John A. Scarlett

Receptor and Postreceptor Defects Contribute to the Insulin Resistance in Noninsulin-dependent Diabetes Mellitus

We have assessed the mechanisms involved in the pathogenesis of the insulin resistance associated with impaired glucose tolerance and Type II diabetes mellitus by exploring, by means of the euglycemic glucose-clamp technique, the in vivo dose-response relationship between serum insulin and the overall rate of glucose disposal in 14 control subjects; 8 subjects with impaired glucose tolerance, and 23 subjects with Type II diabetes. Each subject had at least three studies performed on separate days at insulin infusion rates of 40, 120, 240, 1,200, or 1,800 mU/M2 per min. In the subjects with impaired glucose tolerance, the dose-response curve was shifted to the right (half-maximally effective insulin level 240 vs. 135 microunits/ml for controls), but the maximal rate of glucose disposal remained normal. In patients with Type II diabetes mellitus, the dose-response curve was also shifted to the right, but in addition, there was a posal. This pattern was seen both in the 13 nonobese and the 10 obese diabetic subjects. Among these patients, an inverse linear relationship exists (r = -0.72) so that the higher the fasting glucose level, the lower the maximal glucose disposal rate. Basal rates of hepatic glucose output were 74 +/- 4, 82 +/- 7, 139 +/- 24, and 125 +/- 16 mg/M2 per min for the control subjects, subjects with impaired glucose tolerance, nonobese Type II diabetic subjects, and obese Type II diabetic subjects, respectively. Higher serum insulin levels were required to suppress hepatic glucose output in the subjects with impaired glucose tolerance and Type II diabetics, compared with controls, but hepatic glucose output could be totally suppressed in each study group. We conclude that the mechanisms of insulin resistance in patients with impaired glucose tolerance and in patients with Type II noninsulin-dependent diabetes are complex, and result from heterogeneous causes. (a) In the patients with the mildest disorders of carbohydrate homeostasis (patients with impaired glucose tolerance) the insulin resistance can be accounted for solely on the basis of decreased insulin receptors. (b) In patients with fasting hyperglycemia, insulin resistance is due to both decreased insulin receptors and postreceptor defect in the glucose mechanisms. (c) As the hyperglycemia worsens, the postreceptor defect in peripheral glucose disposal emerges and progressively increases. And (d) no postreceptor defect was detected in any of the patient groups when insulins ability to suppress hepatic glucose output was measured.

Long-term treatment of acromegaly with pegvisomant, a growth hormone receptor antagonist

BACKGROUND Pegvisomant is a new growth hormone receptor antagonist that improves symptoms and normalises insulin-like growth factor-1 (IGF-1) in a high proportion of patients with acromegaly treated for up to 12 weeks. We assessed the effects of pegvisomant in 160 patients with acromegaly treated for an average of 425 days. METHODS Treatment efficacy was assessed by measuring changes in tumour volume by magnetic resonance imaging, and serum growth hormone and IGF-1 concentrations in 152 patients who received pegvisomant by daily subcutaneous injection for up to 18 months. The safety analysis included 160 patients some of whom received weekly injections and are excluded from the efficacy analysis. FINDINGS Mean serum IGF-1 concentrations fell by at least 50%: 467 mg/L (SE 24), 526 mg/L (29), and 523 mg/L (40) in patients treated for 6, 12 and 18 months, respectively (p<0.001), whereas growth hormone increased by 12.5 mg/L (2.1), 12.5 mg/L (3.0), and 14.2 mg/L (5.7) (p<0.001). Of the patients treated for 12 months or more, 87 of 90 (97%) achieved a normal serum IGF-1 concentration. In patients withdrawn from pegvisomant (n=45), serum growth hormone concentrations were 8.0 mg/L (2.5) at baseline, rose to 15.2 mg/L (2.4) on drug, and fell back within 30 days of withdrawal to 8.3 mg/L (2.7). Antibodies to growth hormone were detected in 27 (16.9%) of patients, but no tachyphylaxis was seen. Serum insulin and glucose concentrations were significantly decreased (p<0.05). Two patients experienced progressive growth of their pituitary tumours, and two other patients had increased alanine and asparate aminotransferase concentrations requiring withdrawal from treatment. Mean pituitary tumour volume in 131 patients followed for a mean of 11.46 months (0.70) decreased by 0.033 cm(3) (0.057; p=0.353). INTERPRETATION Pegvisomant is an effective medical treatment for acromegaly.

