Further evidence that insulin resistance exists in patients with chemical diabetes

Insulin resistance was estimated in fifty nonobese subjects who were classified as normal (thirty subjects) or as having chemical diabetes (twenty subjects) on the basis of a standard oral glucose tolerance test. Insulin resistance was determined by administering a constant intravenous infusion of epinephrine, propranolol, glucose and exogenous insulin to each subject for 150 minutes and by measuring the resultant steady state plasma glucose levels attained during the last sixty minutes. Under these conditions, endogenous insulin secretion and hepatic glucose output are suppressed, and similar steady state plasma levels of the exogenously infused insulin are achieved in all subjects. Since all patients are challenged with comparable glucose loads, the height of the steady state plasma glucose response becomes a direct measure of an individual's resistance to insulin-mediated glucose uptake. With this approach we found that the mean (± S.E.) steady state plasma glucose level in patients with chemical diabetes was 224 ± 15 mg. per 100 ml. as compared to 126 ± 12 mg. per 100 ml. in control subjects. This difference was statistically significant (p <.001) and could not be attributed to differences in degree of obesity between the two groups. Furthermore, when individual values were examined it was apparent that relatively little overlap existed between steady state plasma glucose levels in normal and diabetic subjects. These results indicate that resistance to insulin-mediated glucose uptake is a characteristic of nonobese patients with chemical diabetes.

Insulin binding to isolated human adipocytes

In order to study the binding of insulin to insulin-sensitive human tissue, we isolated adipocytes from surgically obtained human fat tissue. Our data demonstrate that insulin readily and specifically binds to isolated adipocytes. The time course of this reaction indicates that steady state binding conditions occur at forty-five minutes, with a subsequent decline in binding later. There is no appreciable insulin or receptor degradation before forty-five minutes, suggesting a true equilibrium. After forty-five minutes the decline in binding can be accounted for by insulin plus receptor degradation. At 3 × 10−11 M 125-I-insulin and 2 × 105 cells per milliliter, 1.8 to 2.4 per cent of the 125-I-insulin was bound. The specificity of this binding reaction is demonstrated by the high concentrations of thyroid stimulating hormone and human growth hormone that are without effect on binding, while proinsulin and desalanine insulin inhibit binding in proportion to their biologic activity. On the other hand, binding can be readily inhibited by porcine insulin at physiologic concentrations, i.e. binding is 13 per cent inhibited at 1 ng. per milliliter and 50 per cent inhibited at 8.6 ng. per milliliter. The kinetic behavior of this reaction can be approximated by Scatchard analysis, which indicates 50,000 high affinity sites per cell with a dissociation constant (Kd) of 1.9 × 10−9 M per liter and 250,000 low affinity sites per cell with a Kd of 8 × 10−9 M per liter.

In conclusion, these studies demonstrate and characterize the binding of insulin to normal isolated human adipocytes; they indicate that the study of insulin-adipocyte binding is a possible means to gain further insight into the mechanism of altered response to insulin in human disease states.

Insulin and glucose responses to identical oral glucose tolerance tests performed forty-eight hours apart

We compared the plasma glucose and insulin responses to two identical oral glucose tolerance tests performed forty-eight hours apart in thirty-one adult patients. The mean results of the two tests were essentially identical, indicating that no consistent difference in glucose or insulin response occurs upon repetitive oral glucose tolerance testing. When the variability of the glucose response curves were analyzed by comparing repeat values to each other, it was found that nine of thirty-one fasting values deviated by more than 10 per cent, but no fasting values deviated by more than 30 per cent. The one hour value was more variable: Twenty-two of thirty-one observations deviated by more than 10 per cent and six of thirty-one deviated by more than 30 per cent. The variability of the insulin response was greater. For example, fourteen of thirtyone fasting values deviated by more than 10 per cent, and three of thirty-one deviated by more than 30 per cent, while twenty-six of thirty-one one hour values deviated by more than 10 per cent and twenty-one of thirty-one deviated by more than 30 per cent. Furthermore, the ratio of the insulin response to the glucose response (I/G ratio) was as variable as was the insulin response alone. The extreme variability of the insulin response to identical glucose loads, coupled with similar variability of the I/G ratio, suggests that factors other than glucose concentration play a role in regulating insulin response.

