Contrasting metabolic effects of continuous and pulsatile growth hormone administration in young adults with Type 1 (insulin-dependent) diabetes mellitus

Summary Plasma growth hormone profiles in adolescents with Type 1 (insulin-dependent) diabetes mellitus are characterized by both increases in pulse amplitude and higher baseline concentrations. To determine which of these abnormalities adversely affect metabolic control, we studied six young adults overnight on three occasions. On each night somatostatin (50–100 g·m^2–1·h^–1) and glucagon (1ng· kg^–1·min^–1) were infused continuously and 18mU/kg of growth hormone was given as either: three discrete pulses of 6 mU·kg^–1· h^–1 at 180-min intervals or a 12-h infusion (1.5 mU·kg^–1· h^–1) or buffer solution only on a control night. Euglycaemia was maintained by an insulin-varying clamp. Blood samples were taken every 15 min for glucose and growth hormone and every hour for intermediate metabolites and non-esterified fatty acids. Comparable normoglycaemic conditions were achieved on all three nights. Growth hormone levels achieved (mean±SEM) on study nights were: 32.8±2.2 mU/l (peak level during growth hormone pulses); 9.8± 0.8 mU/l (continuous growth hormone) and 1.1±0.3 mU/l (control level). Pulsatile growth hormone administration led to an increase in insulin requirements (mean±SEM: 0.17±0.03 vs control 0.09±0.01 mU·kg^–1· min^–1, p < 0.05) whereas insulin requirements following continuous growth hormone administration were unchanged. Cross-correlation confirmed an increase in insulin requirements occurring 135 min after a growth hormone pulse (r=0.21, p < 0.001). Growth hormone administration (continuous and pulsatile) led to a significant increase in B-hydroxybutyrate levels compared to the control night: 0.21±0.01 mmol/l (mean±SEM), 0.29±0.01 mmol/l, 0.08±0.01 mmol/l (p< 0.001) during the night with pulsatile growth hormone, continuous growth hormone and control respectively. Mean plasma non-esterified fatty acids were also increased following growth hormone administration: 0.94±0.04 mmol/l (mean±SEM), 1.09±0.07 mmol/l, 0.61±0.05 mmol/l (p<0.003), during the night with pulsatile growth hormone, continuous growth hormone and control respectively. It appears that the pulsatile and baseline growth hormone signals have contrasting metabolic effects in young adults with Type 1 diabetes mellitus.     

Impaired insulin action in newly diagnosed type 1 (insulin-dependent) diabetes mellitus

Summary Hepatic and peripheral insulin sensitivity were investigated in five newly diagnosed Type 1 (insulin-dependent) diabetic subjects before and after 1 week of twice daily insulin therapy. Eight weight-matched control subjects were also studied. Hepatic glucose production and glucose utilization were measured basally and during two sequential 2-h insulin (25 and 40 mU· kg^–1· h^–1)/glucose infusion periods. In the untreated hyperglycaemic diabetic patients hepatic glucose production was 16.3±2.6, 8.1±1.1 and 3.6±2.8|mol· kg^–1· min^–1 respectively for each of the three periods (mean±SEM), and fell with treatment to 12.5±1.4, 0.5±0.5 and 0.5±0.5 mol· kg^–1· min^–1. Hepatic glucose production for normal subjects was 13.4±0.7, 2.3±0.8 and <0.1 mol-kg^–1· min^–1. Glucose utilization was 12.7±1.4,18.2±0.7 and 22.1±3.4mol· kg^–1· min^–1 before treatment in the diabetic subjects, and 11.8±1.7, 20.9±3.3 and 30.1±3.6 after treatment. These values compare with those in the euglycaemic control subjects (13.4±0.7, 18.7±1.6 and 36.3±2.7 mol · kg^–1· min^–1). The pre-treatment metabolic clearance rate of glucose in all diabetic studies with insulin levels >30mU/l was 2.6 ±0.4 and rose to 3.9 ±0.5 ml· kg^–1· min^–1 following insulin therapy. This was significantly lower than in the control subjects (6.7±0.8 ml· kg^–1 · min^–1; p<0.005). Basal nonesterified fatty acid levels were high in the untreated, but normal in the treated diabetic subjects, and fell in response to insulin infusion. Basal -hydroxybutyrate levels were high in both diabetic groups, but also fell in response to insulin infusion. Erythrocyte insulin receptor binding was normal in the untreated diabetic subjects, and was not changed by treatment. Therefore, treatment of newly diagnosed Type 1 diabetic subjects with insulin reverses the hepatic insensitivity to insulin. In contrast, treatment only partially improves peripheral glucose disposal. Since erythrocyte insulin receptor binding is normal, it is likely that a post-receptor defect in peripheral glucose metabolism exists in Type 1 diabetic patients despite insulin therapy and good diabetic control for a period of 1 week.     

