Stimulation of insulin secretion in man by oral glycerol administration.

The effects of an orally administered glycerol load (1 g/Kg body weight) on blood glucose, plasma FFA, and plasma insulin levels have been determined in eight normal fasting or glucose loaded (1 g/Kg body weight) volunteers. Blood glucose levels were not affected by glycerol loading while glicemia followed the same pattern of a glucose tolerance test in the group treated with glucose plus glycerol. Plasma FFA were significantly lowered only 90 min after glycerol loading while they had markedly and persistently decreased by glycerol plus glucose per os. Finally, though glicemia did not change, insulinemia was markedly increased by glycereol, 90 min after loading; moreover, plasma IRI was significantly higher in the group treated with glycerol plus glucose than in the group treated with glucose alone. These data suggest that the release of insulin may be stimulated by a very small increment of blood glucose, which derives from glycerol.     

Mechanism of action of phenethylbiguanide (phenformin) in man. III. Interrelationship between ethanol and phenethylbiguanide (PBG) in normal and diabetic subjects

A standard 4-hr ethanol infusion (236 mg/min) after a 3-day fast with and without phenformin (25 mg q.i.d.), with blood drawn every hour for 8 hr, was performed on five normal subjects, eight obese nondiabetics, seven obese chemical diabetics, and four nonobese diabetics. Control infusion induced in all subjects a decline in blood sugar levels during and/or after the alcohol challenge, with a parallel decrease in basal plasma insulin. Hypoglycemia and the decrease in insulin secretion were associated with increased plasma free fatty acid concentration. Addition of phenethylbiguanide (PBG) to the preparatory 3-day fast resulted in a greater drop in the blood glucose levels of the normal control subjects, obese and nonobese diabetics; in the obese nondiabetics, however, a significantly lower degree of blood glucose decrease than control was elicited. Furthermore, obese nondiabetics altered their blood glucose-insulin interaction with apparent increased responsivess of the B cells to PBG. The results suggest that effects of phenformin on blood glucose levels are more dependent on the metabolic state of the patient than on a property of the drug itself.     

The effect of different kinds of refeeding on islet glucose phosphorylating activities

The aim of the present work has been to study the regulating effect of different kinds of diet on the activities of enzymes that phosphorylates glucose into glucose 6 phosphate in the islets of Langerhans. The metabolism of glucose in the B cell is controlled by two different enzymes, hexokinase and glucokinase, whose activities are lowered during fasting; this coincides with lowered levels of blood glucose and blood insulin and with a blocking of the insulin-secretory response toward glucose. Refeeding with a high-carbohydrate diet restores glucokinase activity in islet extracts, blood insulin, and blood glucose. By contrast, refeeding with a low-carbohydrate diet restores hexokinase activity in islet extracts, restores poorly blood insulin, and is unable to unblock the insulin secretory response toward glucose. These results support the important role that glucokinase plays in the regulation of glycolytic flux and on insulin secretion in the B cell. Hexokinase could play a role in the regulation of the glycolytic flux when the B cell responds to other secretagogues.     

Regulation of insulin biosynthesis in vivo by glucose in the newborn rat.

The effects of glucose on insulin biosynthesis in vivo were investigated in the 3-day-old rat. The extent of insulin synthesized was determined by following the incorporation of 3H-leucine into immunoreactive insulin (3H-IRI) and pancreatic proteins (3H-PROT). Blood glucose levels increased 2.3 +/- 0.2 g/liter and plasma insulin increased 208 +/- 10 muU/ml after an intraperitoneal injection of 3H-leucine plus glucose (2 g/kg, body wt) in neonates. Hyperglycemia resulted in a decrease of basal plasma specific radioactivity of leucine and 3H-PROT radioactivity by about 40%. Determination of the 3H-IRI/3H-PROT incorporation ratio shows that the glucose load induced a significant stimulation of insulin biosynthesis 120 min after the injections. The present data indicate that (1) short-term variations of blood glucose are efficient in the regulation of insulin biosynthesis, and (2) insulin biosynthesis in vivo is likely to operate at a basal level of blood glucose in the 3-day-old rat. The results suggested that the variations of blood glucose in a concentration range found under physiologic conditions may play a role in the regulation of insulin biosynthesis in vivo.     

Inhibitory effect of somatostatin on dibutyryl cyclic AMP-induced insulin and growth hormone release in human subjects.