Relationship between insulin response during the intravenous glucose tolerance test, rate of fractional glucose removal and the degree of insulin resistance in normal adults

We studied the relationship between the absolute insulin response during the intravenous glucose tolerance test (IVGTT) and both the fractional glucose disposal rate (KG) during the IVGTT and the degree of insulin resistance as measured by a separate infusion study, the pancreatic suppression test (PST), in normal adults. Insulin response during the IVGTT was analyzed in four ways: (1) early (area under the one, three and five minute response curve); (2) middle (ten minute insulin value); (3) late (area under the twenty to sixty minute response curve); and (4) total (area under the entire response curve). Only weak to modest correlations existed between the four measures of insulin response and KG, and the only statistically significant correlation (r = .39, p < .05) existed between the middle insulin response and KG. However, significant correlations were found between the degree of insulin resistance measured during the PST and the early (r = .40, p < .05), middle (r = .59, p < .005), late (r = .71, p < .001) and total (r = .66, p < .001) insulin responses. Whereas the height of the insulin response during an IVGTT has only a modest correlation with the KG, it correlates quite well with the degree of insulin resistance.

Ultrastructural and physiological evidence for corticosteroid-induced alterations in hepatic production of very low density lipoprotein particles

The cause of corticosteroid-induced hyperlipoproteinemia was studied in rats and mice. An ultrastructural morphometric method was utilized to demonstrate alterations in hepatocyte very low density lipoprotein content, and Triton WR 1339-treated rats were used to identify changes in the removal of very low density lipoproteins from plasma. The results show that corticosteroid treatment results in (1) an increase in both plasma triglyceride and cholesterol levels, (2) an increase in rate of accumulation of triglyceride after inhibition of very low density lipoprotein removal by Triton, and (3) an increase in the number and size of Golgi-associated very low density lipoprotein particles in hepatocytes. These combined results suggest that corticosteroids induce hyperlipoproteinemia through increased hepatic production of very low density lipoproteins.

Effects of weight reduction on obesity. Studies of lipid and carbohydrate metabolism in normal and hyperlipoproteinemic subjects

Considerable controversy exists over the purported role of obesity in causing hyperglycemia, hyperlipemia, hyperinsulinemia, and insulin resistance; and the potential beneficial effects of weight reduction remain incompletely defined. Hypertriglyceridemia is one of the metabolic abnormalities proposed to accompany obesity, and in order to help explain the mechanisms leading to this abnormality we have proposed the following sequential hypothesis: insulin resistance --> hyperinsulinemia --> accelerated hepatic triglyceride(TG) production --> elevated plasma TG concentrations. To test this hypothesis and to gain insight into both the possible role of obesity in causing the above metabolic abnormalities and the potential benefit of weight reduction we studied the effects of weight loss on various aspects of carbohydrate and lipid metabolism in a group of 36 normal and hyperlipoproteinemic subjects. Only weak to absent correlations (r = 0.03 - 0.46) were noted between obesity and the metabolic variables measured. This points out that in our study group obesity cannot be the sole, or even the major, cause of these abnormalities in the first place. Further, we have observed marked decreases after weight reduction in fasting plasma TG (mean value: pre-weight reduction, 319 mg/100 ml; post-weight reduction, 180 mg/100 ml) and cholesterol (mean values: pre-weight reduction, 282 mg/100 ml; post-weight reduction, 223 mg/100 ml) levels, with a direct relationship between the magnitude of the fall in plasma lipid values and the height of the initial plasma TG level. We have also noted significant decreases after weight reduction in the insulin and glucose responses during the oral glucose tolerance test (37% decrease and 12% decrease, respectively). Insulin and glucose responses to liquid food before and after weight reduction were also measured and the overall post-weight reduction decrease in insulin response was 48% while the glucose response was relatively unchanged. In a subgroup of patients we studied both the degree of cellular insulin resistance and the rate of hepatic very low density (VLDL) TG production before and after weight reduction. These subjects demonstrated significant decreases after weight reduction in both degree of insulin resistance (33% decrease) and VLDL-TG production rates (40% decrease). Thus, weight reduction has lowered each of the antecedent variables (insulin resistance, hyperinsulinemia, and VLDL-TG production) that according to the above hypothesis lead to hypertriglyceridemia, and we believe the overall scheme is greatly strengthened. Furthermore, the consistent decreases in plasma TG and cholesterol levels seen in all subjects lead us to conclude that weight reduction is an important therapeutic modality for patients with endogenous hypertriglyceridemia.

