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Insulin-induced hypoglycemia leads to far-ranging negative consequences in patients with diabetes. Components of the counterregulatory response (CRR) system that help minimize and reverse hypoglycemia and coordination between those components are well studied but not yet fully characterized. Here, we tested the hypothesis that acyl-ghrelin, a hormone that defends against hypoglycemia in a preclinical starvation model, is permissive for the normal CRR to insulin-induced hypoglycemia. Ghrelin knockout (KO) mice and wild-type (WT) littermates underwent an insulin bolus-induced hypoglycemia test and a low-dose hyperinsulinemic-hypoglycemic clamp procedure. Clamps also were performed in ghrelin-KO mice and C57BL/6N mice administered the growth hormone secretagogue receptor agonist HM01 or vehicle. Results show that hypoglycemia, as induced by an insulin bolus, was more pronounced and prolonged in ghrelin-KO mice, supporting previous studies suggesting increased insulin sensitivity upon ghrelin deletion. Furthermore, during hyperinsulinemic-hypoglycemic clamps, ghrelin-KO mice required a 10-fold higher glucose infusion rate (GIR) and exhibited less robust corticosterone and growth hormone responses. Conversely, HM01 administration, which reduced the GIR required by ghrelin-KO mice during the clamps, increased plasma corticosterone and growth hormone. Thus, our data suggest that endogenously produced acyl-ghrelin not only influences insulin sensitivity but also is permissive for the normal CRR to insulin-induced hypoglycemia. [ABSTRACT FROM AUTHOR]

Objective: While expression of the ghrelin receptor, GHSR (growth hormone secretagogue receptor), by pancreatic islets has been investigated, data regarding the specific islet cell types that express GHSR are inconsistent and incomplete. For instance, transcriptomic studies indicate marked GHSR expression by somatostatin (SST)-secreting delta cells vs. little to no GHSR expression by alpha cells and beta cells, which contrasts studies suggesting direct actions of ghrelin on GHSRs expressed by alpha cells and beta cells. Also, to our knowledge, GHSR expression by pancreatic polypeptide (PP) cells has not previously been directly investigated. The aims of this study were to determine GHSR expression within the four principal islet endocrine cell types and the effects of ghrelin on circulating PP. Methods: Ghsr-IRES-Cre mice, which express Cre recombinase directed by the GHSR promoter, crossed to Cre-dependent ROSA26-YFP reporter mice were used to histologically identify GHSR expression in islets. Double and triple labeling for YFP-immunoreactivity (IR) with insulin-IR, glucagon-IR, SST-IR, and/or PP-IR was performed to determine GHSR co-expression with islet hormones. Pharmacological manipulations were used to examine in vivo effects of acyl-ghrelin on plasma PP. Results: Eighty-five % of YFP-IR cells expressed PP-IR, 50% expressed SST-IR, and none expressed insulin-IR or glucagon-IR. GHSR antagonist raised plasma PP in overnight-fasted C57BL/6N mice. However, acyl-ghrelin administration had no effect on plasma PP in C57BL/6N mice. Conclusions: These results suggest that within mouse pancreatic islets, GHSR expression occurs most abundantly in PP cells and that pharmacologic blockade of ghrelin action raises plasma PP. Disclosure: D. Gupta: None. B.K. Mani: None. K. Shankar: None. S. Osborne-Lawrence: None. N. Metzger: None. J.M. Zigman: Stock/Shareholder; Self; Medtronic. Funding: Diana and Richard C. Strauss Professorship in Biomedical Research; Mr. and Mrs. Bruce G. Brookshire Professorship in Medicine; Kent and Jodi Foster Distinguished Chair in Endocrinology; University of Texas Southwestern Medical Center (to J.M.Z.) [ABSTRACT FROM AUTHOR]

Objective: The hormone ghrelin both regulates and is regulated by food intake and blood glucose. Yet, the mechanisms restricting ghrelin release after meals or upon glucose delivery have remained elusive. Here, we tested whether meal and glucose-associated rises in plasma insulin act via ghrelin cell-expressed insulin receptors to suppress ghrelin release. Methods: We generated ghrelin cell-selective insulin receptor-knock out (GIRKO) mice by crossing ghrelin-Cre mice with floxed-insulin receptor mice. We examined ghrelin release using hyperinsulinemic-hypoglycemic clamps, glucose tolerance tests (GTTs), a fast-refeed protocol, and gastric mucosal cell primary cultures. Results: During clamps, plasma ghrelin fell in WT control groups (by 85%) but not in GIRKO mice, leading to a 29% lower glucose infusion rate in GIRKO mice. During GTTs, plasma ghrelin was suppressed in controls (by 42%) but not in GIRKO mice. During the refeed phase of a fast-refeed protocol, plasma ghrelin fell in controls (by 49%) but not in GIRKO mice. Also, insulin suppressed ghrelin release in isolated ghrelin cells of controls (by 19%) but not of GIRKO mice. Conclusions: These data suggest that insulin, acting via ghrelin cell-expressed insulin receptors, is a key mediator of ghrelin release in response to meals, glucose administration, and insulin infusion. Further, suppressed ghrelin release resulting from direct insulin action on ghrelin cells restricts ghrelin's capacity to protect against hypoglycemia during over-insulinization. Disclosure: K. Shankar: None. S. Takemi: None. D. Gupta: None. B.K. Mani: None. S. Osborne-Lawrence: None. N. Metzger: None. E. Berglund: None. J.M. Zigman: Stock/Shareholder; Self; Medtronic. Funding: National Institutes of Health (R01DK119341-01A1, R01DK103884 to J.M.Z.), (R01DK109408 to E.B.), (R01DK119341 to J.M.Z., E.B.); David and Teresa Disiere Foundation (to J.M.Z.); Diana and Richard C. Strauss Professorship in Biomedical Research; Mr. and Mrs. Bruce G. Brookshire Professorship in Medicine; Kent and Jodi Foster Distinguished Chair in Endocrinology (to J.M.Z.); Uehara Memorial Foundation (to S.T.) [ABSTRACT FROM AUTHOR]