Peripheral and hepatic insulin sensitivity in subjects with impaired glucose tolerance

Summary Recent evidence suggests that the post-prandial hyperglycaemia in impaired glucose tolerance is primarily due to impaired suppression of basal hepatic glucose output. This in turn appears to be secondary to decreased first phase insulin secretion, although decreased hepatic insulin sensitivity, which is a feature of non-insulin-dependent diabetes mellitus, might also play a role. Eight mildly overweight subjects with impaired glucose tolerance and eight closely matched control subjects with normal glucose tolerance underwent an intravenous glucose tolerance test to assess first phase insulin secretion. Insulin sensitivity was examined by a 150-min hyperinsulinaemic-euglycaemic clamp. Somatostatin was infused from 150 min to suppress endogenous insulin secretion, and glucagon and insulin were replaced by constant infusion. Glucose with added dideuterated glucose (labelled infusion technique) was infused to maintain euglycaemia. First phase insulin secretion ( 0–10 min insulin area ÷ 0–10 min glucose area) was significantly decreased in the subjects with impaired glucose tolerance (median [range]: 1.2 [0.2–19.4] vs 9.1 [2.6–14.5] mU·mmol^–1; p<0.01). During the clamp, circulating insulin (93±8 [mean±SEM] and 81±10 mU·l^–1) and glucagon (54±4 and 44±6 ng·l^–1) levels were comparable. Total glucose disposal was decreased in subjects with impaired glucose tolerance (2.78±0.27 vs 4.47±0.53 mg·kg^–1·min^–1; p<0.02), and was primarily due to decreased non-oxidative glucose disposal. However, hepatic glucose output rates were comparable during the clamp (0.38±0.10 and 0.30±0.18 mg·kg^–1·min^–1). Therefore, the main defects in subjects with impaired glucose tolerance are decreased first phase insulin secretion and peripheral non-oxidative glucose disposal, but hepatic glucose output shows normal responsiveness to insulin.Abbreviations FPIS First phase insulin secretion - PG plasma glucose - NIDDM non-insulin-dependent diabetes mellitus - IGT impaired glucose tolerance - HGO hepatic glucose output - IVGTT intravenous glucose tolerance test - OGTT oral glucose tolerance test     

Determination and kinetic analysis of non-insulin mediated glucose uptake in Type 1 (insulin-dependent) diabetes mellitus

Summary In man, total glucose uptake is the sum of insulin mediated glucose uptake and non-insulin mediated glucose uptake. The latter pathway has not been examined in Type 1 (insulin-dependent) diabetes mellitus. In order to assess non-insulin mediated glucose uptake in Type 1 diabetes, we measured steady-state rates of glucose uptake during glucose clamps at 5.27, 9.71 and 12.5 mmol/l using low (0.25 mU· kg^–1·min^–1), intermediate (0.75 mU·kg^–1·min^–1) and high (1.50 mU·kg^–1·min^–1) insulin infusion rates in 10 subjects with Type 1 diabetes. For insulin infusion rates of 0.25, 0.75 and 1.50 mU·kg^–1·min^–1 as plasma glucose rose from 5.27 to 9.71 mmol/l, total glucose uptake increased by 35, 43 and 52 percent respectively (p<0.05 for each insulin infusion rate). For all three insulin infusion rates, there was no significant increase in total glucose uptake as plasma glucose increased from 9.71 to 12.5 mmol/l. At each glycaemic level, glucose uptake correlated significantly with plasma free insulin (r=0.81, p<0.01 at 5.71 mmol/l; r=0.84, p<0.01 at 9.71 mmol/l; r=0.73, p<0.02 at 12.5 mmol/l). Linear regression analysis to a point corresponding to plasma free insulin equalling zero, yielded values for non-insulin mediated glucose uptake (mmol·kg^–1·min^–1) of 0.11,0.14,0.18 at plasma glucose of 5.27, 9.7 and 12.5 mmol/l respectively. Thus, increasing plasma glucose concentrations were associated with increasing rates of non-insulin mediated glucose uptake. For each insulin infusion rate used, the percent of total glucose uptake accounted for by non-insulin mediated glucose uptake remained independent of plasma glucose concentration, but decreased as insulin infusion rate increased. During the insulin infusion at 0.25 mU·kg^–1·min^–1, this percentage ranged from 83.7 to 91.4%. Analysis of glucose uptake data derived for theoretical plasma insulin levels of 0, 40, 80 and 160 U/ml yielded linear Eadie-Hofstee plots (r=– 0.83 to – 0.99), suggesting that insulin increased Vmax but did not alter Km. Hence, in these subjects with Type 1 diabetes, glucose uptake, both insulin mediated and non-insulin mediated can be described by Michaelis-Menten kinetics. Comparison of values obtained for Vmax and Km in the present studies of Type 1 diabetes with those obtained from non-diabetic subjects indicates that non-insulin dependent glucose uptake in Type 1 diabetes is quantitatively similar to that of non-diabetic subjects.     