Characteristics of membrane-bound and free hepatic ribosomes from insulin-deficient rats. I. Acute experimental diabetes mellitus

Membrane-bound and free ribosomes were prepared by discontinuous density gradient centrifugation from livers of rats 2-3 days after receiving alloxan (75 mg/kg) or streptozotocin (100 mg/kg). Hepatocytes from these animals were also examined by electron microscopy and subjected to quantitative morphometric analysis. The results indicated that the two populations of hepatic ribosomes respond differently to acute insulin deficiency. There was an overall reduction (P < 0.001) in total number of bound ribosomes per volume cytoplasm: the remaining bound ribosomes underwent a shift to smaller-sized ribosomal messenger RNA (mRNA) aggregates (P < 0.02); and the proteinsynthetic activity of these bound ribosomes was less than normal (P < 0.02) when protein synthesis was directed by endogenous mRNA. However, there was no difference between bound ribosomes from livers of normal and diabetic rats when protein synthesis was directed by polyuridylic acid. In contrast, free ribosomes were unchanged in number and degree of ribosomal mRNA aggregation, but displayed a significantly increased rate of in vitro protein synthesis (P < 0.01) as compared to normal controls. This increased protein-synthetic activity occurred when amino acid incorporation was directed by endogenous mRNA or polyuridylic acid. These changes in structure and function of bound and free hepatic ribosomes were prevented by the concomitant administration of insulin. The decrease in protein-synthetic activity of bound hepatic ribosomes from acutely diabetic rats seems to be secondary to marked disruption and disaggregation of the rough endoplasmic reticulum (RER) with production of smaller ribosomal mRNA aggregates which incorporate less amino acids into protein. Increased protein synthetic activity of free ribosome appears to be related to the ability of these ribosomes to copy mRNA more efficiently.

A paradoxical effect of chlorpropamide on the plasma glucose and immunoreactive insulin response to intravenous glucose in normal dogs

The effect of various doses of chlorpropamide on plasma glucose and immunoreactive insulin responses to an acute intravenous glucose challenge has been studied in normal dogs. The effects of chlorpropamide were compared to those of placebo in each dog. The administration of 375 mg. of chlorpropamide for four days decreased the ability of the dogs to dispose of a glucose load, and the deterioration in glucose tolerance was accompanied by a marked suppression of the plasma insulin response. Administration of 125 mg. of chlorpropamide for four days led to changes that were qualitatively similar but quantitatively less striking. These results indicate that excessive amounts of chlorpropamide can lead to deterioration in glucose tolerance; this effect seems analogous to induction of glucose intolerance in normal subjects by administration of insulin, as first described by Somogyi. When the chlorpropamide was decreased to a daily dose of 50 mg., dogs were able to dispose of the intravenous glucose load more efficiently than normal. However, this improvement in glucose tolerance was not associated with a significant rise in plasma insulin response. Thus, although it seems reasonable to attribute deterioration in glucose tolerance after excessive doses of chlorpropamide at least partly to inhibition of insulin release, the improvement that occurs after smaller doses does not appear to be due to an absolute increment in the plasma insulin response to glucose.

Do the oral and intravenous glucose tolerance tests provide similar diagnostic information in patients with chemical diabetes mellitus?