The effect of somatostatin on the responses of blood glucose, plasma immunoreactive insulin (IRI), growth hormone (GH), and free fatty acids (FFA) to the injection of dibutyryl cyclic AMP (DBC) was studied in six normal volunteers. DBC, when injected alone, induced a rapid increase in blood glucose and plasma IRI levels, while GH concentrations showed a less marked and more delayed increase and plasma FFA showed a clear downtrend. Somatostatin infusion suppressed the GH and IRI release induced by DBC, potentiated its hyperglycemic effect and changed the pattern of FFA. These results suggest that somatostatin inhibits hormone secretion distal to the generation of cyclic AMP.     

Mechanism of action of phenethylbiguanide (phenformin) in man IV. Effect of phenethylbiguanide (PBG) on glucose tolerance and lactate dynamics in obese nondiabetic and chemical diabetic subjects

The effects of long-term administration of phenethyl-biguanide hydrochloride (PBG) on glucose tolerance (oral and IV) and lactate dynamics were studied in obese nondiabetic and chemical diabetic subjects. In the obese nondiabetic subjects, parameters of glucose tolerance—glucose curve areas and glucose disappearance rates (K_g)—were not significantly altered by PBG. However, significant elevations of immunoreactive insulin (IRI) in the oral glucose tolerance test (OGTT) and intravenous glucose tolerance test (IVGTT) were observed while being treated with PBG, suggesting some degree of “insulin resistance”. Means of blood lactate levels during the OGTT were similar before and while on PBG treatment. Lactate dynamics revealed an increased lactate pool distributed in a larger volume, without significant changes in fasting blood lactate. In the obese chemical diabetic patients, glucose curve areas during the OGTT were corrected in the PBG period, despite the presence of decreased insulin responses to glucose. In the IVGTT, similar glucose curve areas and K_g values were shown in the presence of reduced insulin responses. These changes suggest some degree of PBG-induced facilitation of glucose entry into the cell in obese chemical diabetics. Blood lactate levels in the OGTT were higher at all times during PBG treatment, including fasting values. Lactate dynamics revealed that PBG increased the endogenous production rate of lactate, with expansion of the total lactate pool and no variation in the space of distribution. Plasma lactate concentrations increased significantly. These results show different effects of PBG in obese nondiabetics and obese chemical diabetics, in relation to glucose and lactate dynamics. Furthermore it seems that its effects on lactate dynamics are only exerted in the basal metabolic state of the patients.     

Effects of portacaval anastomosis on glucose tolerance in the dog: evidence of an interaction between the gut and the liver in oral glucose dosposal.

Continuous infusions of glucose (90 min duration) were given into the duodenum (ID), or the portal vein (IP), or a peripheral vein (IV) in conscious dogs, intact dogs, or dogs with portacaval anastomoses. In intact animals ID glucose tolerance was better than IV glucose tolerance, but IP glucose tolerance was not significantly different from IV glucose tolerance. Thus, the superiority of tolerance to ID glucose was not accounted for by relatively high levels of glucose in portal blood. IV glucose tolerance was not significantly affected by protacaval anastomosis with ligation of the portal vein, but ID glucose tolerance was markedly impaired. Disproportionate impairment in tolerance to enterically administered glucose in dogs with portacaval shunts, and the similarity of IP and IV glucose tolerance in intact dogs, suggest that both liver and the gut are important in determination of oral glucose tolerance through mechanisms that have little or no effect on responses to parenterally administered glucose. The results also suggest that the hepatic contribution is not dependent upon portal venous perfusion of the liver, and that a humoral interaction between the gut and the liver is involved which is not simply dependent on endocrine responses of the pancreas.     

Glucose tolerance and fractional extraction of glucose and insulin in patients with peripheral arterial insufficiency. Effect of arterial reconstructive surgery

Glucose tolerance with plasma insulin was determined in 49 nondiabetic patients with peripheral arterial insufficiency before successful arterial reconstruction and 6 mo after the operation. Based on the level of the arterial stenosis, the patients were allocated into two groups, those with stenosis at the level of abdominal aorta or iliac artery (high stenosis) and those with stenosis below the origin of the arteria profunda femoris (low stenosis). In another group of 11 patients with high stenosis and claudication as the only symptom the fractional uptake of insulin and glucose in the leg was determined. Patients with both high and low stenosis had statistically significant lower sums of insulin values during glucose tolerance test than did controls. Sums of glucose values during the test were significantly lower in patients with high stenosis but were unchanged in patients with low stenosis. No significant change occurred in glucose tolerance after operation in patients with low stenosis. In the patients with high stenosis the sums of insulin as well as of glucose values during the glucose challenge had increased 6 mo after the operation. The fractional uptake of insulin (21%) and glucose (8%) was significantly higher in patients with arterial insufficiency than in controls (7% and 3%, respectively). In these patients the insulin:glucose concentration ratio was significantly lower in venous than in arterial blood, indicating a relative extraction of insulin higher than that of glucose in the leg muscles. The results point out that the ratio of insulin and glucose concentration in venous blood, usually used as an index of insulin sensitivity, must be interpreted with caution. The low ratio found in the present work in patients with peripheral arterial insufficiency does not necessarily mean a high insulin sensitivity. Instead, it may be dependent on the increased insulin uptake in the periphery.     

