GLUCOSE PROCESSING DURING THE INTRAVENOUS GLUCOSE-TOLERANCE TEST

The impact of the dynamic changes in plasma glucose and insulin levels observed during a frequently sampled intravenous (IV) glucose toleran ce test (FSIGT) on whole-body glucose processing and muscle glycogen m etabolism is not known. Paired randomized FSIGTs were performed in eig ht healthy subjects (age, 31 years; range, 28 to 35; BMI, 25.4 kg/m(2) ; range, 22.3 to 32.1), one with muscle biopsy samples and one without . The mean time average (0- to 40- and 0- to 120-minute) insulin level s during the test were 26.6 and 11.4 mU/l, respectively. Glucose oxida tion increased following the IV gluclose bolus (basal 1.34 +/- 0.21 v mean value at 0 to 120 minutes 2.09 +/- 0.22 mg/kg fat-free mass [FFM] /min, P < .02). In contrast, fractional glucose-6-phosphate [G-6-P]) ( 0.1/10 mmol/L) skeletal muscle glycogen synthase activity in muscle bi opsies obtained before and following the IV glucose bolus (-30, 30, 60 , and 120 minutes, respectively) were unchanged (38.1% +/- 3.3%, 38.3% +/- 2.9%, 38.1% +/- 2.3%, 35.4% +/- 2.3%, NS). Skeletal muscle glycog en concentration decreased slightly (449 +/- 54, 439 +/- 55, and 383 /- 29, and 438 +/- 48 mmol/kg dry weight, P = .05), indicating no net storage of glucose into glycogen during the FSIGT. G-6-P decreased (0. 77 +/- 0.08, 0.64 +/- 0.07, 0.66 +/- 0.07, and 0.54 +/- 0.04 mmol/kg d ry weight, P < .05). Levels of the insulin-regulatable glucose transpo rter, GLUT-4, were unchanged. Insulin sensitivity (Si), glucose effect iveness, and insulin secretion parameters (O1 and O2) were not affecte d by the muscle biopsy procedure. In conclusion, the FSIGT is associat ed predominantly with increased whole body glucose oxidation with no a pparent activation of muscle glucose storage as glycogen. Thus, the Si measured by the FSIGT, although similar in magnitude to the clamp-der ived parameter, represents primarily glucose oxidation, in contrast to the euglycemic clamp, which involves glucose oxidation and storage. ( C) 1996 by W.B. Saunders Company.