Urinary C-Peptide: An Indicator of ²-Cell Secretion under Different Metabolic Conditions

The relative advantages of assessing ²-cell secretory function by measuring plasma or urinary insulin or Cpeptidewas studied in five athletes during timed periods:1 period, baseline fasting (3 h); period 2, exercise (1.5 h) at 75% maximum heart rate; period 3, rest (2.5 h); period 4, eating (3 h); and period 5, rest (2 h). Blood (collected using a constant withdrawal pump) and urine samples from each period were evaluated for insulin and C-peptide immunoreactivity. Plasma insulin decreased from 0.10 ± 0.01 pmol/ml (13.6 ± 2 μU/ml) in period 1 to 0.06 ± 0.02 pmol/ml (8.7 ±2.1 μU/ml; P < 0.005) in period 2, and increased in period 4 to 0.42 ± 0.08 pmol/ml (58 ± 10.9 μU/ml). Plasma C-peptide declined from 0.55 ±0.11 pmol/ml in period 1 to 0.33 ± 0.07 pmol/ml in period 2 (P < 0.005), and rose in period 4 to 1.9 ± 0.34 pmol/ml (P < 0.005). Urinary insulin decreased from a baseline of 0.05 ± 0.02 pmol/min (7.2 ± 2.1 μU/min) to 0.03 ± 0.01 pmol/min (3.7 ± 0.9 n) in period 2 (P < 0.05), while urinary C-peptide de...