To determine the role of nutrient status on ghrelin regulation of insulin secretion. We hypothesize that ghrelin and glucagon-like peptide-1 (GLP-1)(both are hormones made in the gut,) have differential effects on β-cell function in the fed state. We will compare insulin secretion and glucose turnover during meal ingestion using a dual glucose tracer and mixed meal protocol in subjects receiving ghrelin or saline. We will also determine the role of ghrelin-stimulated GLP-1 levels in this process using the GLP-1 receptor (GLP-1R) antagonist Exendin(9-39) (Ex-9).

We plan to study 20 healthy subjects on 4 occasions where they will receive ghrelin, ghrelin+Ex-9, Ex-9 or saline infusion after an overnight fast in a randomized order; Ex-9 will be used to block GLP-1 action. A 240-minute meal tolerance test (MTT) using a dual glucose tracer method will serve as the foundation of each study visit. One tracer, [6,6-2H2]glucose will be infused intravenously before and during the test meal to quantify fasting endogenous glucose production (EGP), and glucose disappearance during the meal. A second tracer, [U-13C]glucose, will be included in the meal to trace the appearance of oral glucose. The systemic appearance rates of both ingested tracer and total (i.e., ingested and endogenously produced) glucose will be calculated. Using this protocol, we will be able to evaluate a) insulin secretion in response to mixed-meal ingestion, b) glucose appearance and glucose disappearance during meal ingestion, c) the ghrelin effect on these parameters without GLP-1, and d) the effect of GLP-1 in the response based on the effects with and without Ex-9.

This dual-tracer method has been used to assess the ability of an individual to dispose of an oral glucose load, and accurately fractionates the appearance of ingested glucose in plasma (Ra meal), EGP, and peripheral glucose disposal (Rd) in this setting 41-42. The [6,6-2H2]glucose and [U-13C]glucose are stable-isotope tracers and are different from radioactive-isotope tracers in that they do not emit radiation.

All procedures will be performed at the CTRC at the Cincinnati Children's Hospital Medical Center (CCHMC).

Inclusion Criteria:

1. Healthy men and women
2. Ages between 18 and 45 years
3. BMI between 18 and 29 kg/m2

Exclusion Criteria:

1. History or clinical evidence of impaired fasting glucose or diabetes mellitus, myocardial infarction within the past year, history or symptoms of congestive heart failure, uncontrolled hypertension, history or active liver or renal disease (AST or ALT >2x upper limits of normal, calculated glomerular filtration rate [GFR] <60).
2. History of pituitary or adrenal disorders or neuroendocrine tumor.
3. Anemia defined as hematocrit <33%.
4. Use of medications that alter glucose metabolism
5. Pregnancy or lactation.
6. Abnormal Electrocardiogram (ECG): evidence of ischemia or arrhythmia.
7. Women who have a positive pregnancy test at any time during the study period.