Characterization of the insulin sensitivity of ghrelin receptor KO mice using glycemic clamps

Abstract

BackgroundWe and others have demonstrated previously that ghrelin receptor (GhrR) knock out (KO) mice fed a high fat diet (HFD) have increased insulin sensitivity and metabolic flexibility relative to WT littermates. A striking feature of the HFD-fed GhrR KO mouse is the dramatic decrease in hepatic steatosis. To characterize further the underlying mechanisms of glucose homeostasis in GhrR KO mice, we conducted both hyperglycemic (HG) and hyperinsulinemic-euglycemic (HI-E) clamps. Additionally, we investigated tissue glucose uptake and specifically examined liver insulin sensitivity.ResultsConsistent with glucose tolerance-test data, in HG clamp experiments, GhrR KO mice showed a reduction in glucose-stimulated insulin release relative to WT littermates. Nevertheless, a robust 1st phase insulin secretion was still achieved, indicating that a healthy β-cell response is maintained. Additionally, GhrR KO mice demonstrated both a significantly increased glucose infusion rate and significantly reduced insulin requirement for maintenance of the HG clamp, consistent with their relative insulin sensitivity. In HI-E clamps, both LFD-fed and HFD-fed GhrR KO mice showed higher peripheral insulin sensitivity relative to WT littermates as indicated by a significant increase in insulin-stimulated glucose disposal (Rd), and decreased hepatic glucose production (HGP). HFD-fed GhrR KO mice showed a marked increase in peripheral tissue glucose uptake in a variety of tissues, including skeletal muscle, brown adipose tissue and white adipose tissue. GhrR KO mice fed a HFD also showed a modest, but significant decrease in conversion of pyruvate to glucose, as would be anticipated if these mice displayed increased liver insulin sensitivity. Additionally, the levels of UCP2 and UCP1 were reduced in the liver and BAT, respectively, in GhrR KO mice relative to WT mice.ConclusionsThese results indicate that improved glucose homeostasis of GhrR KO mice is characterized by robust improvements of glucose disposal in both normal and metabolically challenged states, relative to WT controls. GhrR KO mice have an intact 1st phase insulin response but require significantly less insulin for glucose disposal. Our experiments reveal that the insulin sensitivity of GhrR KO mice is due to both BW independent and dependent factors. We also provide several lines of evidence that a key feature of the GhrR KO mouse is maintenance of hepatic insulin sensitivity during metabolic challenge.

Effect of ghrelin on glucose regulation in mice

“…Higher glucose disappearance rate observed in ghrelin Ϫ/Ϫ and Ghsr Ϫ/Ϫ than in WT during postabsorptive steady state despite any difference in blood glucose level supports the increased glucose disposal in these mice models. Measurement of whole body insulin sensitivity and insulin tolerance in ghrelin Ϫ/Ϫ and Ghsr Ϫ/Ϫ demonstrated that these mice models are less insulin resistant compared with WT (42,49). This is in line with our present data of higher hepatic insulin sensitivity index and reduced insulin resistance in ghrelin Ϫ/Ϫ and Ghsr Ϫ/Ϫ compared with WT mice.…”

“…Studies using glucose clamp demonstrated reduced insulin requirements for higher glucose disposal and a lower glucose-stimulated insulin release in the absence of ghrelin signaling (31,42). Ghrelin reversed leptin's inhibiting effect on insulin secretion, implying lower insulin in the absence of ghrelin (7,25,27,39,41,47).…”

Acylated and unacylated ghrelin do not directly stimulate glucose transport in isolated rodent skeletal muscle

“…In rodents, animals lacking ghrelin or the ghrelin receptor are resistant from the induction of impaired glucose tolerance and insulin sensitivity induced by a high fat diet (Qi et al. 2011). In contradiction to these findings, Gagnon et al.…”

“…Furthermore, ghrelin KO and ghrelin receptor KO mice are protected from the negative effects of a high fat diet on glucose tolerance and insulin sensitivity (Qi et al. 2011). In contradiction to these findings, AG injection has recently been shown to improve blood glucose clearance during an oral glucose tolerance test in mice, which was linked to the ability of AG to stimulate the secretion of glucagon‐like peptide‐1 (GLP‐1) (Gagnon et al.…”

Ghrelin – Physiological Functions and Regulation

“…Although some studies found a negative correlation between ghrelin level and insulin resistance, 83,84 there is plenty of data suggesting the reverse, as ablation of ghrelin and ghrelin receptors improves insulin sensitivity. 51,84,85 and ghrelin infusion in humans impairs glucose tolerance in hyperinsulinaemic euglycaemic clamp studies. Ghrelin has recently been shown to be directly regulated by glucagon.…”