Transient insulin resistance following infusion of adrenaline in Type 1 (insulin-dependent) diabetes mellitus

Summary Insulin resistance was assessed after an intravenous infusion of adrenaline (50 ng·kg^–1·min^–1) or saline (control study) given between 08.00 and 08.30 hours in nine patients with Type 1 (insulin-dependent) diabetes mellitus. The blood glucose level during a somatostatin (100g/h)-insulin (0.4mU·kg^–1·min^–1)-glucose (4.5 mg·kg^–1·-min^–1)-infusion-test performed between 1030 and 14.30 hours served as an indicator of the total body insulin resistance. Blood glucose was maintained around 7 mmol/l between 08.00 and 10.30 hours by a constant infusion of regular insulin (0.57 mU·kg^–1· min^–1) and a variable infusion of a 20% glucose solution. The infusion of adrenaline raised plasma adrenaline to 2.7±0.3 nmol/l (mean±SEM) at the end of the infusion; thereafter it returned to its basal level within 30 min. The plasma levels of free insulin, glucagon, cortisol and growth hormone were similar in the adrenaline and the control studies from 08.00 to 14.30 hours. In comparison with the control study the infusion of adrenaline decreased the need for intravenous glucose significantly over the initial 2 h. Furthermore, during the somatostatin-insulin-glucose infusion test the blood glucose rose significantly (p<0.05) over the initial 2h; thereafter no significant differences between the two studies were seen. It is concluded that a short term infusion of adrenaline, resembling the adrenergic hormone response to hypoglycaemia, induces a diabetogenic effect which subsides within 6 h after omission of the adrenaline infusion.     

Artificial induction of intravascular lipolysis by lipid-heparin infusion leads to insulin resistance in man

Summary Although extensive evidence indicates that free fatty acids can decrease glucose utilization in vitro, it is still controversial how an increase in lipolysis affects glucose metabolism in man. To test the hypothesis that an increase in lipolysis is related to insulin resistance, we examined the effect of lipid-heparin infusion on glucose metabolism in ten normal subjects by the euglycaemic glucose clamp technique and isotopic determination of glucose turnover. In the control euglycaemic clamp studies with insulin infusion at 0.2 and 1.0 mU·kg^–1·min^–1, endogenous glucose production was suppressed from the basal rate of 2.0±0.3 mg· kg^–1min^–1 to 1.1±0.7 mg·kg^–1·min^–1 and -0.4±0.7mg· kg^–1min^–1 respectively. Glucose utilization increased from the basal rate of 2.0±0.3 mg·kg^–1min^–1 to 2.3±0.5mg· kg^–1min^–1 and 5.9±1.8 mg·kg^–1min^–1 respectively. When the euglycaemic clamp studies were coupled with lipid-heparin infusion at comparable low and high rates of insulin infusion, endogenous glucose production increased (1.8± 0.7 mg·kg^–1·min^–1, p<0.001, and 0.3±0.6 mg·kg^–1· min^–1, p<0.05, respectively), and glucose utilization decreased (2.1±0.3 mg·kg^–1·min^–1, not significant, and 3.2±0.7 mg·kg^–1·min^–1, p<0.001 respectively). These data suggest that the artificial induction of intravascular lipolysis by lipid-heparin infusion leads to a state of insulin resistance in man.     

Decreased hepatic glucagon responses in Type 1 (insulin-dependent) diabetes mellitus

Summary The effect of glucagon infusion on hepatic glucose production during euglycaemia was evaluated in seven Type 1 (insulin-dependent) diabetic patients and in ten control subjects. In the diabetic subjects normoglycaemia was maintained during the night preceding the study by a variable intravenous insulin and glucose infusion. During the study endogenous insulin secretion was suppressed by somatostatin (450 g/h) and replaced by insulin infusion (0.15 mU·kg^–1·min^–1). ³H-glucose was infused for isotopic determination of glucose turnover. Plasma glucose was clamped at 5 mmol/1 for 2 h 30 min and glucagon (1.5 ng· kg^–1·min^–1) was then infused for the following 3 h. Hepatic glucose production and glucose utilisation were measured during the first, second and third hour of the glucagon infusion. Basal hepatic glucose production (just prior to glucagon infusion) was similar in diabetic (1.2±0.3 mg·kg^–1·min^–1) and control (1.6±0.1 mg·kg^–1·min^–1) subjects. In diabetic patients hepatic glucose production rose slowly to 2.1±0.5 mg·kg^–1·min^–1 during the first hours of glucagon infusion and stabilized at this level (2.4±0.5 mg·kg^–1·min^–1) in the third hour. In control subjects hepatic glucose production increased sharply to higher levels than in the diabetic subjects (3.4±0.3 mg·kg^–1·min^–1) during the first and second hour of glucagon infusion (p<0.05) and then gradually fell (2.9±0.4 mg·kg^–1·min^–1) during the third hour. In conclusion, when stimulated with glucagon at a physiologic plasma concentration diabetic patients had 1) an overall reduced hepatic glucose production response and 2) an abnormal sluggish response pattern. These abnormalities may imply inappropriate counter-regulation following a hypoglycaemic episode.     

