Mechanism of hypertriglyceridaemia in diabetic patients with fasting hyperglycaemia

Several aspects of lipid metabolism were studied to define the mechanism of hypertriglyceridaemia in insulin-independent diabetic patients with fasting hyperglycaemia. Patients with insulin-independent diabetes were more obese (p < 0.001) and had a significantly (p < 0.001) higher mean (+/- SEM) fasting plasma triglyceride concentration (387 +/- 66 mg/dl) than did either insulin-dependent diabetics (133 +/- 11 mg/dl) or normal (73 +/- 1 mg/dl) subjects. Very low density lipoprotein secretion rate was also significantly (p < 0.01 - < 0.001) higher in patients with insulin-independent diabetes (14.65 +/- 1.37 mg/kg/h) as compared to 7.64 +/- 0.60 mg x kg/h and 9.86 +/- 0.75 mg/kg/h in normal subjects and patients with insulin-dependent diabetes, respectively. However, the relationship between plasma triglyceride concentration and very low density lipoprotein-triglyceride secretion was similar in diabetics and in normals. The diabetic groups had equivalent degrees of fasting and postprandial hyperglycaemia, and comparable elevations of fasting plasma nonesterified free fatty acid levels (insulin-independent = 0.72 +/- 0.07 mmol/L, insulin-dependent = 0.63 +/- 0.08 mmol/L). Postprandial plasma insulin concentrations, however, reached normal levels in insulin-independent diabetics and were higher (p < 0.001) than in insulin-dependent diabetics. Thus, hypertriglyceridaemia in insulin-independent diabetics with fasting hyperglycaemia was associated with increased hepatic very low density lipoprotein-triglyceride secretion, and normal plasma insulin levels. The lower triglyeride levels in the insulin-dependent diabetics is assumed to be due to their relative hypoinsulinaemia.

Effects of age on various aspects of glucose and insulin metabolism

Age is known to be associated with the development of glucose intolerance. In this review an effort has been made to differentiate between the effects of age per se on glucose tolerance, as distinguished from those of such age-related variables as obesity, diet, development of frank diabetes, etc. At the same time, an attempt was made to evaluate the evidence implicating abnormalities of insulin secretion and/or insulin action in the development of glucose intolerance with age. It is concluded that the questions being asked are far from simple, and that available data do not provide unequivocal answers.

Hyperinsulinemia in fructose-induced hypertriglyceridemia in the rat

Rats were fed diets containing (as percent of calories) 66% glucose or fructose, 22% casein, and 12% lard, for 1 wk. The effects of these diets on plasma triglyceride, glucose, and insulin concentrations were compared to those of control rats eating regular rat chow. Plasma triglyceride levels increased from a mean (+/- SE) control level of 85 +/- 7 to 142 +/- 9 (p less than 0.001) and 380 +/- 38 (p less than 0.001) mg/dl in dextrose- and fructose-fed rats, respectively. Plasma insulin concentrations demonstrated a similar increase, rising from a mean (+/- SE) control value of 29 +/- 4 microunits/ml to 55 +/- 10 microunits/ml in dextrose-fed rats and 85 +/- 12 microunits/ml in rats eating the fructose diet. Plasma glucose concentrations of the three groups were comparable. These results indicate that fructose-induced hypertriglyceridemia is associated with significant hyperinsulinemia.

Evidence for multiple causality in the development of diabetic hypertriglyceridaemia

The relationship between varying degrees of insulin deficiency and hypertriglyceridaemia in rats have been examined. Rats were studied 7--10 days after injection with streptozotocin, and plasma glucose concentrations used to classify rats as having either moderate (200--350 mg/dl) or severe diabetes (< 350 mg/dl). A 2- to 3-fold rise in plasma triglyceride (TG) concentration developed in six week old insulin deficient rats associated with elevated plasma non esterified fatty acid (NEFA) concentrations and decreased very low density lipoprotein secretion. Perfused liver from six week old rats with either moderate or severe diabetes were incapable of increasing hepatic TG secretion when perfusate NEFA concentrations were raised from 0.4 to 1.8 mmol/l. In one year old, spontaneously obese rats, an equivalent degree of hypertriglyceridaemia could be produced with a lesser degree of insulin deficiency, and in this instance very low density lipoprotein secretion was increased over control values. Hepatic TG secretion by perfused livers from these rats with moderate diabetes approximately doubled when perfusate NEFA condentration was raised from 0.40 to 0.85 mmol/l. These results emphasize the complex causality of diabetic hypertriglyeridaemia in situations characterised by comparable degrees of fasting hyperglycaemia.

