Glucagon-like peptide 1 (GLP-1) is a crucial regulator of glucose homeostasis and appetite, with GLP-1 receptor agonists (GLP-1RAs) effectively treating type 2 diabetes (T2D) and obesity. Endogenous GLP-1 secretion, modulated nutritionally, offers a potential alternative to GLP-1RAs, potentially mitigating side effects and engaging local sensory nerves. However, the mechanisms by which protein digestion products stimulate GLP-1 release remain poorly understood. Previous research identified L-valine as the most potent luminal stimulator of GLP-1 release from the rat small intestine, suggesting its potential as a nutritional strategy to enhance endogenous GLP-1 secretion. This study aimed to elucidate the molecular details of L-valine's stimulatory effect on gut hormone secretion, focusing on its absorption and intracellular mechanisms of action. The researchers hypothesized that the mechanism involves uptake and/or intracellular processes because L-valine only stimulated GLP-1 secretion when infused luminally. They investigated the role of sodium co-transport, amino acid oxidation, and calcium influx.

The literature review section extensively cites prior research on GLP-1's role in glucose homeostasis and appetite regulation, highlighting the therapeutic success of GLP-1RAs. It emphasizes the limitations in understanding how protein digestion products stimulate GLP-1 release, given the complex mixture of oligopeptides and amino acids involved. Previous work from the authors' group is cited, detailing the potent GLP-1 stimulatory effect of protein hydrolysates and the identification of L-valine as a particularly strong stimulator. This sets the stage for the current investigation into L-valine's mechanism of action.

The study used in vivo and in vitro approaches. In vivo, male C57BL/6JRj mice received oral L-valine (1 g/kg), D-glucose (2 g/kg), or water. Blood samples were collected to measure blood glucose and active GLP-1 levels. In vitro experiments used isolated perfused rat small intestines and GLUTag cells. The perfused small intestine allowed for luminal and vascular infusions of L-valine and inhibitors (nifedipine, diazoxide). GLUTag cells were employed to investigate intracellular calcium mobilization using Fluo-4 assay and the effects of inhibitors on L-valine-induced calcium increase. Perfused rat colon was also studied to assess L-valine's effects on the distal intestine. The methodology section provides detailed descriptions of animal handling, surgical procedures, perfusion protocols, and biochemical assays for hormone and glucose measurements, including the use of specific inhibitors and the statistical analyses employed.

Oral L-valine (1 g/kg) in mice increased plasma active GLP-1 similarly to oral glucose (2 g/kg), indicating its in vivo potency. Luminal L-valine (50 mM) in perfused rat small intestine robustly stimulated GLP-1 secretion (p<0.0001). Nifedipine (10 µM), a voltage-gated Ca²⁺-channel blocker, significantly inhibited this response (p<0.01), implicating Ca²⁺ channels. Sodium depletion did not affect L-valine-induced GLP-1 secretion, suggesting that Na⁺ co-transport is not crucial for membrane depolarization. Diazoxide (250 µM), a KATP-channel opener, completely blocked the L-valine response (p<0.05), suggesting that L-valine metabolism leads to KATP-channel closure and subsequent depolarization. L-valine also tended to stimulate GLP-1 and PYY release from the perfused rat colon, although less potently than in the small intestine. In GLUTag cells, L-valine increased intracellular calcium, an effect reduced by nifedipine, diazoxide, and EDTA, further supporting the proposed mechanism. L-valine was absorbed in the colon but less effectively than in the small intestine.

The results demonstrate that L-valine is a potent GLP-1 secretagogue in rodents, acting through a mechanism involving intracellular metabolism, closure of ATP-sensitive potassium channels (KATP), membrane depolarization, and subsequent opening of voltage-gated calcium channels (VGCC). The lack of effect from sodium depletion suggests that Na⁺-coupled transport is not essential for this process. The less potent effect in the colon compared to the small intestine may be due to differences in L-valine absorption. The use of GLUTag cells, while useful, has limitations due to the loss of cell polarization. The study highlights the potential of L-valine as a nutritional strategy to modulate GLP-1 secretion, but further research is needed to confirm these findings in humans, exploring the postprandial effects of L-valine preloads.

L-valine potently stimulates GLP-1 secretion in rodents via a mechanism involving intracellular metabolism, KATP channel closure, membrane depolarization, and VGCC activation. This suggests L-valine as a potential nutritional approach for GLP-1 modulation, although further human studies are needed to validate these findings and explore its effect on postprandial glucose control.

The study's limitations include the use of high L-valine concentrations (50 mM) in the perfused intestine, potentially higher than physiological levels after a meal. The second L-valine response was lower than the first in all experiments, possibly due to the short washout period causing receptor desensitization or saturation. The study only used male animals, potentially limiting the generalizability of the findings. The use of GLUTag cells, lacking physiological polarity, might have affected the amino acid sensing mechanisms. The total amino acid assay used to measure L-valine absorption may not provide completely accurate L-valine measurements.