Uncontrolled diabetes mellitus and hyperglucagonemia associated with an islet cell carcinoma.

A 53 year old woman presented with diabetes mellitus, hyperglucagonemia (600 to 1,500 pg/ml), clinical hyperparathyroidism and an abdominal mass diagnosed on biopsy as an islet cell carcinoma. Glucagon content of the tumor was 0.78 mug/g wet weight. Hourly blood samples during a 24 hour period revealed a direct correlation between plasma glucose and glucagon. The oral administration of glucose paradoxically increased whereas the intravenous administration decreased plasma glucagon. Circulating glucagon levels were markedly increased with arginine and epinephrine infusion. Both short- and long-term administration of alpha adrenergic blockade depressed the glucagon response to epinephrine infusion. In contrast, long-term alpha adrenergic blockade increased glucagon secretion despite improved glucose tolerance during a second 24 hour study. Although the patient demonstrated overt clinical and chemical findings of hyperparathyroidism, parathyroid hormone (PTH) was not detected in her plasma. The pattern of tumor growth was consistent with an origin from pancreatic islets. We conclude that (1) the tumor was responsive to physiologic stimuli known to affect glucagon secretion; (2) elevations of plasma glucagon levels with oral and dietary glucose suggest regulation of secretion by intestinal factors; and (3) improvement of glucose tolerance with alpha adrenergic blockade may be related to increased insulin secretion.     

Factors altering platelet monoamine oxidase. The influence of oral glucose intake

The specific activity of human platelet monoamine oxidase from control subjects undergoing glucose tolerance tests is reduced drastically. Three hours after intake of 100 g of glucose only 25%–30% of the MAO-baseline activity was measured with tryptamine, β-phenylethylamine and p-tyramine as substrates. At about 5 hr, platelet MAO activity has increased again. Inhibition was not due to small molecular weight inhibitors or other diffusible factors. Studies of other platelet enzymes, including succinate dehydrogenase and isocitrate dehydrogenase (NADP⁺ dependent) showed no parallel reductions; hGH, insulin, blood glucose and platelet glycogen concentrations did not correlate with platelet MAO activity. The changes of MAO activity in respect with p-tyramine and tryptamine as substrates 24 hr after glucose ingestion suggest changes of the lipid microenvironment of this enzyme of the outer mitochondrial membrane.     

Influence of prostaglandins on plasma glucagon levels in the rat.

The effect of PGE1and PGA1 intravenous infusion (2/mug/min) on plasma glucose and glucagon levels was investigated in normal, sympathectomized and propranolol-treated rats. PGE1 infusion significantly increased glucose and glucagon levels, while PGA1 had no effect. Since the dose of PGE1 used in this study was able to reduce the arterial blood pressure by about 20%, the possibility that PGE1 acted indirectly through a reflex sympathetic overactivity was tested. The increases in plasma glucagon induced by PGE1 occurred also in sympathectomized or in beta-blocked animals. Thus, it was possible to exclude a sympathetic mediation or a direct stimulation of pancreatic beta-receptors as a likely mechanism of PGE1 action.     

Failure of insulin infusion during euglycemia to influence endogenous basal insulin secretion

Despite some evidence of self-regulation of insulin secretion, it is unclear whether endogenous insulin influences insulin secretion independently of blood glucose. The aim of the present study was to examine this question in humans. Seven healthy fasting men were given two-hour porcine insulin infusions (40 mU/min) with and without maintenance of euglycemia (glucose clamp). Intravenous glucose required to maintain basal blood glucose levels (4.2 ± 0.1 mmole/liter) during insulin infusion was 34.3 ± 3.0 gm with a mean rate of 273 ± 29 mg/min in the second hour of insulin infusion. During the glucose clamp, mean C-peptide levels were not significantly altered from fasting levels of 1.91 ± 0.24 ng/ml, but when blood glucose levels fell by approximately 1 mmole/liter, C-peptide fell to 0.37 ± 0.07 ng/ml. Plateau insulin levels were significantly higher during euglycemia than during mild hypoglycemia (53.2 ± 5.6 mU/liter versus 38.5 ± 3.6 mU/liter, P < 0.01). Plasma nonesterified fatty acids were suppressed equally in the two studies. However, a rise in plasma glucagon seen during mild hypoglycemia was absent when euglycemia was maintained. We conclude that insulin self-regulation (either direct or neurally mediated) is not physiologically important in the basal state in normal humans and that the blood glucose-insulin feedback loop dominates in the short-term control of basal insulin secretion.     