Differential effects of insulin therapy on hepatic and peripheral insulin sensitivity in Type 2 (non-insulin-dependent) diabetes

Summary Hepatic glucose production and metabolic clearance rate of glucose were measured using (3-³H) glucose at steady state, basally and during two sequential 2 h insulin (25 and 40mU · kg^–1 · h^–1)/glucose(2 and 3mg · kg^–1 · min^–1) infusion periods. Eight diabetic subjects were studied before and after 1 week of twice daily insulin therapy; six control subjects matched for age, weight and degree of obesity were also studied. In the diabetic patients, pre-treatment hepatic glucose production was 20.0 ± 2.2, 9.9 ± 2.9, and 1.4 ± 0.8 mol · kg^–1 · min^–1 respectively (± SEM) for each of the three periods, and fell significantly with treatment to 12.8 ± 1.7,4.0 ± 1.5 and 1.9 ± 1.0 mol · kg^–1 · min^–1. Hepatic glucose production in normal subjects was 13.2 ± 0.6, 2.2 ± 0.8 and < 1 mol · kg^–1 · min^–1. The pre-treatment metabolic clearance rate in all diabetic studies with insulin levels 30 mU/l was 1.10 ± 0.14 ml · kg^–1 · min^–1 and remained virtually unchanged following insulin therapy; this was significantly lower than in the control subjects (6.83 ± 1.02, p < 0.001). Basal non-esterified fatty acid levels were higher (p < 0.02) in the pre-treated diabetic patients compared to post-treated diabetic patients and control subjects. Non-esterified fatty acids in each group fell to similar levels during the insulin infusions, but the rate of fall was slower in the pre-treated diabetic patients. Insulin receptor binding to erythrocytes was normal in the diabetic subjects and unchanged by treatment. Therefore, following insulin treatment of uncontrolled Type 2 (non-insulin-dependent) diabetes, the initially increased basal hepatic glucose production, and decreased hepatic sensitivity, return towards normal. However, the glucose clearance remains low, despite good diabetic control, and appears to be a major factor in the continuing glucose intolerance. As insulin receptor binding is normal, the defect of glucose clearance in Type 2 diabetes appears compatible with a post-receptor defect of glucose metabolism.     

Studies on the insulin-antagonistic effect of catecholamines in normal man

Summary The insulin-antagonistic effect of adrenaline was studied in seven healthy subjects with the euglycaemic clamp technique using two insulin infusion rates (40 and 1200 mU (m²)^–1 min^–1). The adrenergic receptor mediating the adrenaline effect was characterized by concomitant infusion of propranolol ₁+ ₂-antagonist) or metoprolol ( ₁-antagonist). Each subject was studied four times (placebo, adrenaline, adrenaline + propranolol, adrenaline + metoprolol). Glucose turnover was measured with D(3-³H)-glucose. Similar plasma insulin levels were reached in all studies with the two insulin infusion rates (mean; placebo 51 ± 3 and 7421 ± 337 mU/l respectively). Glucose production was completely inhibited by the low insulin level during placebo infusion. Adrenaline antagonized this effect so that a significant glucose production was seen at the low but not at the high insulin level. Propranolol, but not metoprolol, reversed this insulin-antagonistic effeet of adrenaline. Glucose utilization increased from 2.53 ±0.17 to 7.28 ± 0.88 mg · kg^–1 · min^–1 during placebo when the insulin levels were increased from 4± 0.3 to 51 ± 3 mU/l. Increasing the insulin levels 150-fold to 7500 mU/l only doubled the glucose utilization (14.68 ±1.14 mg·kg^–1·min^–1). Adrenaline induced a pronounced inhibition of glucose utilization at both insulin levels (78% and 37% inhibition respectively). Propranolol, but not metoprolol, prevented this effect of adrenaline. Thus, physiological adrenaline levels exert a pronounced insulin-antagonistic effect which is mediated by ₂-receptor stimulation. The inhibitory effect on glucose uptake is maintained even at high insulin levels when hepatic glucose production is completely abolished.     

No evidence for isotope discrimination of tritiated glucose tracers in measurements of glucose turnover rates in man