Effect of age on glucose oxidation by isolated rat islets

Islets were isolated from pancreases of 2-month and 12-month-old rats, and the oxidation of 14C-glucose to 14CO2 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.

Evidence that microtubules play a permissive role in hepatocyte very low density lipoprotein secretion

To determine whether a minimum number of assembled microtubules is required for very low density lipoprotein (VLDL) triglyceride TG) secretion in hepatocytes, antimicrotubule drugs of different concentrations were given to rats. Hepatic VLDL-TG release was subsequently measured by a liver perfusion system, and hepatocyte ultrastructural changes were analyzed by quantitative ultrastructural methods. The results demonstrate a tight coupling between the reduction in hepatocyte microtubule content and the reduction in hepatic VLDL-TG secretion which is related to the dose of colchicine or vinblastine administered. The various estimates imply that a minimum number of microtubules is necessary for hepatic VLDL secretion to proceed normally and that hepatic VLDL secretion rates reach their nadir (10--30% of control) when microtubules comprise less than 0.005% of the cytoplasm (or less than 10% of control values) when microtubules comprise less than 0.005% of the cytoplasm (or less than 10% of control values). At this point, hepatocyte Golgi complexes are also greatly altered; Golgi complexes with recognizable dictyosomal membranes are reduced to 15% of control values and the region is filled with large numbers of electron-dense bodies which appear to be lysosomes in the process of digesting VLDL. There is a predilection for the remaining Golgi complexes to be associated with a few segments of microtubules, even when no microtubules can be measured in random samplings of hepatocytes. Clusters of vacuoles containing VLDL are also present throughout the cytoplasm; the limiting membranes of 25% of these vacuoles are studded with ribosomes. These findings demonstrate that the administration of antimicrotubule agents results in decreases in hepatic VLDL-TG secretion which are associated with loss of microtubules and alteration of existing Golgi complexes.

Effects of differences in amount and kind of dietary carbohydrate on plasma glucose and insulin responses in man

The effect of variations in kind and amount of dietary carbohydrate on plasma glucose and insulin responses was studied in normal subjects and in patients with chemical diabetes. Plasma glucose and insulin responses fell when the proportion of total calories given as carbohydrate were decreased by 15% (coincidental with a comparable increase in dietary fat). Associated with the lower plasma glucose and insulin concentrations was a fall in fasting and postprandial triglyceride levels. Plasma glucose and insulin responses were also lower when equivalent carbohydrate challenges were given as part of a mixed meal, as compared to a drink. Furthermore, carbohydrate given as starch also led to an attenuated glucose and insulin response when compared to an equivalent amount of glucose administered as either dextrose or sucrose. Finally, significant differences were also seen after the ingestion of different kinds of starch, with potato being the most like dextrose, and rice the least. These results indicate that differences in amount and kind of ingested carbohydrate can modify ensuing plasma glucose and insulin responses, and raise the possibility that such dietary manipulation may have some therapeutic utility in patients with abnormal carbohydrate and lipid metabolism.

Is the hypertriglyceridemia associated with insulin deficiency caused by decreased lipoprotein lipase activity?

