Hartmut Tillil

Abnormal Patterns of Insulin Secretion in Non-Insulin-Dependent Diabetes Mellitus

To determine whether non-insulin-dependent diabetes is associated with specific alterations in the pattern of insulin secretion, we studied 16 patients with untreated diabetes and 14 matched controls. The rates of insulin secretion were calculated from measurements of peripheral C-peptide in blood samples taken at 15- to 20-minute intervals during a 24-hour period in which the subjects ate three mixed meals. Incremental responses of insulin secretion to meals were significantly lower in the diabetic patients (P less than 0.005), and the increases and decreases in insulin secretion after meals were more sluggish. These disruptions in secretory response were more marked after dinner than after breakfast, and a clear secretory response to dinner often could not be identified. Both the control and diabetic subjects secreted insulin in a series of discrete pulses. In the controls, a total of seven to eight pulses were identified in the period from 9 a.m. to 11 p.m., including the three post-meal periods (an average frequency of one pulse per 105 to 120 minutes), and two to four pulses were identified in the remaining 10 hours. The number of pulses in the patients and controls did not differ significantly. However, in the patients, the pulses after meals had a smaller amplitude (P less than 0.03) and were less frequently concomitant with a glucose pulse (54.7 +/- 4.9 vs. 82.2 +/- 5.0, P less than 0.001). Pulses also appeared less regularly in the patients. During glucose clamping to produce hyperglycemia (glucose level, 16.7 mmol per liter [300 mg per deciliter]), the diabetic subjects secreted, on the average, 70 percent less insulin than matched controls (P less than 0.001). These data suggest that profound alterations in the amount and temporal organization of stimulated insulin secretion may be important in the pathophysiology of beta-cell dysfunction in diabetes.

Quantitative study of insulin secretion and clearance in normal and obese subjects.

The secretion and hepatic extraction of insulin were compared in 14 normal volunteers and 15 obese subjects using a previously validated mathematical model of insulin secretion and rate constants for C-peptide derived from analysis of individual decay curves after intravenous bolus injections of biosynthetic human C-peptide. Insulin secretion rates were substantially higher than normal in the obese subjects after an overnight fast (86.7 +/- 7.1 vs. 50.9 +/- 4.8 pmol/m2 per min, P less than 0.001, mean +/- SEM), over a 24-h period on a mixed diet (279.6 +/- 24.2 vs. 145.8 +/- 8.8 nmol/m2 per 24 h, P less than 0.001), and during a hyperglycemic intravenous glucose infusion (102.2 +/- 10.8 vs. 57.2 +/- 2.8 nmol/m2 per 180 min, P less than 0.001). Linear regression analysis revealed a highly significant relationship between insulin secretion and body mass index. Basal hepatic insulin extraction was not significantly different in the normal and obese subjects (53.1 +/- 3.8 vs. 51.6 +/- 4.0%). In the normal subjects, fasting insulin did not correlate with basal hepatic insulin extraction, but a significant negative correlation between fasting insulin and hepatic insulin extraction was seen in obesity (r = -0.63, P less than 0.02). This finding reflected a higher extraction in the six obese subjects with fasting insulin levels within the range of the normal subjects than in the nine subjects with elevated fasting insulin concentrations (61 +/- 3 vs. 45 +/- 6%, P less than 0.05). During the hyperglycemic clamp, the insulin secretion rate increased to an average maximum of 6.2-fold over baseline in the normal subjects and 5.8-fold in the obese subjects. Over the same time, the peripheral insulin concentration increased 14.1-fold over baseline in the normals and 16.6-fold over baseline in the obese, indicating a reduction in the clearance of endogenously secreted insulin. Although the fall in insulin clearance tended to be greater in the obese subjects, the differences between the two groups were not statistically significant. Thus, under basal, fasting conditions and during ingestion of a mixed diet, the hyperinsulinemia of obesity results predominantly from increased insulin secretion. In patients with more marked basal hyperinsulinemia and during intense stimulation of insulin secretion, a reduction in insulin clearance may contribute to the greater increase in peripheral insulin concentrations that are characteristic of the obese state.+

