Possible mechanism by which somatostatin-induced glucagon suppression improves glucose tolerance during insulinopaenia in man

Summary The present study examines the question of whether lowering the basal plasma glucagon concentration alters the response of the liver to an intravenous glucose load under conditions where insulin is present at near-basal concentrations. Acute hyperglycaemia of 220–240 mg/dl was induced by peripheral venous glucose infusion in two groups of normal men who had undergone hepatic vein catheterization. Somatostatin (0.9 mg/h) was infused in both groups together with an infusion of insulin (0.15 mU/kg/min) to maintain arterial insulin levels at 7–12 U/ml. Glucagon (1.5 ng/kg/min) was infused in one group resulting in a rise in plasma glucagon levels from 148±37 to 228±25 pg/ml, thus mimicking basal portal glucagon concentrations, whereas in the second group glucagon was not replaced, resulting in a fall in circulating glucagon levels from 132±21 to 74±15 pg/ml. In the glucagon-deprived group, net splanchnic glucose production (NSGP) fell from 143±31 to –72.5±39 mg/ min (p<0.01), indicating that net splanchnic glucose uptake had occurred. By contrast, NSGP did not change significantly (137±20 vs 151±60 mg/min) in the group in which both insulin and glucagon were replaced during hyperglycaemia. These data thus suggest that during hyperglycaemia, when the insulin concentration is fixed at basal levels, glucagon may play an important role in determining whether or not the liver diminishes its output of glucose and stores glucose in response to a glucose load.     

Relationship between glucose oxidation and glucose tolerance in man

The study was performed to determine the influence of peripheral glucose utilization on glucose tolerance. Glucose oxidation was measured in a group of 6 normal subjects by means of continuous indirect calorimetry during a 100 g oral glucose tolerance test for 3 hr, comparing the control state with experimental inhibition or stimulation of glucose oxidation. Suprabasal oxidation, corresponding to oxidation in response to the load, mainly by insulin-dependent tissue, was obtained by subtracting basal oxidation (essentially by non-insulin dependent tissues) from total oxidation. Suprabasal oxidation of glucose was inhibited by a neutral fat infusion, and stimulated by means of dichloracetate. In the control test, from the 100 g glucose administered, 18 g participated to suprabasal oxidation during the 3 hr of the test. A neutral fat infusion, started 2 hr before the glucose load and lasting throughout the test, decreased suprabasal oxidation to 7.5 g, i.e. to 42% of the control value. With the fat infusion, a larger fraction of the energy consumption was shown to originate from lipid oxidation (37% versus 25% in controls, p < 0.05) at the expense of carbohydrate (CHO) oxidation (44% versus 60% in controls, p < 0.05). However, these major changes in peripheral glucose oxidation were accompanied by only a moderate decrease in glucose tolerance. Dichloracetate administered prior to the test increased suprabasal oxidation to 25 g glucose oxidized in the 3 hours following the glucose load, i.e. an increment of 39% above the control value. A larger fraction of energy consumption was derived from carbohydrates (77% versus 60% in controls, p < 0.05). However, no significant change was observed in glucose tolerance. These results indicate that marked changes of peripheral glucose oxidation have little influence on glucose tolerance and suggest that another mechanism, i.e. glucose storage, plays a larger role in regulating plasma glucose levels during oral glucose tolerance tests.     

Effect of glucose ingestion on peripheral glucose metabolism in normal man

This study was designed to determine the forearm exchange of energetic substrates (glucose and free fatty acids) and the carbohydrate and lipid oxidation rates of normal individuals, in the postabsorptive state and after an oral glucose challenge. Seventeen healthy male volunteers were studied after an overnight fast (12-14 h) and during 3 hours after ingestion of 75 g of glucose. The forearm glucose uptake rate reached the maximum at 60 minutes (0.935 +/- 0.098 mg/100 ml of forearm.min), while 114.9 +/- 9.4 and 90.1 +/- 8.4 mg/100 ml of forearm were the total and incremental peripheral uptake of glucose in 3 hours of study, respectively. After the ingestion of 75 g of glucose, an increase in the oxidation rate of this hexose was observed, corresponding to an increment in the basal oxidation of 21.0 +/- 2.6 mg/100 ml of forearm in 3 hours. The arterial and venous plasma levels of free fatty acids decreased significantly after the glucose meal, associated with a decrease in the lipid oxidation rate. In conclusion, the results of this study showed that, in normal subjects after ingestion of 75 g of glucose, 30.3 +/- 2.1 g (40.4% of the ingested load) of this substrate were processed by the skeletal muscle in the body as a whole and 8.1 +/- 0.6 g were oxidized in this tissue during 3 hours.     