Questions have been raised as to whether reduced lipoprotein lipase (LPL) activity can account for the defect in plasma removal of very low density lipoprotein (VLDL) associated with insulin deficiency. In order to study this issue, streptozotocin-induced insulin deficiency was produced in 2-mo-old rats. Three groups of rats were studied: control, moderate diabetes (glucose &gt; 200 &lt; 350 mg/dl), and severe diabetes (glucose &gt; 350 mg/dl). One week later food was withdrawn at 8 a.m., and the following measurements made at 2 p.m. (6 h after removal of food): (1) plasma glucose, insulin, and triglyceride (TG) levels; (2) VLDL-TG secretion rate; and (3) adipose tissue and skeletal muscle LPL activity. Rats with moderate diabetes had higher glucose and lower insulin levels than did control rats, and these differences were accentuated in rats with severe diabetes. Plasma TG levels were elevated (P &lt; 0.001) in rats with moderate (↑99%) and severe diabetes (↑126%), and this was associated with a fall (P &lt; 0.001) in VLDL-TG secretion. An increase in plasma TG levels, despite a decrease in VLDL-TG secretion, indicates the presence of a profound defect in VLDL-TG removal from plasma. However, muscle LPL of rats with severe diabetes and moderate diabetes was equal to that of control rats, and adipose tissue LPL was only moderately (P &lt; 0.05) reduced (MD = ↓25%, SD = ↓32%). These results indicate that plasma TG levels rise in young, insulin-deficient rats as a result of a defect in removal of VLDL-TG from plasma. This removal defect is associated with normal muscle and only moderately reduced adipose tissue LPL activity, suggesting that the VLDL-TG removal defect associated with insulin deficiency may not be a simple function of a decrease in total (muscle + adipose tissue) LPL activity.

Effect of age on glucose-stimulated insulin release by the beta-cell of the rat

To assess the effect of age on beta-cell insulin release, collagenase-isolated islets of Langerhans were obtained from rats aged 2--18 mo and incubated with increasing concentrations of glucose. Similar islets were analyzed for insulin content or subjected to morphometric measurements to identify both the number of beta-cells and the volume of beta-granules per islet. In parallel studies, the islet content of intact pancreata was also determined. The results showed that beta-cell number increased from 2,300 t0 5,000 cells as rats aged from 2 to 18 mo and islet insulin content doubled. However, glucose-stimulated insulin release decreased progressively with age, and this was especially striking when considered in terms of the increase in number of beta-cells/islet; e.g., mean (+/- SEM) insulin secretion (nanounits per minute per beta-cell) of islets incubated with 450 mg/dl of glucose was 1.3 (+/- 0.02), 1.0 (+/- 0.1), 0.4 (+/- 0.05), and 0.3 (+/- 0.01), respectively for 2-, 6-, 12-, and 18-mo-old rats. Thus, insulin secretion per beta-cell was decreased, despite increased stores of insulin per cell. These findings demonstrate that the aging process leads to a profound defect in glucose-stimulated insulin release from the beta-cell. Whether this is a global secretory defect, or solely a failure of the beta-cell to respond to glucose, remains to be defined.

Effect of a high carbohydrate diet on insulin binding to adipocytes and on insulin action in vivo in man

We studied the effects of short-term (5 days) and long-term (2 wk) high carbohydrate (75%) feedings on insulin binding to isolated adipocytes and insulin sensitivity in vivo in normal subjects. Ingestion of the high carbohydrate diet led to daylong hyperinsulinemia in both short- and long-term groups. Insulin binding to isolated adipocytes was decreased in both groups; in the short-term groups this decrease in insulin binding was caused by a decrease in the receptor affinity, whereas in the long-term group it was caused by a decrease in receptor number. On the other hand, despite this decrease in insulin binding, total in vivo insulin sensitivity was markedly improved in both groups. In conclusion, (1) the short-term adaptive response of the insulin receptor is a decrease in binding affinity whereas the long-term response is a decrease in receptor number, (2) sustained and chronic hyperinsulinemia can lead to a decrease in the number of cellular insulin receptors, (3) high carbohydrate diets lead to a general increase in insulin's ability to promote glucose removal from plasma, and (4) the paradox of enhanced insulin sensitivity in the face of decreased insulin binding can be explained if high carbohydrate diets also lead to an increase in the activity of steps in glucose metabolism distal to the insulin receptor.

Characterization of a model of dietary-induced hypertriglyceridemia in young, nonobese rats