Insulin Secretion and Clearance: Comparison After Oral and Intravenous Glucose

Insulin secretion and clearance in response to the administration of oral and intravenous glucose was investigated in nine normal men. C-peptide metabolic kinetics were calculated by analysis of individual C-peptide decay curves after the bolus injection of biosynthetic human C-peptide. Glucose was administered to the subjects on three occasions: as a 75-g oral dose, a 75-g i.v. infusion, and an intravenous glucose infusion at a variable rate adjusted to mimic the peripheral glucose levels obtained after the oral glucose load (matching experiment). Glucose, insulin, and C-peptide concentrations were measured for the subsequent 5 h. The glucose level after the oral glucose load (115.9 ± 2.6 mg/dl, mean ± SE) closely approximated that after the matching experiment (120.5 ± 2.5 mg/dl) but was significantly lower than after 75 g i.v. glucose (127.7 ± 3.4 mg/dl, P < .05). Analysis of the areas under the peripheral concentration curves (60-360 min) showed that the responses of both insulin (52.7 ± 5.6 and 46.5 ± 4.5 pmol · ml−1 300 min1) and C-peptide (252.7 ± 27.5 and 267.0 ± 21.6 pmol · ml−1 · 300 min1) were not significantly different after the oral and 75-g i.v. glucose studies, respectively, whereas in the matching experiment, both the insulin (26.1 ± 3.9 pmol · ml−1 · 300 min−1) and C-peptide (178.0 ± 18.9 pmol ml−1 300 min−1) responses were lower (P < .05) than in the other two studies. Insulin secretory rates were derived from peripheral C-peptide concentrations with an open two-compartment model and individually derived model parameters. The basal insulin secretion rate was 86.8 ± 2.9 pmol/min. The insulin secretory response over the 300 min was 66.2 ± 4.8 nmol after oral glucose. This was similar to that after 75 g i.v. glucose (72.4 ± 4 . 1 nmol), whereas that secreted in response to the matching experiment was lower (47.6 ± 4.1 nmol, P ± .05). As a measure of the clearance of endogenous insulin, the ratio between the area under the insulin secretory curve and the area under the peripheral insulin concentration curve was calculated. This ratio was similar (1906 ± 149 ml/min) during the baseline period and the matching glucose infusion (2042 ± 245 ml/min) but was significantly lower after oral glucose (1330 ±112 ml/min, P < .05). The incretin effect calculated based on the insulin secretion rate (25 ± 9.2%) appeared to be less than if the calculations were based on peripheral insulin levels. These data demonstrate that equivalent doses of glucose administered orally and intravenously elicit an equivalent insulin secretory response. However, when the arterialized plasma glucose curve after 75 g oral glucose is matched by an intravenous glucose infusion, only 35.6 ± 2.9 g glucose was infused, and the intravenous glucose resulted in a lower secretory response. Furthermore, after oral administration of 75 g glucose a significant reduction in insulin clearance resulted. These data provide evidence that the hyperinsulinemia seen after oral glucose is due both to enhanced insulin secretion and diminished insulin clearance.

Reduction of Insulin Clearance During Hyperglycemic Clamp: Dose-Response Study in Normal Humans

Insulin secretion and clearance were studied in eight normal subjects who underwent hyperglycemie clamp studies at plasma glucose levels of 120, 225, and 300 mg/dl on three occasions. Insulin secretion rates were calculated during a 1-h baseline period and during 3 h of glucose clamping from a two-compartmental analysis of peripheral C-peptide concentrations with individual kinetic parameters derived after intravenous bolus injections of biosynthetic human C-peptide. At the 300-mg/dl clamp level, the insulin secretion rate increased to a value 9.9 ± 0.7 times that of basal at the end of the clamp (mean ± SE), whereas over the same period, the peripheral insulin concentrations increased to a greater extent, reaching a value 15.4 ± 1.2 times that of basal (P = .002). This greater relative increase in the insulin concentration in comparison with the corresponding insulin secretion rate suggests a reduction in the clearance of endogenous insulin. A similar trend was seen at the 225-mg/dl clamp level, but the relative increase in the insulin concentration (9.9 ±1.5 times that of basal) was not significantly higher than the relative increase in the insulin secretion rate (8.1 ± 0.5 times that of basal, P = .17). At the 120-mg/dl clamp level, the relative increases in the insulin secretion rate (2.7 ± 0.2 times that of basal) and the insulin concentration (2.4 ± 0.2 times that of basal) were similar (P = .26), indicating no reduction in endogenous insulin clearance during moderate stimulation of insulin secretion. In conclusion, a reduction in endogenous insulin clearance occurs during greater stimulation of insulin secretion at higher glucose-clamp levels. These data suggest that endogenous insulin clearance is nonlinear and shows evidence of saturation at high physiologic insulin concentrations.