Effect of somatostatin on duodenal glucose absorption in man

OBJECTIVE: The hyperglycemic hyperinsulinemic clamp technique using intraduodenally infused glucose is an attractive tool for studying postprandial glucose metabolism under strictly controlled conditions. Because it requires the use of somatostatin (SST), we examined, in this study, the effect of SST on intestinal glucose absorption. CONTEXT: Twenty-six normal volunteers were given a constant 3-h intraduodenal infusion of glucose (6 mg.kg(-1).min(-1)) labeled with [2-(3)H]glucose for glucose absorption measurement. During glucose infusion, 19 subjects received iv SST at doses of 10-100 ng.kg(-1).min(-1) plus insulin and glucagon, and seven subjects were studied under control conditions. In the controls, glucose was absorbed at a rate that, after a 20-min lag period, equaled the infusion rate. RESULTS: With all the doses of SST tested, absorption was considerably delayed but equaled the rate of infusion after 3 h. At that time, only 5 +/- 2% of the total amount of infused glucose was unabsorbed in the control subjects vs. 36 +/- 2% (P < 0.001) in the SST-infused subjects. In the latter, the intraluminal residue was almost totally absorbed within 40 min of the cessation of SST infusion. At the lowest dose of SST tested (10 ng.kg(-1).min(-1)), suppression of insulin secretion was incomplete. CONCLUSION: These properties of SST hamper the use of intraduodenal hyperglycemic hyperinsulinemic clamps as a tool for exploring postprandial glucose metabolism.     

Effect of long chain triglyceride infusion on glucose metabolism in man

The effect of long chain triglyceride infusions (Intralipid 20%, 1 ml/min) on total body glucose uptake, glucose oxidation and glucose storage was examined in 25 healthy young volunteers by employing the euglycemic insulin clamp technique in combination with indirect calorimetry. Insulin was infused at three different rates (0.5, 2 and 4 mU/kg min) to achieve steady state hyperinsulinemic plateaus of 60 ± 4, 170 ± 10 and 420 ± 15 μU/ml. Prior to Intralipid infusion, the mean basal plasma free fatty acid concentration of all subjects was 385 ± 8 μmole/l. Following 90 min Intralipid infusion, the mean plasma free fatty acid level was increased to 760 ± 20 μmole/l (p < 0.001). At each insulin dose level, hyperlipidemia caused a significant reduction in total glucose uptake (5.9–3.5, 9.9–7.1, 11.1–8.8 mg/kg min, all p < 0.001. The decrease in total body glucose uptake was reflected by a decrease in both total glucose oxidation (2.4–1.6, 3.4–2.2, 3.7–2.8 mg/kg min, all p < 0.001) and glucose storage (3.6–1.9, 6.5–4.9, 7.4–5.9 mg/kg min, all p < 0.001). Basal glucose oxidation (1.3 ± 0.1 mg/kg min) fell by about 30% following 90 min of Intralipid infusion (0.9 ± 0.1 mg/kg min). Six additional subjects were studied with a lower infusion rate of Intralipid (0.5 ml/min). In these studies, insulin was infused at two different doses (0.5 and 2 mU/kg min) to achieve steady state plasma levels of 62 ± 2 and 171 ± 4 μU/ml. Intralipid caused again a significant reduction in total body glucose uptake during both the low (5.9 to 4.5 mg/kg min, p < 0.001) and the high (9.9–8.7 mg/kg min, p < 0.01) insulin clamp studies. This decrease in total glucose uptake was again the combined effect of an inhibition of both glucose storate (p < 0.05) and glucose oxidation (p < 0.001). In both high and low dose Intralipid infusion protocols, a strong inverse correlation was noted between the plasma free fatty acid concentration during the insulin clamp study and total body glucose uptake (r = 0.92, p < 0.001), glucose oxidation (r = 0.95, p < 0.001), and glucose storage (r = 0.90, p < 0.01). These results indicate that the inhibitory effect of free fatty acids on glucose utilization involves the biochemical pathways regulating both glucose oxidation and glycogen synthesis.     

