Weight loss medications alter how the brain and kidneys manage body fluids

Medications like liraglutide help treat diabetes and obesity, but they also influence how the body handles water and sodium. A new study shows that these drugs directly lower levels of a key water-conserving hormone, altering chemical signals sent from the brain to the kidneys. The findings were published in Science Advances.

Drugs known as glucagon-like peptide-1 receptor agonists mimic a natural digestive hormone. These medications make people feel full and prompt insulin release to manage blood sugar. Doctors notice that patients taking these drugs often lose water and sodium in their urine. Common side effects include nausea and diarrhea, which can lead to dehydration.

Patients are often advised to drink plenty of fluids to counter this water loss. Until now, researchers have not fully understood how these medications communicate with the biological systems governing basic fluid balance.

The primary way the human body regulates water involves a hormone called arginine vasopressin. Produced by specialized neurons in the brain, this hormone travels to the pituitary gland for storage. When the body needs to conserve water, the pituitary releases the hormone into the bloodstream.

The hormone then travels to the kidneys, prompting them to absorb water back into the blood rather than excreting it as urine. Michael P. Greenwood, a researcher at the University of Bristol, along with a team of colleagues, wanted to know if liraglutide disrupts this chain of events.

The researchers first conducted a small study involving sixteen healthy human adults. They took blood and urine samples from the volunteers before and after administering a single dose of liraglutide. The participants also consumed a slightly salty drink to prompt mild changes in their hydration levels.

After analyzing the samples, the researchers found that liraglutide acutely reduced the amount of arginine vasopressin in the blood. They also saw drops in a secondary marker molecule called copeptin, which the body releases alongside the primary water-conserving hormone. The drug also led to lower plasma potassium levels and higher amounts of sodium in the volunteers’ urine.

Because humans take these medications for extended periods, the researchers turned to an animal model to observe longer physiological changes. They injected male and female rats with either liraglutide or a saline placebo over the course of fourteen days. They then tracked the animals’ daily food and water consumption.

During the first few hours, the drug predictably reduced both food and water intake. Over the full fourteen days, the rats receiving the medication started consuming more water relative to their food intake compared to the control group. The medication essentially scrambled their normal daily drinking patterns, indicating a disturbance in their overall fluid regulation.

The team then examined the specific tissues involved in producing and storing the water-conserving hormone in these animals. They looked at the hypothalamus, the brain region that manufactures the hormone. They found no changes in the activity of the genes responsible for the hormone’s initial production.

However, when they looked at the pituitary gland where the hormone is stored, they found a decrease in the actual amount of the stored hormone. In female rats, they also observed lower levels of the marker molecule copeptin circulating in the blood after fourteen days. The drug appeared to be halting the release of the hormone from its storage site.

To understand why the pituitary gland was holding onto less hormone, the researchers analyzed the entire landscape of proteins within the tissue. They extracted tissue from the rat pituitary glands at four hours and fourteen days after drug administration. Using advanced protein-scanning techniques, they mapped both the abundance of different proteins and the chemical tags attached to them.

Cells often modify their proteins by attaching phosphate groups to them after they are built. These chemical tags act like molecular on-off switches, changing how a protein behaves or interacts with other molecules. By scanning for these tags, the researchers could see exactly which cellular machines were activated or deactivated by the drug.

The protein landscapes looked vastly different depending on the duration of the treatment and the sex of the animal. At the four-hour mark, both male and female rats showed broad changes in protein tagging. After fourteen days, female rats exhibited a massive shift in protein abundance and tagging, while male rats showed fewer alterations.

Many of the modified proteins belonged to a category of molecules that control how cells package and release hormones. Specifically, the researchers noticed heavy modifications to proteins guiding the microscopic storage bubbles that hold arginine vasopressin. Two proteins known to manage these storage bubbles acquired distinct chemical tags in the female rats following the fourteen-day treatment.

To test if these specific protein modifications directly alter hormone release, the researchers engineered a laboratory cell culture. They grew cells in a dish and introduced synthetic genes to mimic the exact protein modifications they had observed in the rat pituitary glands. They then measured how well these engineered cells could release a glowing marker molecule designed to imitate arginine vasopressin.

Altering these specific release-guiding proteins directly inhibited the cells’ ability to secrete the glowing marker. Mutating the proteins to block the chemical tags prevented the cells from holding back the secretion. This confirmed that the drug changes how the pituitary gland physically manages its hormone reserves at a microscopic level.

Finally, the researchers looked at the end of the line in the signaling pathway, which is the kidney. They examined kidney tissue from the treated rats to see how the organs were responding to the altered hormone levels. Under normal conditions, arginine vasopressin tells the kidneys to deploy tiny water channels called aquaporins to reabsorb fluid.

The team found that the genes responsible for building these water channels were less active in the treated rats. They also observed changes to the chemical tags on the aquaporin proteins themselves. These specific changes mark the channels for destruction rather than deployment. The drug was actively dampening the kidney’s ability to retain water.

The study outlines how these diabetes medications alter fluid dynamics, but it has some limitations. The human portion of the research was a small study involving only healthy individuals taking a single dose of the drug. The human trial did not have enough participants to measure potential differences between men and women.

In the animal studies, the researchers did not directly measure the volume of urine the rats produced. Future research will need to examine how these hormonal shifts affect patients taking the drug over months or years. Understanding these mechanisms could eventually help doctors assess kidney and cardiovascular risks for patients with fluid retention issues.

The study, “A clinical and experimental investigation of liraglutide effects on the brain-kidney axis,” was authored by Michael P. Greenwood, Mohammed I. Alotaibi, Soledad Bárez-López, Nathan W. P. Cantley, Sharon Merritt, Danijela Tatovic, and David Murphy.