Rapamycin, an immunosuppressant used in human transplantation, impairs β-cell function, but the mechanism is unclear. Chronic (24 h) exposure to rapamycin concentration dependently suppressed 16.7 mM glucose-induced insulin release from islets (1.65±0.06, 30 nM rapamycin versus 2.35±0.11 ng/islet per 30 min, control, n=30, P<0.01) without affecting insulin and DNA contents. Rapamycin also decreased α-ketoisocaproate-induced insulin release, suggesting reduced mitochondrial carbohydrate metabolism. ATP content in the presence of 16.7 mM glucose was significantly reduced in rapamycin-treated islets (13.42±0.47, rapamycin versus 16.04±0.46 pmol/islet, control, n=30, P<0.01). Glucose oxidation, which indicates the velocity of metabolism in the Krebs cycle, was decreased by rapamycin in the presence of 16.7 mM glucose (30.1±2.7, rapamycin versus 42.2±3.3 pmol/islet per 90 min, control, n=9, P<0.01). Immunoblotting revealed that the expression of complex I, III, IV, and V was not affected by rapamycin. Mitochondrial ATP production indicated that the respiratory chain downstream of complex II was not affected, but that carbohydrate metabolism in the Krebs cycle was reduced by rapamycin. Analysis of enzymes in the Krebs cycle revealed that activity of α-ketoglutarate dehydrogenase (KGDH), which catalyzes one of the slowest reactions in the Krebs cycle, was reduced by rapamycin (10.08±0.82, rapamycin versus 13.82±0.84 nmol/mg mitochondrial protein per min, control, n=5, P<0.01). Considered together, these findings indicate that rapamycin suppresses high glucose-induced insulin secretion from pancreatic islets by reducing mitochondrial ATP production through suppression of carbohydrate metabolism in the Krebs cycle, together with reduced KGDH activity.

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin secreted from enteroendocine K cells after nutrient ingestion. Fat strongly induces GIP secretion, and GIP hypersecretion is involved in high-fat diet-induced obesity and insulin resistance. Aging also induces GIP hypersecretion, but its effect on body weight gain and insulin sensitivity remains unclear. In the present study, we investigated the effect of GIP on age-related body weight gain and insulin resistance using GIP-knockout homozygous (GIP^{−/ −}) and heterozygous (GIP^{+/ −}) mice, which have entirely absent and 50% reduced GIP secretion compared to wild-type (WT) mice, respectively. Under 12% fat-containing normal diet feeding condition, body weight was significantly lower in GIP^{−/ −} mice compared to that in WT and GIP^{+/ −} mice from 38 weeks of age, while there was no significant difference between WT and GIP^{+/ −} mice. Visceral and s.c. fat mass were also significantly lower in GIP^{−/ −} mice compared to those in WT and GIP^{+/ −} mice. During oral glucose tolerance test, blood glucose levels did not differ among the three groups. Insulin levels were significantly lower in GIP^{−/ −} mice than those in WT and GIP^{+/ −} mice. During insulin tolerance test, GIP^{−/ −} mice showed higher insulin sensitivity than that of WT and GIP^{+/ −} mice. Adiponectin mRNA levels were increased and leptin mRNA levels tended to be decreased in adipose tissue of GIP^{−/ −} mice. These results demonstrate that GIP is involved in age-related obesity and insulin resistance and that inhibition of GIP secretion alleviates age-related fat mass gain and insulin resistance under carbohydrate-based diet feeding condition.

Glucose-dependent insulinotropic polypeptide (GIP), a gut hormone secreted from intestinal K-cells, potentiates insulin secretion. Both K-cells and pancreatic β-cells are glucose-responsive and equipped with a similar glucose-sensing apparatus that includes glucokinase and an ATP-sensitive K⁺ (K_ATP) channel comprising KIR6.2 and sulfonylurea receptor 1. In absorptive epithelial cells and enteroendocrine cells, sodium glucose co-transporter 1 (SGLT1) is also known to play an important role in glucose absorption and glucose-induced incretin secretion. However, the glucose-sensing mechanism in K-cells is not fully understood. In this study, we examined the involvement of SGLT1 (SLC5A1) and the K_ATP channels in glucose sensing in GIP secretion in both normal and streptozotocin-induced diabetic mice. Glimepiride, a sulfonylurea, did not induce GIP secretion and pretreatment with diazoxide, a K_ATP channel activator, did not affect glucose-induced GIP secretion in the normal state. In mice lacking K_ATP channels (Kir6.2 ^−/− mice), glucose-induced GIP secretion was enhanced compared with control (Kir6.2 ^{+ / +} ) mice, but was completely blocked by the SGLT1 inhibitor phlorizin. In Kir6.2 ^−/− mice, intestinal glucose absorption through SGLT1 was enhanced compared with that in Kir6.2 ^{+ / +} mice. On the other hand, glucose-induced GIP secretion was enhanced in the diabetic state in Kir6.2 ^{+ / +} mice. This GIP secretion was partially blocked by phlorizin, but was completely blocked by pretreatment with diazoxide in addition to phlorizin administration. These results demonstrate that glucose-induced GIP secretion depends primarily on SGLT1 in the normal state, whereas the K_ATP channel as well as SGLT1 is involved in GIP secretion in the diabetic state in vivo.