Summary Under non-steady-state conditions, glucose turnover rates determined with tritiated glucose tracers are often underestimated. To examine whether isotope discrimination or a tracer contaminant can contribute to this, we compared the turnover rates of unlabelled and tritiated glucose under isotopic steady-state conditions. The turnover rates were measured in 20 healthy subjects at two insulin concentrations (79±3 mU·l^–1 and 704±62 mU·l^–1). Euglycaemia was maintained by infusing unlabelled glucose mixed with (3³H)-or (6³H)-glucose. In both studies, the isotopically determined glucose disposal rate was virtually identical to the exogenous glucose infusion rate (low insulin 7.66±0.48 vs 7.58±0.44 mg·kg^–1·min^–1, high insulin 13.36±0.74 vs 13.55±0.98 mg·kg^–1·min^–1). The individual values were correlated in both the low (r = 0.85, p<0.001) and high dose insulin (r=0.81, p<0.001) studies. Tritiated glucose specific activities were also compared in arterialized and deep venous blood across forearm tissues during the high-dose insulin infusion. Glucose specific activities were similar in arterilized and deep venous blood when analysed with HPLC and conventional methods. In summary: (1) Under isotopic steady-state conditions the turnover rates of unlabelled and labelled glucoses are similar. (2) Unlabelled and labelled glucose are handled identically across forearm tissues. (3) We found no tracer impurity in our tritiated glucose preparations. We conclude that (3³H)- and (6³H)-glucose tracers can be used to reliably measure glucose turnover rates in man.     

Importance of glucagon in the control of futile cycling as studied in alloxan-diabetic dogs

Summary In order to determine the role of glucagon in futile or substrate cycling in diabetes, we measured tracer determined glucose kinetics during a combined infusion of 2-³H-glucose (total glucose production) and 6-³H-glucose (glucose production) in six alloxan-diabetic dogs. The animals received either a 420 min infusion of (1) somatostatin alone (0.3 g·kg^–1· min^–1), (2) somatostatin with insulin replacement (100 U·kg^–1min^–1) or (3) glucagon (6 ng·kg^–1· min^–1) together with somatostatin and transient insulin replacement. When somatostatin was given alone, plasma glucagon (p<0.004) and insulin (p<0.0001) were suppressed. Glucose production and disappearance and plasma glucose concentrations fell (p<0.0001), but the metabolic clearance of glucose did not change significantly. In the basal state, futile cycling comprised 29±4%, 33±4% and 33±3% of total glucose production in the three goups of studies, which is high compared to normal dogs. The absolute rate of futile cycling fell slightly but significantly from 10.0±1.7 to 8.3±1.7 mol·kg·^–1min^–1 (p<0.0008). When insulin replacement was given during somatostatin infusion to correct for the small somatostatin-induced insulin suppression, there were similar changes in plasma glucagon, glucose concentrations and glucose kinetics as seen during the infusion of somatostatin alone. Futile cycling decreased to a slightly greater extent from 12.8±2.8 to 9.5±1.7mol·kg^–1·min.^–1 (p<0.02). When glucagon was infused together with somatostatin and insulin replacement, plasma glucagon (p<0.0002) increased and plasma glucose levels rose (p<0.001) due to a transient increase in glucose production. Metabolic clearance of glucose did not change significantly. There was a marked increase in futile cycling from 12.2±1.7 to 21.7±1.7mol· kg^–1·min^–1 (p<0.0001) in response to exogenous glucagon excess. There was a slight (p<0.01) drop in free fatty acid levels with somatostatin. Free fatty acid levels nearly doubled (p<0.025) with the infusion of glucagon together with somatostatin. In conclusion, (a) futile cycling was increased in alloxan-diabetic dogs; (b) glucagon suppression can suppress futile cycling only if total insulin deficiency is prevented; and (3) hyperglucagonaemia increases futile cycling, and this effect is more pronounced during insulin deficiency.     

Increased glucose carbon recycling in severely insulin deficient Type 1 (insulin-dependent) diabetic subjects

Summary Six Type 1 (insulin-dependent) diabetic subjects were studied in order to determine the contribution of recycling of glucose carbon to the overproduction of glucose which is characteristic of the fasting hyperglycaemia produced by insulin withdrawal. The subjects were studied on two occasions, once after an overnight insulin infusion and once following 24 h of insulin withdrawal. The difference in turnover rates of 1-¹⁴C-glucose and 3-³H-glucose was used as a measure of glucose recycling. Insulin withdrawal caused a marked metabolic derangement with a rise in non-esterified fatty acids from 0.69±0.23 to 1.11±0.21 mmol/l (mean±SEM, p<0.05), total ketones from 0.27±0.06 to 2.06±0.51 mmol/l (p<0.01), cortisol from 341±43 to 479±31 nmol/l (p<0.05) and growth hormone from 1.1±0.3 to 19+5-mu/l (p<0.05). Glucose turnover rose from 13.8±2.3 mol·kg^–1·min^–1 at a glucose of 6.9±0.7 mmol/l in the insulin infused study to 25.8±4.4 mol·kg^–1·min^–1 (p<0.05) at a glucose of 16.4±0.7 mmol/l in the insulin withdrawn study. Recycling also rose from 3.0±0.4 mol· kg^–1·min^–1 to 9.4±2.2 mol·kg^–1·min^–1 (p<0.05) when insulin withdrawn, accounting for 23±3% and 36±3% of glucose turnover, respectively. We conclude that in the severely insulin deficient Type 1 diabetic subject recycling of glucose carbon is a major contributor to the excess glucose production.     