Effect of Temporal Distribution of Calories on Diurnal Patterns of Glucose Levels and Insulin Secretion in NIDDM

The effect of different temporal patterns of calorie intake on plasma glucose, serum insulin, and insulin secretion rates was examined in six patients with moderately well controlled non-insulin-dependent diabetes mellitus (NIDDM). Patients were studied on three separate occasions over 26 h. Total calories and food composition (50% carbohydrate, 15% protein, and 35% fat) were kept constant, but the pattern of calorie intake was varied. In study A (similar meal size), calories were distributed as 30, 40, and 30% at breakfast, lunch, and dinner, respectively. In study B (3 snacks, 3 meals), each subject ate three meals of 20, 20, and 30% of calories for breakfast, lunch, and dinner, respectively, and three snacks, each comprising 10% of calories, presented 2.5 h after the meal. In study C (large dinner), 10% of calories were consumed at breakfast, 20% at lunch, and 70% at dinner. Glucose, insulin, and Cpeptide concentrations were measured at 15- to 30-min intervals. Insulin secretion rates were calculated from C-peptide levels with individually derived C-peptide clearance parameters. The different eating patterns were associated with only modest differences in overall levels of glucose and insulin secretion. Daytime insulin secretion was lowest when most of the daily calorie intake occurred in the form of a large dinner. Overnight levels of glucose and insulin secretion rates did not differ for the three eating patterns, and the morning glucose levels were also unaffected by the pattern of calorie intake on the previous day. A morning rise of glucose of >0.28 mM occurred consistently only when dinner was of moderate size (30% of total calories). We conclude that if total calorie content and food composition are kept constant, overall blood glucose control is only modestly affected by the temporal pattern of food intake. The partition of food intake into meals and snacks does not improve glucose control, and a large dinner does not result in significantly higher overnight or next-morning glucose levels.

Reevaluation of urine C-peptide as measure of insulin secretion.

Urine C-peptide (UCP) has been proposed as a measure of insulin secretion, because insulin and C-peptide are cosecreted in equimolar concentrations by the pancreatic β-cell. The validity of this approach was tested by comparing insulin secretion rates, calculated by application of a two-compartmental analysis of peripheral C-peptide concentrations, with UCP excretion rates. Insulin secretion and UCP excretion with subjects on a mixed diet were simultaneously measured over a 24-h period in 13 patients with non-insulin-dependent diabetes mellitus and in 14 matched nondiabetic control subjects. The fraction of secreted C-peptide that was excreted in the urine (fractional C-peptide excretion) showed considerable intersubject variability in the diabetic (11.3 ± 1.6%, range 3.9–20.8) and control (8.0 ± 1.7%, range 1.1–27.9, P = .07) subjects (means ± SE). UCP clearance demonstrated a similar degree of variability and was not significantly different (P = .07) between diabetic (23.8 ± 3.0 ml/min) and control (16.5 ± 2.7 ml/min) subjects. In control subjects, the 24-h insulin secretion rate correlated more closely with the fasting insulin secretion rate (r = .97, P = .0001), fasting C-peptide (r = .81, P = .0005), and fasting insulin (r = .80, P = .0005) concentrations than with the 24-h UCP excretion rate (r = .62, P = .02). Similar results were obtained in the diabetic patients. The mean coefficient of variation of fractional UCP excretion in 7 nondiabetic control subjects who were studied on a mixed diet over a 24-h period on two occasions was 28.4 ± 10.5%, that of UCP clearance was 28.9 ± 8.6%, and that of simultaneously measured creatinine clearance was 7.8 ± 3.5%. In summary, the fraction of secreted C-peptide that appears in the urine varies considerably between subjects and in the same subject studied repeatedly. UCP excretion does not correlate as well with 24-h insulin secretion as does the fasting insulin secretion rate or the fasting C-peptide or fasting insulin concentration. We conclude that, because the fraction of secreted C-peptide that is excreted in the urine varies considerably between subjects and in the same subject studied on different occasions, UCP is of only limited value as a quantitative measure of endogenous insulin secretion.