The defective glucose sensitivity of the B cell in non insulin dependent diabetes. Improvement after twenty hours of normoglycaemia

In non insulin dependent diabetics (N.I.D.D.) of normal body weight, the acute insulin response to glucose is defective while that to pharmacologic agents such as tolbutamide is less impaired. This specific B-cell insensitivity to glucose results from unknown and perhaps multiple mechanisms. Hyperglycemia may by itself aggravate this phenomenon. To test this hypothesis acute insulin release (Δl: sum of increment at 2, 5, 10 min) after intravenous glucose and tolbutamide injection was studied in 5 N.I.D.D. with fasting blood glucose averaging 12.1 mM/l (range 10.7–13.7) before and after 20 hours of glycemic normalization by an artificial pancreas. Intravenous injection of .3 g/k glucose did not elicit an acute insulin or C-peptide response, but following Tolbutamide (20 mg/kg) Δl was 44 ± 21 μU/ml and ΔC-peptide 0.84 ± 0.37 nM/l. After 20 hr of normoglycemia a response to glucose was apparent (Δl 60 ± 24 and ΔCP 0.86 ± 26) that to Tolbutamide was unchanged (Δl 58 ± 26 and ΔCP 0.97 ± 0.27). These results suggest that 20 hr of normoglycemia improve significantly the “glucoreceptor” function of the B-cell in N.I.D.D.     

Evidence for in vivo compartmentation of amino acids between blood cells and plasma in man with liver disease during constant infusion of L (U-14C) tyrosine

L (U-¹⁴C) tyrosine tracer was infused at a constant rate for 8 hrs in a series of five patients with liver disease on two occasions, the first when diet consisted of intravenous glucose, and the second when aminoacids were added. Plateau labelling of both plasma and intracellular blood cell free tyrosine was obtained by 6 hr of each infusion. However, the intracellular specific activities were on average 50% lower (p < 0.05) when diet was glucose alone, and 54% lower (p < 0.01) when glucose and aminoacids were given. Change in diet did not significantly affect these differences. The results provide evidence for significant in-vivo compartmentation of aminoacids between plasma and blood cells in man with liver disease, and indicate that whole blood cannot be used in the conventional measurement of whole body protein     

Physical training in human hyperplastic obesity. IV. Effects on the hormonal status.

Severly obese subjects and sex- and age-matched controls underwnet physical training during a 6-wk period. Evidence of training was shown in all subjects by increased aerobic power. Before training the obese subjects were characterized by the following abberations: decreased glucose tolerance, hyperinsulinemia, elevated blood glycerol and plasma free fatty acids, and a blunted plasma growth hormone response during glucose tolerance. Noradrenaline output was elevated, a finding of potential interest for the explanation of increased lipolysis, blood pressure, and heart size in obesity. With training the following changes were found:In the controls there was evidence for the beginning of a decrease of adipose tissue mass. In the obese, however, body weight, body fat, or fat cell size did not decrease during training. Plasma insulin decreased, and a corresponding increase of plasma glycerol was seen. Glucose tolerance was not changed, and this, together with decreased plasma insulin, indicated an increase insulin sensitivity of the periphery. Changes in noradrenaline or growth hormone during training could not explain this increased sensitivity. Urinary cortisol output was found to decrease after training in the obese; this might be interpreted as a decrease in cortisol secretion allowing a more effective insulin action on the periphery.     

Preliminary report: the effects of acute acidosis on alanine and glucose metabolism across the liver, gut, kidney, and muscle in the dog

Acute metabolic acidosis (AMA) in the dog results in a 48% increase in arterial alanine concentration and an 11% fall in blood glucose. Extraction/production of glucose and alanine were studied by A/V sampling and electromagnetic blood flow probe measurements. Alanine and total amino-N release by muscle increased in acidosis Hepatic glucose release was reduced. More marked changes in metabolism occur in AMA in the dog than previously considered.     