Acetyl-salicylic acid impairs insulin-mediated glucose utilization and reduces insulin clearance in healthy and non-insulin-dependent diabetic man

Summary The effect of acetyl-salicylic acid (ASA, 3 g per day for 3 days) on glucose utilization and insulin secretion was studied in healthy volunteers and Type 2 diabetic patients using the hyperglycaemic and euglycaemic insulin clamp technique. When in healthy subjects arterial plasma glucose was acutely raised and maintained at +7 mmol/l above fasting level, the plasma insulin response was enhanced by ASA (70±7 vs. 52±7mU/l), whereas the plasma C-peptide response was identical. Despite higher insulin concentrations, glucose utilization was not significantly altered (control, 61±7; ASA, 65±6mol·kg^–1·min^–1) indicating impairment of tissue sensitivity to insulin by ASA. Inhibition of prostaglandin synthesis was not likely to be involved in the effect of ASA, since insulin response and glucose utilization were unchanged following treatment with indomethacin. In the euglycaemic insulin (1 mU·kg^–1·min^–1) clamp studies, glucose utilization was unaltered by ASA despite higher insulin concentrations achieved during constant insulin infusion (103±4vs. 89±4mU/l). In Type 2 diabetic patients, fasting hyperglycaemia (10.6 ±1.1 mmol/l) and hepatic glucose production (15±2 mol·kg^–1·min^–1) fell upon ASA treatment (8.6±0.7 mmol/l; 13±1 mol·kg^–1· min^–1). During the hyperglycaemic clamp study, the plasma response of insulin, but not of C-peptide, was enhanced by ASA, whereas tissue sensitivity to insulin was reduced by 30 percent. It is concluded that in healthy and Type 2 diabetic man, ASA impairs tissue sensitivity to the action of insulin. This effect is counterbalanced by an augmented plasma insulin response to glucose, which results from a reduced insulin clearance rate. In Type 2 diabetic patients, the reduction in hepatic glucose production may be responsible for the amelioration of hyperglycaemia following ASA treatment.     

The regulation of glucose-induced insulin secretion by Pre-stimulus glucose level and tolbutamide in normal man

Summary The relationship between the pre-stimulus glucose level and immunoreactive insulin responses to a glucose challenge (20-g IV) was studied in normal subjects. When the steady-state pre-stimulus glucose concentration was lowered by a 0.33 mU· kg^-1· min^-1 insulin infusion or raised by a 900mg/ min glucose infusion, no effect on first phase insulin secretion (mean 3–5 min insulin level) was observed. In contrast, the second phase response (10–60 min insulin area after glucose pulse) to intravenous glucose fell during insulin infusion and increased during the glucose infusion. Overall, a linear relationship was found between the change of pre-stimulus glucose level from the control to that during the insulin or glucose infusion and the change in second phase response (r=0.65, n = 14, p<0.02). The effect of tolbutamide infusion (7 mg·m^-2· min^-1) when compared with saline control was to increase both first phase (+54 ±13 mU/1, n=8, p <0.001, mean±SEM) and second phase (+972±256mU · mur^-1 · 1^-1, P<0.01) insulinsecretion. It is concluded that the first phase response to a glucose pulse is independent of the steady-state pre-stimulus glucose concentration and is directly enhanced by tolbutamide; in contrast, second phase is related to both the steady-state pre-stimulus glucose level and tolbutamide. These findings suggest that changes in basal or pre-stimulus plasma glucose during therapy with sulphonylurea drugs may be expected to influence the second phase insulin responses to glucose challenge.     

The circulating molecular weight forms of infused recombinant insulin-like growth factor-I and effects on glucose and fat metabolism in lambs

Summary We have investigated the relationship between the plasma distribution of infused recominant insulin-like growth factor-I across the insulin-like growth factor binding proteins and the resultant effects on glucose and fat metabolism. The studies were performed in 24-h fasted ram lambs which received primed constant infusions of ³H labelled glucose tracer. When isotopic equilibrium had been reached, the animals received 90-min infusions of human insulin-like growth factor-I at various doses (2.5, 20, 40 and 120 g· kg^–1·h^–1, n=3 for each dose). Total plasma insulin-like growth factor-I was significantly elevated by infusion at a rate of 40 g·kg^–1·h^–1 (from 185±14 g/l to 442±41 g/l, p<0.05) and 120g·kg^–1h^–1 (from 181±2 g/l to 953±39 g/1, p<0.005). The plasma concentrations of insulin-like growth factor-I not associated with binding proteins remained undetectable (<15 g/l) at the end of the 2.5 and 20 g·kg^–1·h^–1 doses, but were significantly elevated at the end of the 40 and 120 g·kg^–1·h^–1 infusions (to 71±14 g/l, p<0.05 and 176±55 g/l, p<0.01 respectively). The infused insulin-like growth factor-I associated primarily with 35–60 kilodalton binding proteins. Glucose kinetics were significantly altered only by the highest dose infusion, during which there was a fall in plasma glucose concentration from 3.5±0.2 mmol/l to 1.9±0.2 mmol/l (p<0.05). This was due to a 51% increase in the rate of glucose clearance. There was no significant change in the rate of glucose production. The plasma concentrations of glycerol and non-esterified fatty acid were not changed by any of the doses infused. We conclude that the hypoglycaemic action of infused recombinant insulin-like growth factor-I relates to a marked elevation of free insulin-like growth factor-I in the plasma, but that a threshold concentration of free insulin-like growth factor-I must be exceeded before this action is observed. The hypoglycaemic action of recominant insulin-like growth factor-I results primarily from an increase in glucose clearance while glucose metabolism was more sensitive than fat metabolism to infused recominant insulin-like growth factor-I. Both these actions contrast with those of insulin, and suggest that the acute metabolic effects of recombinant insulin-like growth factor-I are not mediated simply by cross-reaction with insulin receptors.     

