High protein diet reduces food intake and improves glucose homeostasis in rodents and humans with obesity and diabetes, but the mechanistic site (s) of action remain unclear. Small intestinal lipid and glucose sensing regulates energy and glucose homeostasis, but the metabolic role of gut protein sensing remains elusive. In this study, we infused 8% casein hydrolysate as protein source into the upper small intestine (USI) and performed fasting-refeeding studies. We found that USI casein infusion (1.28 kcal/min) suppressed food intake up to 6h in chow but not high fat (HF) - fed rats (chow USI: sal 47±2 vs. cas 36±2 kcal, n=18, p<0.01. HF USI: sal 39±3 vs. cas 40±3 kcal, n=11) . Interestingly, we discovered USI casein infusion instead improved intravenous glucose tolerance effectively in both chow and HF rats (AUC, chow USI: sal 1152±112 vs. cas 850.2±37, n=7, p<0.05. HF USI: sal 1172±54 vs. cas 758.3±42 kcal, n=9, p<0.05) . Next, we evaluated whether small intestinal calcium sensing receptor (CaSR) mediates protein sensing and first discovered that CaSR was expressed in the small intestine with higher expression in the ileum vs. USI (USI 1.0±0.1 vs. ileum 5.8±1, n=8, p<0.01) . Next, we inhibited CaSR with CaSR inhibitor NPS2143 (56 µM) and found that its USI co-infusion with casein prevented the effects on glucose tolerance in both chow and HF rats. Finally, we injected lentivirus expressing shRNA of CaSR or mismatch sequence into the USI and CaSR expression was reduced by ˜40% (n=6, p<0.001) . Importantly, USI casein effect in both chow and HF rats on glucose tolerance were lost (chow: MM cas 799.2±51 vs. LV-CaSR cas 1069±64, n=6, p<0.01; HF: MM cas 771.8±20 vs. LV-CaSR + cas 1240±89, n=4, p<0.05) .
In summary, these data illustrate that USI protein sensing increases glucose tolerance in chow and HF rats via CaSR, and lowers feeding in chow but not HF rats. Future studies are needed to dissect the metabolic role of CaSR not only for USI but also ileal protein sensing in energy balance and glucose homeostasis in obesity and diabetes.
R.J.W.Li: None. D.Barros: None. Y.Lim: None. S.Zhang: None. A.Gao: None. T.K.Lam: None.