Systemic and skeletal muscle metabolic signatures associated with early insulin resistance in just-weaned non-obese Goto-Kakizaki diabetic rats

The Goto-Kakizaki (GK) rat is a well-established experimental model for spontaneous non-obese type 2 diabetes mellitus (T2DM). However, the mechanisms underlying the early onset of the disease remain poorly understood. In this study, we investigated systemic and skeletal muscle metabolism in just-weaned (21-day-old) GK rats to identify early metabolic alterations preceding overt hyperglycemia. Male and female GK rats were compared with age- and sex-matched Wistar controls. All animals were bred and maintained under controlled environmental conditions. Offspring remained with the dams until postnatal day 21 (PND21), which served as an acclimatization period before experimental procedures. On PND21, non-fasted blood glucose and insulin levels were measured to avoid metabolic stress during weaning, followed by euthanasia and serum collection for biochemical and metabolomic analyses. Capillary and serum glucose levels, together with the HOMA-IR and QUICKI indices, confirmed an insulin-resistant state in both male and female GK rats. Serum NMR-based metabolomics revealed elevated concentrations of alanine, lysine, proline, lactate, fatty acyl chains, cholesterol, and glycerophosphocholine in both sexes, while triacylglycerols and glucose were increased only in males. The absolute and normalized wet and dry masses of the extensor digitorum longus and soleus muscles were significantly lower in GK rats of both sexes. The HR-MAS NMR analysis of muscle tissues revealed decreased alanine, arginine, and creatine levels in both sexes, with additional reductions in glycine, glutamine, lactate, and ATP detected exclusively in male GK rats. These findings demonstrate that systemic and skeletal muscle metabolic alterations are already established at the weaning stage, preceding the onset of overt diabetes. The identified metabolic signatures, particularly those reflecting enhanced glycolysis and protein catabolism, highlight early biomarkers of insulin resistance and reveal sex-related differences in the onset of non-obese T2DM.