Transplanted Rat Guts Rewire Their Absorption While Transit Stays on Schedule

Intestinal transplantation is one of the most demanding operations in medicine, reserved for patients whose bowels have failed so completely that intravenous feeding can no longer sustain them. Yet even when the surgery succeeds and the immune system is held in check, many recipients continue to struggle with persistent gut dysfunction that clinicians cannot fully explain or treat. A new experimental study in rats, published in the journal Transplant International, offers a rare quantitative window into what actually happens inside a transplanted intestine once the acute surgical storm has passed, revealing a surprising split between how the graft moves and how it absorbs.

Researchers at KU Leuven and UZ Leuven performed orthotopic intestinal transplantations in inbred Lewis rats, a syngeneic Lewis-to-Lewis setup deliberately chosen to eliminate immunological rejection as a confounding variable. By using genetically identical donors and recipients, the team could isolate the purely physiological consequences of transplantation itself: the severing of lymphatic vessels, the loss of autonomic innervation from the brain and spinal cord, and the ischemia-reperfusion injury that occurs when the gut is deprived of blood during procurement and then flooded with oxygenated blood upon reperfusion. Thirteen transplants were completed, with donor procedures lasting on average 48 minutes, cold ischemia times of about an hour, and recipient operations of roughly two and a half hours.

The centerpiece of the study was an elegant tracer technique based on fluorescently labeled dextrans, non-digestible sugar polymers that can be given by oral gavage and then tracked in the bloodstream by measuring plasma fluorescence. The trick lies in molecular weight. A small 4-kilodalton dextran tagged with fluorescein (FITC4) crosses the intestinal lining mainly through the paracellular route, squeezing between epithelial cells through the tight junctions that normally seal the barrier. A bulky 70-kilodalton dextran tagged with rhodamine (TRITC70) can only enter via the transcellular pathway, riding on energy-dependent endocytosis through the enterocytes themselves. Because the two tracers report on structurally distinct absorption mechanisms, comparing their blood concentrations at 3, 6, and 24 hours after gavage allows researchers to dissect which pathway has been altered.

The timing of the blood draws also maps onto gut geography. In rats, small intestinal transit takes roughly two to six hours, so the 3-hour measurement reflects uptake in the small bowel, the 6-hour point blends small intestinal and colonic absorption, and the 24-hour sample captures late colonic permeability. As an added bonus, the team could follow the red-colored 70-kilodalton tracer in the feces, timing its excretion as a functional readout of total gastrointestinal motility. Each animal served as its own control, receiving the dextran cocktail two days before transplantation and again four weeks afterward, with paired statistical comparisons throughout.

The motility results were, on the surface, reassuring. Fecal excretion of the large tracer took 9.58 hours on average before transplant and 11.02 hours four weeks after, a difference that did not reach statistical significance. Despite the complete severing of sympathetic and parasympathetic nerves, leaving the graft dependent on its intrinsic enteric nervous system and circulating hormones, the transplanted bowel kept food moving at essentially its normal pace. Histology reinforced this picture of preserved wiring: the numbers of ganglia and neurons in both the myenteric and submucosal plexuses were indistinguishable from baseline four weeks after surgery, echoing earlier reports of preserved neuronal markers and even partial reinnervation lasting hundreds of days in similar models.

Absorption told a very different story. Plasma levels of the small paracellular tracer FITC4 were more than threefold higher in transplanted animals at 3 hours (0.72 versus 0.23 micrograms per milliliter, p = 0.041) and remained elevated at 6 hours (p = 0.031). The total exposure over time, quantified as the area under the plasma concentration curve, was nearly four times greater after transplantation (8.84 versus 2.39 microgram-hours per milliliter, p = 0.029). In stark contrast, the large transcellular tracer showed no significant change at any time point, and even trended downward at 24 hours. In other words, the graft became selectively leakier between its cells while the active, cell-crossing machinery of absorption carried on undisturbed.

What could drive this selective permeability shift? The histological analysis pointed toward a structural explanation. Villus length grew from 350 to 491 micrometers and villus width from 79 to 99 micrometers, pushing the calculated absorptive surface area per villus from 0.087 to 0.153 square millimeters, a 76 percent expansion. Because paracellular flux is passively driven by concentration gradients across the intercellular space, more lining means more potential leakage, whereas transcellular transport is self-limited by the slower, energy-hungry machinery of the enterocytes. Consistent with remodeling of the intercellular compartment, occludin immunohistochemistry revealed a significant increase in tight-junction protein staining after transplantation. The authors also raise the possibility of graft hypercontractility at the local level: accelerated small-bowel transit could shorten mucosal contact time, penalizing the slower transcellular route more than the passive one, and could even functionally mimic a shortened bowel, triggering the compensatory mucosal growth the team observed.

One cell population did not fare so well. Paneth cells, the antimicrobial peptide-secreting sentinels of the crypts that also nurture intestinal stem cells and reinforce the epithelial barrier, dropped significantly from 44 to 32 cells per high-power field four weeks after transplant. Crypt-to-villus ratios were unchanged, suggesting the overall architecture of renewal remained intact, but the persistent Paneth cell deficit mirrors findings in ischemia-reperfusion injury and in human inflammatory conditions such as Crohn’s disease and graft-versus-host disease. Whether this depletion reflects lingering regeneration after reperfusion injury or a stable new steady state of the graft remains an open question with potential implications for barrier defense and microbiome interactions.

The authors are candid about the limits of their pilot study. Only six animals completed the tracer experiments, the four-week follow-up may be too short to capture long-term adaptation such as lymphatic reconnection, which typically occurs within four to ten weeks, and dextran permeability does not directly predict the absorption of real nutrients or drugs, which travel mainly through transcellular pathways shaped by many additional factors. The design also omitted immunosuppression, a central feature of clinical intestinal transplantation, and lacked a sham surgery group to separate graft-specific effects from the stress of major abdominal surgery alone.

Even so, the study delivers a methodological gift to the field: a paired, in vivo technique that simultaneously quantifies gastrointestinal transit, paracellular permeability, and transcellular permeability in the same animal before and after transplantation. The central message, that a denervated, re-plumbed gut can maintain normal motility while quietly remodeling its absorptive lining toward greater paracellular flux, reframes how clinicians might think about the persistent enteric dysfunction seen in human recipients. If the same pathway-specific shift occurs in patients, it could inform nutritional strategies, drug dosing, and targeted therapies aimed at tight-junction regulation, and it sets the stage for longer studies probing how the transplanted intestine continues to adapt, and perhaps compensate, in the months and years after it finds a new home.

Subject of Research: Long-term motility and permeability changes in rat intestinal grafts after transplantation

Article Title: Changes in rat intestinal motility and absorption following intestinal transplantation: an experimental study

Article References: Dubois, A., Leenders, A., De Hertogh, G., Rustichelli, A., Boelhouwer, C., Wylin, T., Heedfeld, V., Bonaccorsi-Riani, E., Pirenne, J., Vanuytsel, T., Farré, R., & Ceulemans, L. J. (2026). Changes in rat intestinal motility and absorption following intestinal transplantation: an experimental study. Transplant International, 39, Article 17141. https://doi.org/10.3389/ti.2026.17141

Image Credits: AI Generated

DOI: 10.3389/ti.2026.17141

Keywords: intestinal transplantation, intestinal permeability, paracellular transport, transcellular absorption, fluorescent dextrans, gastrointestinal motility, enteric nervous system, tight junctions, occludin, Paneth cells, villus remodeling, ischemia-reperfusion injury