title: Fasting hyperglycemia impairs glucose- but not insulin-mediated suppression of glucagon secretion.
abstract: OBJECTIVE
  Our aim was to assess the effect of chronic hyperglycemia on glucose- and insulin-mediated suppression of glucagon secretion by the alpha-cell.
  
  
  METHODS
  Thirty subjects with normal glucose tolerance, 27 with impaired fasting glucose and/or impaired glucose tolerance, and 32 type 2 diabetic subjects were studied with oral glucose tolerance test (OGTT) and euglycemic hyperinsulinemic clamp. Fasting plasma glucagon concentration and plasma glucagon concentration during the OGTT and insulin clamp were measured.
  
  
  RESULTS
  During the OGTT, the decrement in the plasma glucagon concentration (area under the curve) was correlated inversely with the fasting plasma glucose concentration (r = -0.35; P < 0.001). As the fasting glucose level increased, the suppression of plasma glucagon progressively diminished. In contrast, during the euglycemic insulin clamp, the suppression of plasma glucagon was not correlated with the fasting plasma glucose concentration and was similar in subjects with normal glucose tolerance, subjects with impaired fasting glucose/impaired glucose tolerance, and diabetic subjects: 18, 23, and 18%, respectively.
  
  
  CONCLUSIONS
  Insulin-mediated suppression of glucagon secretion is unrelated to the fasting plasma glucose concentration and is not impaired by chronic hyperglycemia. Thus, the defect in plasma glucagon suppression during the OGTT most likely results from impaired glucose-mediated glucagon suppression. The close correlation between fasting plasma glucose concentration and reduced glucagon suppression suggests a glucotoxic effect on alpha-cell function.
authors.affiliation: Diabetes Division, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229, USA. albarado@uthscsa.edu
authors.forename: Muhammad
authors.initials: M
authors.lastname: Abdul-Ghani
authors.forename: Ralph A
authors.initials: RA
authors.lastname: DeFronzo
mesh_terms: D000328:Adult; D001786:Blood Glucose / Q000378:metabolism*; D002096:C-Peptide / Q000378:metabolism; D003924:Diabetes Mellitus, Type 2 / Q000097:blood; D005215:Fasting / Q000502:physiology; D005260:Female; D005934:Glucagon / Q000097:blood*; D015309:Glucose Clamp Technique; D005951:Glucose Tolerance Test; D006801:Humans; D006943:Hyperglycemia / Q000097:blood*; D007328:Insulin / Q000097:blood / Q000502:physiology*; D008297:Male; D017058:Mexican Americans
chemical_list: D001786:Blood Glucose; D002096:C-Peptide; D007328:Insulin; D005934:Glucagon