title: A stable isotope method for in vivo assessment of human insulin synthesis and secretion.
abstract: OBJECTIVE
  In vitro, beta cells immediately secrete stored but readily releasable insulin in response to a rise of glucose. During a prolonged insulin response, this is followed by newly synthesized insulin. Our aim was to develop an in vivo test to determine the ratio between readily available and newly synthesized insulin after a stimulus in humans by labelling newly synthesized insulin.
  
  
  METHODS
  A stable isotope tracer of 1.0 g13C leucine with C-peptide as target peptide was administered 45 min prior to 75 g glucose load of a frequently blood sampled 210-min oral glucose tolerance test (OGTT). Our OGTT also encompassed collection of urine, which has a high content of C-peptide. Prior, the optimal conditions under which the tracer13C leucine was administered for enrichment of (pre) proinsulin were established. Also, techniques to obtain urinary C-peptide under highly purified circumstances were set up. Our main outcome measure was the stable isotope enrichment of de novo C-peptide, which we related to early plasma insulin and glucose AUC. Twelve healthy Caucasian individuals (M4F8, age 41.8 ± 2.3, BMI 28.3 ± 1.7) with normal glucose tolerance underwent our OGTT.
  
  
  RESULTS
  We found that during a 75-g OGTT, newly synthesized insulin contributed approximately 20 % of total insulin secretion. The pattern of isotope enrichment obtained by collecting multiple urine voids was suggestive that the newly synthesized insulin contributes to the late phase of insulin secretion. De novo C-peptide correlated negatively with both early plasma insulin AUC (r = -0.629, P = 0.028) and early plasma glucose AUC (r = -0.605, P = 0.037).
  
  
  CONCLUSIONS
  With stable isotope technique added to OGTT, we were able to measure newly synthesized insulin in healthy individuals. This new technique holds the promise that it is feasible to develop a direct in vivo beta cell function test.
authors.affiliation: Department of Internal Medicine, Erasmus MC - University Medical Center Rotterdam, Room Na512, PO Box 2040, 3000 CA, Rotterdam, The Netherlands.
authors.forename: Sjaam
authors.initials: S
authors.lastname: Jainandunsing
authors.affiliation: Department of Internal Medicine, Erasmus MC - University Medical Center Rotterdam, Room Na512, PO Box 2040, 3000 CA, Rotterdam, The Netherlands.
authors.forename: Joram N I
authors.initials: JNI
authors.lastname: van Miert
authors.affiliation: Department of Internal Medicine, Erasmus MC - University Medical Center Rotterdam, Room Na512, PO Box 2040, 3000 CA, Rotterdam, The Netherlands.
authors.forename: Trinet
authors.initials: T
authors.lastname: Rietveld
authors.affiliation: Department of Internal Medicine, Erasmus MC - University Medical Center Rotterdam, Room Na512, PO Box 2040, 3000 CA, Rotterdam, The Netherlands.
authors.forename: J L
authors.initials: JL
authors.lastname: Darcos Wattimena
authors.affiliation: Department of Internal Medicine, Erasmus MC - University Medical Center Rotterdam, Room Na512, PO Box 2040, 3000 CA, Rotterdam, The Netherlands.
authors.forename: Eric J G
authors.initials: EJG
authors.lastname: Sijbrands
authors.affiliation: Department of Internal Medicine, Erasmus MC - University Medical Center Rotterdam, Room Na512, PO Box 2040, 3000 CA, Rotterdam, The Netherlands. f.derooij@erasmusmc.nl.
authors.forename: Felix W M
authors.initials: FWM
authors.lastname: de Rooij
mesh_terms: D000328:Adult; D001786:Blood Glucose / Q000032:analysis; D002096:C-Peptide / Q000378:metabolism; D002846:Chromatography, Affinity / Q000379:methods*; D005240:Feasibility Studies; D005260:Female; D015309:Glucose Clamp Technique / Q000379:methods; D005951:Glucose Tolerance Test / Q000379:methods; D006801:Humans; D007328:Insulin / Q000032:analysis / Q000096:biosynthesis / Q000378:metabolism; D007333:Insulin Resistance / Q000502:physiology; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000502:physiology*; D007553:Isotope Labeling / Q000379:methods*; D007930:Leucine / Q000032:analysis / Q000378:metabolism; D008297:Male; D015203:Reproducibility of Results
chemical_list: D001786:Blood Glucose; D002096:C-Peptide; D007328:Insulin; D007930:Leucine
keywords: Beta cell function; C-peptide; Oral glucose tolerance test; Stable isotope