Healthy, nonobese, young rats developed hypertriglyceridemia (mean triglyceride levels of 250 mg/dl) following consumption of a sucrose-lard diet. The hypertriglyceridemia was apparent three days after start of the diet and persisted throughout the 4-week experimental period. Body weight, liver weight, and serum glucose levels were similar in animals eating either the sucrose-lard diet or standard rat chow. On the other hand, serum free fatty acid levels were slightly increased and serum insulin levels were substantially increased in animals eating the sucrose-lard diet. Determination of very low density lipoprotein turnover revealed that total triglyceride secretion in rats eating the sucrose-lard diet was significantly (P < 0.01) increased over that of rats eating standard chow. Direct measurement of hepatic and intestinal very low density lipoprotein secretion indicated that the observed rise in total triglyceride secretion was secondary to increased secretion of very low density lipoproteins by the liver. Finally, lipoprotein lipase activity of adipose tissue from rats eating the sucrose-lard diet was equal to, or greater than (depending upon sampling time), the activity of the enzyme from adipose tissue of rats eating the control diet. These data indicate that young, nonobese, rats develop hypertriglyceridemia when they ingest a sucrose-lard diet, and that the rise in plasma triglyceride levels results from an increase in hepatic very low density lipoprotein secretion. The dietary-induced hypertriglyceridemia is associated with elevated serum insulin levels, and, as such, may provide a useful animal model to use in studies aimed at defining the pathogenesis of endogenous hypertriglyceridemia in man.-Reaven, G. M., T. R. Risser, Y-D. I. Chen, and E. P. Reaven. Characterization of a model of dietary-induced hypertriglyceridemia in young, nonobese rats.

Defect in glucose removal in nonketotic diabetic patients with fasting hyperglycemia

The present experiments were performed to determine the ability of insulin to stimulate glucose uptake in normal subjects as compared with nonobese patients with fasting hyperglycemia at similar steady state plasma glucose and insulin concentrations. The studies were carried out at two different steady state plasma glucose levels (approximately 250 and 350 mg/100ml), and, in both instances, patients with fasting hyperglycemia removed approximately one-half as much glucose from the plasma as did normal subjects. Since the plasma insulin levels were comparable in the two groups (approximately 100 μU/ml), these results demonstrate that patients with fasting hyperglycemia are more insulin resistant than are normal subjects. Furthermore, the differences in glucose removal were observed under conditions in which the plasma glucose levels in the normal and the diabetic groups were similar; therefore, the insulin resistance of patients with fasting hyperglycemia is independent of differences in plasma glucose pool size. Finally, glucose removal increased proportionately in both experimental.groups at the higher plasma glucose levels, suggesting that saturation of the glucose transport system does hot occur at these plasma glucose concentrations. These results further document the fact that nonobese patients with significant fasting hyperglycemia are less responsive than normal subjects to the acute action of insulin that stimulates glucose removal from plasma.

The effect of ten days of fasting on various aspects of carbohydrate metabolism in obese diabetic subjects with significant fasting hyperglycemia

Patients with nonketotic diabetes mellitus, who were both obese and had significant fasting hyperglycemia (mean plasma glucose = 310 mg/100 ml), were fasted for 10 days. There was a prompt drop of plasma glucose levels as the result of the caloric deprivation with a mean fall of approximately 200 mg/dl at the end of the fast. The drop in plasma glucose level that occurred during the fast was associated with a drop in plasma insulin level. Fasting plasma glucose concentration rose immediately after food intake was resumed and stabilized within 3--4 days at levels halfway between the initial and the lowest value. At this time, the patients also seemed capable of disposing of an oral glucose load more efficiently. The apparent improvement in carbohydrate homeostasis observed after the fast could not be attributed to an increase in insulin response, but was associated with some amelioration of the insulin resistance that characterizes these patients. Unfortunately, the beneficial effects of the 10-day period of caloric deprivation were transitory and fasting plasma glucose values had returned to prefast levels in most patients within a few months.

Effect of obesity on the relationship between very low density lipoprotein production rate and plasma triglyceride concentration in normal and hypertriglyceridemic subjects

The effect of obesity on the relationship between very low density lipoprotein (VLDL)-triglyceride (TG) production rate and concentration was studied in 80 subjects whose TG concentrations ranged from 41-1315 mg/100 ml and whose relative weight varied from 0.74-1.46. There was a positive correlation between VLDL-TG production rate and plasma TG concentration in all 80 patients (r = 0.78), which was highly statistically significant (p less than 0.001). Both male and female subjects were subdivided into quartiles on the basis of relative weight; the relationship between VLDL-TG production rate and plasma TG concentration was found to be comparable in all subgroups. These observations suggest that moderate degrees of obesity do not affect VLDL-TG kinetics in patients whose plasma TG levels vary over an extremely wide range.