Two hormone systems that the biggest weight drugs switch on reshaped the mouse heart in different directions, a team at Lund University reported. One inflated the heart's main pumping chamber. The other thickened its wall. Which change showed up depended on the receptor, the sex of the animal, and whether the mouse was lean or obese.
The study, published September 14 in the American Journal of Physiology-Heart and Circulatory Physiology ↗, matters because those two hormones are the active targets of the biggest drugs in medicine right now. GLP-1 (glucagon-like peptide-1) is the target of semaglutide ↗, sold as Ozempic and Wegovy. GIP (glucose-dependent insulinotropic polypeptide) is the second target of tirzepatide ↗, sold as Mounjaro and Zepbound, which switches on both receptors at once. The GLP-1 arm has a track record of protecting the heart. The GIP arm does not. Whether stimulating the GIP receptor is good, bad, or neutral for the heart is still an open question, and this study is a direct attempt to look.
What they did
The researchers gave lean mice a month of continuous infusion of either a GIP-receptor agonist or a GLP-1-receptor agonist, at four doses from 0.1 to 1.0 milligrams per kilogram per day. A separate group ate a high-fat diet for two months to become obese, then received an effective dose (0.8 mg/kg/day) of one agonist or the other. Throughout, the team measured blood pressure with a tail cuff, imaged the heart with MRI, ran metabolic tests, and drew blood for two markers that rise when heart muscle is under strain, ST2 and NT-proBNP.
In the lean mice, the two drugs pulled the heart in different directions. The GIP-receptor agonist increased end-diastolic volume, the amount of blood the main chamber holds when full, and it did so more at higher doses. The GLP-1-receptor agonist instead increased the mass of the left ventricle, the muscle of the wall itself, and that showed up in the males. Neither change dented the heart's pumping strength. And here is the part that makes the finding hard to read: neither strain marker moved. The hearts were being remodeled with no sign of overt stress.
Why the sex split is the interesting part
In the obese mice, both drugs looked helpful on the numbers clinicians care about. Both lowered mean arterial pressure. Both improved systolic function, the squeeze of the pump. NT-proBNP, the strain marker, fell on both, consistent with a heart working under easier loading conditions. But the structural signature still split by sex. The GIP-receptor agonist mainly enlarged chamber volume in males. The GLP-1-receptor agonist mainly added wall mass in females.
That is the sentence worth arguing with. Cardiac remodeling is usually treated as one thing that is either good (the heart drifting back toward normal) or bad (the thickening and dilation that run ahead of failure). This study says the two incretin arms produce different kinds of remodeling, and the kind you get depends on which sex is taking the drug. If that holds in people, it complicates the tidy story that these drugs are simply cardioprotective.
The honest caveats
This is mice, not patients. It is one month of exposure, not the years a person spends on a weight drug. The agonists were generic research compounds delivered by an infusion pump, not the actual clinical molecules taken by weekly injection or daily pill, so the doses and kinetics do not map onto a prescription. And the most provocative result, the remodeling in lean mice, came with no change in the strain markers at all. The study cannot say whether a bigger chamber or a heavier wall in an otherwise healthy heart is a problem or a harmless adaptation.
What the work does is draw a map. Drugmakers are pushing GIP-receptor agonists, GIP-receptor blockers, and triple agonists into trials, and the assumption underneath much of that pipeline is that the GIP arm behaves at the heart the way the GLP-1 arm does. This study says do not assume it. The receptors are different. The remodeling is different. And, at least in mice, the sex of the animal changes the answer.
On peptidemodel, the drugs that carry these two arms are tirzepatide, the dual agonist, and semaglutide, the GLP-1-only agonist, both hosted against the GIP receptor ↗ and GLP-1 receptor ↗ targets. The cards that map onto GIP are worth watching as the arm moves from a supporting role in tirzepatide toward standalone candidates, because the heart, this study suggests, can tell the two receptors apart.