Increased glucose turnover and glucose cycling in acromegalic patients with normal glucose tolerance

Summary To characterize the diabetogenic effects of growth hormone, we simultaneously measured glucose turnover with 2-³H- and 6-³H-glucose in six acromegalic patients with normal fasting blood glucose and oral glucose tolerance tests. Eight healthy volunteers served as controls. All subjects were studied under both basal conditions and during glucose infusion (2 mg · kg^–1 · min^–1). We determined true glucose production and irreversible glucose uptake using 6-³H-glucose and glucose cycling (difference between 2-³H- and 6-³H-glucose). After an overnight fast, glucose production was higher than normal in the acromegalic patients (2.18±0.15 vs 1.85±0.03 mg · kg^–1 · min^–1,p < 0.05) despite hyperinsulinaemia. The metabolic clearance rate was normal. During the glucose infusion, glucose production was not suppressed as effectively in the acromegalic patients as in controls nor was glucose uptake augmented, while metabolic clearance rate was decreased. In acromegaly, basal glucose cycling was increased (0.44 ± 0.08 vs 0.25 ± 0.07 mg · kg^–1 · min^–1, p < 0.05). Furthermore cycling of endogenous glucose measured during glucose infusion was also augmented (0.41 ± 0.05 vs 0.24 ± 0.05 mg · kg^–1 · min^–1, p < 0.05). Hence the increase of glucose cycling (70%) was much more pronounced than that of glucose production (17%). In conclusion, small defects in glucose metabolism in acromegaly can be detected with sensitive tracer methods. These derangements are confined to the liver under fasting conditions, but are of both hepatic and extrahepatic origin during glucose loading.     

Contribution of glucose/glucose 6-phosphate cycle activity to insulin resistance in Type 2 (non-insulin-dependent) diabetes mellitus

Summary It has been suggested that increased glucose/glucose 6-phosphate substrate cycling impairs net hepatic glucose uptake in Type 2 (non-insulin-dependent) diabetes mellitus and contributes to hyperglycaemia. To investigate glucose/glucose 6-phosphate cycle activity and insulin action in Type 2 diabetes we studied eight patients and eight healthy control subjects, using the euglycaemic glucose clamp and isotope dilution techniques with purified [2-³H]- and [6-³H] glucose tracers, in the post-absorptive state and eight patients and five healthy control subjects during consecutive insulin infusions at rates of 0.4 and 2.0 mU·kg^–1·min^–1. [2-³H]glucose and [6-³H]glucose radioactivity in plasma samples were determined using selective enzymatic detritiation, allowing calculation of glucose turnover rates for each isotope, the difference being glucose/glucose 6-phosphate cycling. Endogenous glucose production ([6-³H]glucose) was greater in diabetic than control subjects in the post-absorptive state (15.6±1.5 vs 11.3±0.4 mol·kg^–1·min^–1, p<0.05) and during the 0.4 mU insulin infusion (10.1±1.3 vs 5.2±0.3 mol·kg^–1·min^–1, p<0.01) indicating hepatic insulin resistance. Glucose/glucose 6-phosphate cycling was significantly greater in diabetic than in control subjects in the post-absorptive state (2.6±0.4 vs 1.6±0.2 mol·kg^–1·min^–1, p<0.05) but not during the 0.4 mU insulin infusion (2.0±0.4 vs 2.0±0.3 mol·kg^–1·min^–1). During the 2.0 mU insulin infusion endogenous glucose production was suppressed to a similar degree in both groups (2.6±0.5 vs 3.4±0.7 mol · kg^–1·min^–1) but glucose disappearance was lower in the diabetic subjects (30.8±2.0 vs 52.4±4.6 mol·kg^–1·min^–1, p<0.01). During the 2.0 mU insulin infusion glucose/glucose 6-phosphate cycling was greater in the diabetic subjects (3.8±0.7 vs 0.8±0.6 mol·kg^–1·min^–1, p<0.05). In conclusion, both hepatic and peripheral insulin action are impaired in Type 2 diabetes. Increased glucose/glucose 6-phosphate cycling is seen in the post-absorptive state and also during marked hyperinsulinaemia, when insulin resistance is predominantly due to reduced peripheral tissue glucose uptake.     

