, depending on a range of factors including age, sex, weight, and health.
If we went for just a few days without a drop, we would die. Such is the vital importance of water.
It keeps tissues such as the eyes and inside of the mouth moist, regulates body temperature, lubricates joints, and acts as a shock absorber. Water also dissolves minerals and nutrients, which makes them available for biological processes, and transports them around the body.
With this in mind, it may come as quite a surprise that controlling the level of water in the body is predominantly reliant upon just one molecule: vasopressin.
Vasopressin and water management
Vasopressin is found in most mammals, and its role as the water-regulating molecule has been known for 70 years. Until recently, it was thought to only work in a classic, negative feedback loop: when we are dehydrated, levels of vasopressin rise, which causes urine to become concentrated in the kidneys, freeing up more water to be used in the body. Conversely, when we have too much water in our body, vasopressin levels decrease, and urine is diluted.
However, new research demonstrates that this molecule has much more to offer than the feedback loop described. Dr. Charles Bourque of McGill University in Montreal, Canada, uncovered an unexpected pathway that vasopressin uses to keep us properly hydrated.
He presented his findings at the 2017 Canadian Neuroscience Meeting, an annual meeting of the Canadian Association for Neuroscience, held in Montreal.
Dr. Bourque demonstrated that not only is there a vasopressin feedback loop, but "it's also involved in feed-forward mechanisms. [They] determined that this molecule is produced in the brain right before [people] go to bed, and during sleep, in anticipation of the dehydrating effect of sleep."
Their research showed that mice increased water intake just before sleep. Rather than being motivated by a physiological need for water, the drinking response was solely based on the animal's circadian rhythms.
Circadian rhythms, which drive the sleep-wake cycle, are controlled by a tiny brain region within the hypothalamus called the suprachiasmatic nucleus (SCN). This pre-sleep drinking behavior was shown to be under the control of vasopressin neurons that run from the SCN to thirst neurons in the organum vasculosum lamina terminalis, which is an area important for fluid regulation.
Vasopressin is known to work as a hormone, but this discovery demonstrates that the molecule also works as a neurological signal. Its role is clearly more complex than originally believed.
The importance of studying dehydration
Dehydration is an important topic of study. A range of conditions are linked with changes in vasopressin. As Dr. Bourque explains, "We've known for some time that [vasopressin] is involved in critical conditions like congestive heart failure and some types of lung cancer and other ailments like bed-wetting or increased urination at night. Our recent work has shown it also plays a key role in salt-dependent hypertension."
In fact, body fluid disorders are some of the top reasons for patient admission to emergency departments in the United States. Dr. Bourque hopes that his work will spur more studies into the importance of vasopressin in sickness and health.
"We know [vasopressin] is one of the most important molecules for regulating body hydration, but we still can't use it to improve primary care."
Dr. Charles Bourque
One of his key goals is to design a sensitive vasopressin test for healthcare workers. If levels can be assessed rapidly, it could help doctors to understand how the body is managing water, giving clues as to how to deal with various illnesses. Dr. Bourque plans to work with an interdisciplinary team of scientists to make this test a reality.