In order to evaluate the diagnostic reliability of the oral glucose tolerance test (OGTT) and the intravenous glucose tolerance test (IVGTT), the results of these two teats have been-compared both to each other as well as to a third test of carbohydrate tolerance termed the pancreatic suppression test (PST), in forty-five subjects. When results of the IVGTT and OGTT are compared, there is a 40 per cent incidence of discordance in the diagnosis of diabetes mellitus. There is no significant correlation between the results of the PST and the IVGTT, but a significant correlation is obtained between results of the PST and the OGTT (P &lt; 0.01). Furthermore, the PST could not detect differences between patients classified as normal or diabetic on the basis of the IVGTT, but differences could be detected between subjects divided into diabetic or normal on the basis of the OGTT (P &lt; 0.001). Although neither test is an ideal diagnostic tool, our results suggest that the OGTT is more meaningful than its counterpart in estimating the efficiency of glucose disposal in patients with mild abnormalities of glucose tolerance.

The effect of experimental diabetes mellitus and insulin replacement on hepatic ultrastructure and protein synthesis

The following study was conducted in order to define the specific alterations in hepatic ultrastructure responsible for the decrease in hepatic protein synthesis associated with experimental diabetes. Rats received intravenous alloxan (70 mg/kg) and 48 h later were either sacrificed or given insulin for 1, 2, 4, 6, or 24 h. Specimens for electron microscopic evaluation and morphometric analysis were taken from the same livers used to isolate ribosomes for measurement of in vitro protein synthesis. Our results show that hepatocytes from animals with untreated alloxan diabetes show varying degrees of disorganization and loss of rough endoplasmic reticulum (RER) which is directly related to the severity of the alloxan diabetes. A significant correlation existed between the severity of ultrastructural changes as judged by the loss of both membrane and polysome components of the RER and degree of inhibition of protein synthesis (P < 0.001). Abnormalities of hepatic ultrastructure and protein synthesis were reversed within 24 h of insulin administration. The data are consistent with the view that it is the relative decrease in hepatic polysomes that results from the loss of RER in alloxan diabetes that is responsible for the decrease in hepatic protein synthesis.

Comparison of the effects of metabolic acidosis and acute uremia on carbohydrate tolerance

In an attempt to assess the role of metabolic acidosis in the glucose intolerance of uremia, we have estimated insulinmediated glucose uptake in normal dogs, in acidotic dogs with normal renal function, and in uremic dogs. Mean steady state plasma glucose and insulin levels were measured in eighteen dogs (pH 7.4 ± .01) during constant infusion of glucose and insulin, while endogenous insulin secretion was suppressed by infusion of propranolol and epinephrine. The dogs were then divided into three groups: moderate acidosis was produced by giving 7 gm. NH4Cl/ day for four days; severe acidosis, 12 gm. NH4Cl/day for four days; and uremia (serum creatinine 15.7 ± 0.2 mg. per cent) by anastomosing both ureters to the vena cava. Duplicate infusions were then performed and the results compared to control values. Plasma insulin levels were identical, despite the presence of either acidosis or uremia. The mean increase in plasma glucose in moderate acidosis (pH 7.2 ± 0.1) over controls was 39 mg./l00 ml. (p &lt; .05); in severe acidosis (pH 7.0 ± .04) plasma glucose increased by 69 mg,/100 ml. (p &lt; .01). However, the mean increase in plasma glucose (149 mg./100 ml.) in uremia (pH 7.3 ± .03) was significantly greater (p &lt; .005) than that following acidosis, although the pH did not significantly fall. Thus, although metabolic acidosis results in deterioration of glucose tolerance, the magnitude of the change does not seem able to account for the carbohydrate intolerance of uremia.

Effect of ammonium chloride acidosis on the oral glucose tolerance and insulin sensitivity of normal rats

Moderate systemic acidosis (pH 6.97-7.20) was produced in rats by the oral administration of ammonium chloride. The rats were treated for two and one-half days, and during this period had free access to food. The effect of this degree of metabolic acidosis on glucose tolerance was studied by determining the plasma glucose response to an oral glucose load (3 gm./kg. body weight) and the effect of acidosis on insulin sensitivity was estimated by measuring in vitro glucose oxidation of isolated epididymal fat pads. It was not possible to demonstrate a significant effect of acidosis in either instance, and it is concluded that moderate acidosis in the absence of starvation, has a relatively modest effect on glucose tolerance and on adipose tissue responsiveness to insulin.