Influence of nicotinic acid on the rates of turnover and oxidation of plasma glucose in man

The effects of antilipolysis induced by nicotinic acid on the rates of turnover and oxidation of plasma glucose were studied in normal overnight fasted, obese overnight fasted, and obese starved subjects, using a ¹⁴C-glucose infusion technique. Changes induced by nicotinic acid were similar whatever the nutritional state of the subjects. Plasma FFA levels and blood concentrations of glycerol and ketones decreased by about 60%; glycemia remained essentially unchanged but both the removal rate of plasma glucose and the hepatic glucose output increased by about 25%. Moreover, the fraction of glucose taken up by tissues and promptly oxidized and the fraction of expired CO₂ derived from plasma glucose increased, respectively, by 18% and 33%. This enhancement of glucose utilization occurred despite a small but significant decrease in plasma IRI concentration, indicating that nicotinic acid increased sensitivity to insulin. The above-mentioned results were obtained in 8 of the 10 patients studied. The remaining two subjects did not respond to the administration of nicotinic acid by any significant decrease in FFA nor in glycerol concentrations and showed no change in the rates of glucose turnover and oxidation. These data indicate that the effects of nicotinic acid on glucose metabolism may be partly mediated through changes in plasma FFA concentration and are consistent with the idea that the “glucose-fatty acid cycle” plays a significant role in the control of glucose metabolism in man.     

Effect of neural blockade on somatostatin-induced inhibition of exocrine pancreatic secretion

In vivo somatostatin inhibits exocrine pancreatic secretion. In in vitro experiments, such as the isolated perfused pancreas, somatostatin fails to inhibit exocrine pancreatic secretion. This suggests an indirect action of somatostatin, such as modulation of neural regulation. In the anesthetized rat we tested the inhibitory capacity of somatostatin in the presence of neural blockade in vivo. Neither the drugs given intravenously--phentolamine, propranolol, atropine and naloxone--nor vagotomy were able to prevent somatostatin-induced inhibition of exocrine pancreatic secretion. We conclude that somatostatin-induced inhibition of exocrine pancreatic secretion is not dependent on intact extrinsic innervation.     

Effect of age on glucose oxidation by isolated rat islets

Summary Islets were isolated from pancreases of 2-month and 12-month-old rats, and the oxidation of¹⁴C-glucose to¹⁴CO₂ determined at various medium D-glucose concentration. Islets from 12-month-old rats oxidized significantly less glucose than those from 2-month-old rats at glucose concentrations of 150, 300, and 450 mg/dl, and this was true when islets were selected by hand or by Ficoll density gradient separation. The effect of age on glucose oxidation was seen when islets were incubated with [U-¹⁴C], [1-¹⁴C], or [6-¹⁴C] glucose. The results raise the possibility that previously reported age-related defects in glucose-stimulated insulin secretion may be secondary to the effect of age on islet glucose catabolism.     

Effect of elevated free fatty acids on glucose oxidation in normal humans

In vitro studies indicating an inverse relationship between free fatty acid (FFA) availability and glucose oxidation led to proposal of the glucose-fatty acid cycle. In vivo studies have yielded conflicting results regarding the effect of FFA on glucose oxidation. In the present study the effect of FFA on glucose oxidation was determined in six normal volunteer subjects. The rate of glucose uptake was fixed by using a constant glucose infusion to suppress endogenous glucose production. Glucose was infused continuously overnight and during the four hour study at 8 mg/kg X min to ensure use of glucose as an energy substrate by virtually all tissues. Following a two-hour baseline glucose infusion, infusion of 20% IV fat emulsion at 1.0 mL/min plus heparin was added to the constant glucose infusion for two additional hours. Total carbohydrate oxidation was determined by indirect calorimetry, and the direct oxidation of the infused (plasma) glucose was measured by the use of U-13C-glucose. Glycogen oxidation was calculated as the difference between total carbohydrate oxidation and the oxidation of plasma glucose. Glucose uptake was calculated from the infusion rate, corrected for any changes in plasma and/or urine glucose concentration. Glucose uptake closely approximated the rate of IV glucose infusion and was unchanged by fat infusion. The percent of CO2 production from U-13C-glucose oxidation (74.5 +/- 12.3, mean +/- SD) was not affected by FFA, nor was the percent of glucose uptake oxidized (37.5 +/- 4.0).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of 50 and 100 g glucose loads on peripheral muscle glucose metabolism in normal man.