Effect of buformin on splanchnic carbohydrate and substrate metabolism in healthy man

The effect of buformin (100 mg b.i.d. for 5 days) on carbohydrate metabolism, both splanchnic glucose output (SGO) and net substrate exchange were studied in 6 healthy male volunteers in the basal state and following glucose ingestion (100 g). Control studies without buformin were also performed in 5 men. Splanchnic glucose and substrate exchange was determined by means of the hepatic venous catheter technique. SGO was 154 ± 18 (SEM) mg/min in the postabsorptive state and increased 33.3 ± 2.8 g above the basal level during the 150 min period following glucose ingestion. Buformin administration did not alter basal SGO (157 ± 26 mg/min), nor the splanchnic exchange of pyruvate, alanine, glycerol, OH-butyrate and acetoacetate. Splanchnic lactate balance was altered by buformin and net lactate output occurred. Following glucose ingestion the rise in splanchnic lactate output was increased, whereas no change in SGO ($\text{32.9 ± 3.5 g}\text{150 min}$) and splanchnic exchange of the other substrates was observed. The increase in arterial blood glucose concentration following oral glucose loading was reduced by buformin pretreatment (p < 0.0005). The insulin production rate (basal, 16 ± 2 mU/min; following oral glucose, $\text{13 ± 2 U}\text{150 min}$) as calculated from C-peptide release from the splanchnic area was unchanged by buformin. Except for a marked rise in splanchnic lactate production, buformin did not alter splanchnic carbohydrate metabolism after orally ingested glucose in healthy man. The diminished increase in arterial blood glucose concentration associated with unaltered insulin production suggests that buformin facilitates glucose utilization by peripheral tissues.     

Effects of arginine infusion in infants: increased urea synthesis associated with unchanged ammonia blood levels.

Infusion of L-arginine hydrochloride in infants and children (ages ranging from 1 day to 12 yr) at a dosage of 0.5 g/kg body weight resulted in a dramatic increase in the arginine plasma concentration, with highest values of approximately 7 mmole/liter immediately after the end of the infusion; 120 min later the mean plasma level of the amino acid had decreased to mean values of 1 mmole/liter. These fluctuations were paralleled by increased ornithine concentrations, although the mean plasma levels of this amino acid remained far below those of arginine, i.e., 0.73 and 0.22 mmole/liter after 30 and 90 min, respectively. When expressed on a molar basis, arginine administration resulted in an almost stoichiometric rise in urinary urea excretion. These findings indicate that arginine is rapidly metabolized via urea and ornithine, the latter being transformed to glucose, as evidenced by a significant rise in the blood glucose concentration. Blood gas analyses and serum urea and blood ammonia concentrations determined after the load showed no significant deviations from preinfusion levels. Thus, in contrast to the effects to be expected form studies with tissue culture homogenates, even when administered to newborn infants, arginine does not impair the turnover of the urea cycle.     

Effects of ethanol on carbohydrate metabolism: I. Influence on oral glucose tolerance test.

To study the effect of ethanol on glucose tolerance test, a series of experiments were performed on Sprague-Dawley male rats weighing 250 g. After 18-hr fast, each rat was given, at random, the following test substances (one test substance at a time) intragastrically in a volume of 0.5 ml/100 g body weight: saline; glucose 0.75 g/kg (3 kcal); ethanol 0.4 g/kg (3 kcal); and ethanol with glucose. Saline or ethanol alone produced no significant changes in blood glucose or plasma insulin concentrations. Addition of ethanol to glucose load resulted in glucose intolerance as well as a lower and sluggish insulin response when compared to these with glucose load alone. The results suggest that ethanol per se has no effect on blood glucose or plasma insulin. However, when given together with glucose load, ethanol produces glucose intolerance as well as inhibition of glucose mediated insulin response.     

The effect of ingestion of meat on hepatic extraction of insulin and glucagon and hepatic glucose output in conscious dogs

The effect of ingestion of protein on hepatic extraction of insulin and glucagon and hepatic glucose output were investigated in conscious dogs. The ingestion of meat stimulated both insulin and glucagon secretion but the glucagon response was much more rapid and greater than that of insulin. Secretion of glucagon demonstrated a biphasic pattern while insulin release was monophasic. The fractional hepatic extraction of glucagon increased gradually from the basal value of 15 +/- 3% to a peak of 36 +/- 5% at 90 min, and that of insulin increased from the basal level of 41 +/- 2% to 54 +/- 4% at 45 and 60 min. The increased hepatic extraction of glucagon and insulin after meat ingestion may be explained by neural or hormonal signals from the gut. The blood glucose and hepatic glucose output did not increase significantly despite the significant decrease of the portal vein insulin to glucagon molar ratio as well as the significant decrease of the molar ratio of the hepatic uptake of these hormones. The absence of greater hepatic glucose production despite the augmented glucagon secretion and decreased portal vein insulin to glucagon molar ratio could reflect down regulation by glucagon.