Renal and splanchnic exchange of human biosynthetic C-peptide in Type 1 (insulin-dependent) diabetes mellitus

Summary Biosynthetic human C-peptide or NaCl (154 mmol·l^–1) was given intravenously to 13 Type 1 (insulin-dependent) diabetic patients to determine the renal and splanchnic exchange of C-peptide. Catheters were inserted percutaneously into an artery and a renal and hepatic vein. Infusions of C-peptide were given for 60 min at two dose levels (5 and 30 pmol·kg^–1·min^–1). Insulin was infused throughout the study (0.5 mU·kg^–1·min^–1) and plasma glucose was kept constant by a variable glucose infusion. The regional blood flows were measured by indicator dilution techniques. In 11 of the 13 patients basal C-peptide levels were not detectable. The arterial steady-state C-peptide concentration was 0.81±0.10 nmol·l^–1 and 2.33±0.30 nmol·l^–1 at the low and high rate infusions, respectively. Renal uptake was 124±18 pmol·min^–1 at the low infusion corresponding to 39% of the infused amount. At the higher dose C-peptide infusion renal uptake increased to 155±21 pmol·min^–1 (p<0.05). Urinary excretion of C-peptide was 7±2 pmol·min^–1 at the low dose infusion and increased to 34±6 pmol·min^–1 at the high dose infusion (p<0.01). The proportions of infused amount excreted were fairly constant and between 2% and 3%. No net exchange of C-peptide was found across the splanchnic vascular bed. The rate of glucose infusion had to be increased by 35% during the low dose C-peptide, but not during NaCl infusion in order to maintain a constant plasma glucose concentration. Arterial plasma concentrations of noradrenaline increased by 15–25% during both C-peptide and NaCl infusions. It is concluded that in patients with Type 1 diabetes (a) the kidney is the primary site of C-peptide removal, (b) renal metabolism rather than urinary excretion is the dominating process for C-peptide elimination (c) the excreted proportions of an infused amount of C-peptide were fairly constant between 2% and 3% and (d) no hepatic C-peptide catabolism could be detected.     

Effects of endogenous and exogenous secretin on plasma pancreatic polypeptide concentrations in dogs

Summary The effects of exogenous and endogenous secretin with or without intravenous glucose infusion upon islet hormone secretion were studied in four conscious mongrel dogs fitted with a duodenal fistula. Intravenous infusion of secretin for 1 h at doses of 0.5 and 4 U/kg raised plasma secretin concentrations to physiological and pharmacological levels respectively, without affecting plasma insulin and pancreatic polypeptide concentrations. In contrast, bolus injections of secretin at high concentrations produced significant increases of plasma insulin at 0.5 U/kg and 4 U/kg and of pancreatic polypeptide at 4 U/kg. Plasma glucagon did not change during intravenous infusion of low dose secretin (0.5 U · kg^–1 · h^–1), but decreased during infusion of 4 U · kg^–1 · h^–1 or bolus injection of secretin (0.5 U/kg). Intravenous infusion of glucose together with secretin (0.5 U/kg and 4 U/kg) did not affecf plasma insulin, glucagon, or pancreatic polypeptide levels significantly compared with the changes caused by glucose infusion alone. Intraduodenal instillation of HCl, which produced plasma secretin concentrations similar to those evoked by intravenous infusion of secretin (4 U · kg ^–1 · h^–1), led to a rise in plasma pancreatic polypeptide. It is concluded that the stimulatory effects of secretin on insulin and pancreatic polypeptide and the inhibitory effect on glucagon are pharmacological, and that increase of plasma pancreatic polypeptide after intraduodenal infusion of HCl is not mediated by endogenous secretin.     

Islet amyloid polypeptide amide causes peripheral insulin resistance in vivo in dogs

Summary Islet amyloid polypeptide is a 37 amino acid hormone-like peptide which is the major protein component of islet amyloid deposits commonly found in patients with Type 2 (non-insulin-dependent) diabetes mellitus. Recent studies indicate that a physiologically active form of this peptide appears to be carboxyamidated and secreted from the insulin-producing beta cell. In order to clarify the possible in vivo actions of islet amyloid polypeptide, we have studied the effects of synthesized islet amyloid polypeptide-amide on peripheral glucose utilization by performing hyperinsulinaemic euglycaemic glucose clamp studies on dogs. Exogenously administered islet amyloid polypeptide-amide (an infusion from 1.0 to 100 g·kg^–1·h^–1, over 2 h) inhibited the insulin-stimulated glucose disposal rate in a dose dependent manner. Twenty-five g·kg^–1·h^–1 of islet amyloid polypeptide-amide infused via a peripheral vein significantly lowered the glucose disposal rate by 20% (from 17.4±1.7 to 14.4±1.7 mg·kg^–1·min^–1, n = 5, p<0.01). These findings suggest that islet amyloid polypeptide-amide causes peripheral insulin resistance in vivo.