The present study was designed to determine the effects of 50 and 100 g glucose loads on forearm muscle glucose uptake, oxidation and nonoxidative glucose metabolism in normal man. Ten healthy male subjects were studied during the postabsorptive state (12-14 h overnight fast) and for 3 hours following glucose ingestion. Peripheral glucose metabolism was analysed by the use of the forearm technique to estimate muscle exchange of substrate combined with indirect calorimetry. Greater forearm muscle uptake and nonoxidative metabolism of glucose were observed in the subjects during the G.100 study than during the G.50 study (G.100 = 178.9 +/- 19.7 and 155.3 +/- 23.0 vs 103.5 +/- 16.6 and 85.2 +/- 16.7 mg/100 g forearm muscle. 3 h, respectively). the muscle glucose oxidation did not show significant difference after the two glucose loads. Insulin levels reached after 100 g glucose ingestion were significantly higher than after the 50 g glucose load. In conclusion, this study revealed a dose-dependent metabolic response in the muscular tissue of normal subjects to oral glucose loads of 50 and 100 g, with respect to forearm muscle glucose uptake and nonoxidative glucose metabolism. The oxidative responses of the muscle tissue to the oral glucose challenges seem not to be directly proportional to the ingested meals.     

Non-esterified fatty acids regulate lipid and glucose oxidation and glycogen synthesis in healthy man

Summary We examined the interrelationship of lipid and glucose metabolism in the basal state and during insulin stimulus in 19 healthy men (27±2 years, body mass index 23.6±0.6 kg/m²). In each subject, we performed a 4-h euglycaemic (5.3±0.1 mmol/l) hyperinsulinaemic (647±21 pmol/l) insulin clamp with indirect calorimetry in the basal state and during insulin infusion, and muscle biopsies before and at the end of the clamp. In the basal state, serum non-esterified fatty acid levels correlated directly with lipid oxidation (r =0.56, p<0.05) and indirectly with glucose oxidation (r = –0.80, p<0.001). Lipid and glucose oxidation rates were inversely related in the basal state (r = –0.47, p<0.05) and during insulin infusion (r = –0.65, p<0.01). Basal lipid oxidation and glycogen synthase total activity correlated inversely (r = –0.54, p<0.05). Lipid oxidation both in the basal state (r = –0.61, p<0.01) and during insulin infusion (r = –0.62, p<0.05) was inversely related to muscle glycogen content after the insulin clamp. Fasting plasma triglyceride concentration correlated directly to fasting insulin (r =0.55, p<0.05) and C-peptide (r =0.50, p<0.03) concentrations and inversely to non-oxidative glucose disposal rate at the end of clamp (r = –0.54, p<0.05). In conclusion: 1) Serum non-esterified fatty acid concentration enhances lipid and reduces glucose oxidation. 2) Lipid oxidation is inversely related to total glycogen synthase activity. 3) Lipid oxidation both in the basal state and during insulin stimulus correlates inversely with muscle glycogen content after insulin infusion. 4) Even in normotriglyceridaemic subjects, plasma triglycerides reduce insulin-stimulated non-oxidative glucose disposal. These data suggest that serum non-esterified fatty acids in physiologic concentrations have an important role in the regulation of lipid and glucose oxidation as well as glucose storage as glycogen. [Diabetologia (1994) 37: 202–209] Received: 2 June 1993 and in revised form: 27 August 1993     

Effect of intermittent physiologic hyperglucagonemia on postprandial plasma glucose levels in normal man

The ability of glucagon to impair glucose tolerance has been questioned by studies involving infusion of exogenous glucagon during a glucose load. Since such hormone administration may not reflect the physiologic pattern of glucagon secretion and may result in hepatic downregulation to glucagon, the present experiments have examined the effects of intermittent andogenous hyperglucagonemia (induced by episodic infusion of arginine) on plasma glucose profiles of normal man following ingestion of mixed meals. In control studies following meal ingestion, plasma glucose, insulin and glucagon increased respectively 15–30 mg/dl, 30–60 uU/ml and 25–50 pg/ml. When meals were accompanied by arginine infusions, plasma glucagon responses were augmented three to fourfold (p < 0.05). Amplitudes of glycemic excursions during infusion of arginine (345 ± 40 mg/dl) were significantly augmented compared to those observed in control studies (286 ± 34 mg/dl, p < 0.02). These results indicate that intermittent increases in plasma glucagon within the physiologic range can adversely affect postprandial glucose profiles in normal man despite concomitant hyperinsulinemia and suggest that such hyperglucagonemia may contribute to impaired postprandial glucose tolerance in diabetic individuals in whom insulin secretion is deficient.     

Effect of lipid oxidation on the regulation of glucose utilization in obese patients

The effect of changes in lipid oxidation on glucose utilization (storage and oxidation) was studied in seven nondiabetic obese patients. They participated in three protocols in which: (1) Intralipid (to raise plasma FFA concentrations), (2) -pyridylcarbinol [a precursor of nicotinic acid, to lower plasma free fatty acids (FFA) concentrations], or (3) isotonic saline were infused over 2 h. Thereafter, these infusions were discontinued, and a 2-h cuglycemic, hyperinsulinemic clamp was performed to measure glucose uptake. All studies were carried out in combination with indirect calorimetry to measure oxidative and nonoxidative glucose disposal (glucose storage). The high plasma FFA concentrations (1024±57 mol/l) and lipid oxidation rates (1.1±0.1 mg/kg·min) found at the end of the Intralipid infusion and the low plasma FFA concentrations (264±26 mol/l and lipid oxidation rates 0.7±0.1 mg/kg·min) found at the end of the -pyridylcarbinol infusions resulted in significantly different rates of total and nonoxidative glucose disposal during the insulin clamp. The values were 2.6±0.6 mg/kg·min after Intralipid and 4.1±1.0 mg/kg·min after -pyridylcarbinol for total glucose disposal, and 0.4±0.4 and 1.6±0.8, respectively for nonoxidative glucose disposal. In conclusion, these observations show that changes in lipid oxidation rates preceding a glucose load influence glucose disposal and glycogen storage in obese subjects.     

Effect of epinephrine infusion and adrenergic blockade on glucose oxidation in conscious dogs

The effect of combined alpha- and beta-adrenergic blockade, and the effect of epinephrine infusion, on the rate of glucose oxidation has been tested in conscious dogs. The dogs were prepared seven to ten days before the experiment with chronic catheters and tracheostomy. Glucose oxidation was measured by means of the primed-constant infusion of U-14C-glucose and indirect calorimetry. Six experimental groups were tested. In all groups, insulin and glucagon concentrations were held constant throughout by the inhibition of their secretion with somatostatin and intraportal replacement at basal rates. All experiments consisted of two two-hour periods. In half of the experiments, combined alpha- and beta-adrenergic blockade was administered in the second period, and in the other experiments, epinephrine was infused in the second period. The adrenergic blockade or epinephrine infusion (E) was performed in three different groups. In one, free fatty acid (FFA) levels were allowed to change spontaneously in response to blockade or E; in a second group, liposyn and heparin were infused throughout periods 1 and 2 in order to maintain FFA levels at high, constant levels; and in a third group, FFA levels were maintained at a constant low level in periods 1 and 2 by means of an infusion of nicotinic acid. In all cases tested, epinephrine infusion caused an increase in the rate of glucose oxidation. Adrenergic blockade also caused an increase in glucose oxidation when FFA levels were allowed to spontaneously fall; but when FFA levels were held constant (either high or low), there was no effect on glucose oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of phentolamine on the somatostatin-induced inhibition of glucagon and insulin secretion.

Synthetic cyclic somatostatin was infused into either the cranial pancreaticoduodenal artery or the femoral vein of anesthetized dogs with or without previous administration of phentolamine. Somatostatin infused into the pancreatic artery at a dose of 50 ng/kg/min for 10 min caused significant decreases in blood flow and plasma basal concentrations of both glucagon and insulin in the cranial pancreaticoduodenal vein, resulting in a profound decline of bihormonal output during the infusion. Arterial plasma glucose was not reduced during the administration of somatostatin in the pancreatic artery. These somatostatin-induced decreases failed to be eliminated by a 0.2 mg/kg injection of phentolamine into the femoral vein followed by a 9-min infusion of this alpha-adrenergic blocker (0.02 mg/kg/min) into the pancreatic artery immediately prior to the somatostatin administration. An inhibition of glucagon and insulin output and a fall of plasma glucose caused by somatostatin (1.7 microgram/min) infused into the femoral vein for 30 min also were not abolished by a prolonged and simultaneous infusion of phentolamine (0.2 mg/min) into the femoral vein over a period of 2 hr. These results indicate that alpha-adrenergic receptor mechanisms do not play a major role in the inhibition of islet glucagon and insulin secretion by somatostatin.