The Acute and Chronic Effects of Sulfonylurea Therapy in Type II Diabetic Subjects

Although sulfonylurea agents have been used in the clinical management of type II diabetes (non-insulin-dependent diabetes mellitus, NIDDM) for over two decades, the mechanisms responsible for their hypoglycemie action remain controversial. We have quantitated glycemie control, endogenous insulin secretion in response to mixed meals, adipocyte insulin binding, insulin-mediated peripheral glucose disposal, and basal hepatic glucose output in 17 type II diabetic subjects before and after 3 mo of therapy with the second-generation, sulfonylurea compound glyburide in an attempt to identify the factors responsible for the clinical response to the drug. In addition, 9 subjects were treated for an additional 15 mo to see if the response to the drug changed with time. The mean fasting serum glucose level fell from an initial value of 264 ± 17 mg/dl to 178 ± 16 mg/dl after 3 mo of drug therapy. Endogenous insulin secretion increased in all subjects, but the increase was most marked in those subjects who continued to exhibit fasting hyperglycemie (fasting serum glucose > 175 mg/dl) after 3 mo of therapy. Adipocyte insulin binding was unchanged after 3 mo of therapy, while the maximal rate of peripheral glucose disposal was increased by 23%, indicating enhancement of peripheral insulin action at a postreceptor site(s). Basal hepatic glucose output showed a significant correlation with the fasting serum glucose level both before and after therapy (r = 0.86, P < 0.001) and fell from 141 ±12 mg/m2/min before therapy to 107 ± 11 mg/m2/min after 3 mo of therapy. A significant correlation was also found between the decrease in the fasting glucose level and both the reduction in basal hepatic glucose output (r = 0.81, P < 0.001) and the enhancement of postreceptor function in peripheral tissues (r = 0.68, P < 0.005). After 18 mo of therapy, those subjects exhibiting an initial good response to the drug demonstrated a slight decrease in endogenous insulin secretion compared with the levels seen at 3 mo, adipocyte insulin binding had increased to the normal range, postreceptor function was further enhanced, and basal hepatic glucose output remained unchanged from the levels observed after 3 mo of therapy. We conclude that (1) glyburide therapy increases endogenous insulin secretion, increases adipocyte insulin binding after 18, but not 3, mo of therapy, enhances peripheral insulin action by acting primarily at a post-receptor site, and reduces basal hepatic glucose output; (2) the increase in postreceptor function and the reduction of basal hepatic glucose output appear to be the crucial determinants of the clinical response to the sulfonylurea agent; and (3) the response pattern to sulfonylurea compounds in terms of these various parameters can vary as a function of the duration of treatment.

Insulin Treatment Reverses the Insulin Resistance of Type II Diabetes Mellitus

Type II diabetic subjects are both insulin-deficient and insulin-resistant. Recent studies suggest that the insulin resistance is due to a combined receptor and postreceptor defect with the postreceptor defect being the predominant lesion. In the present study, we examined the effects of exogenous insulin therapy upon these defects in insulin action in six untreated type II diabetic subjects. Glycemic control and adipocyte insulin binding were measured and in vivo insulin dose-response curves for overall glucose disposal and suppression of hepatic glucose output were constructed before treatment. Following these initial studies, the diabetic subjects were treated with twice-daily injections of regular and NPH purified pork insulin for 14 days and the pretreatment studies repeated. Glycemic control was significantly improved by this treatment regimen. The mean fasting serum glucose level (±SE) fell from 287 ± 20 to 125 ± 13 mg/dl, the mean glycosylated hemoglobin level (± SE) decreased from 14.2 ± 1.1% to 8.3 ± 0.5%, and the mean 24-h urinary glucose excretion (±SE) declined from 65.6 ± 40.3 to 0.6 ± 0. 1 g/24 h. Adipocyte insulin binding did not change significantly during the treatment period. In contrast, the 14-day period of insulin treatment produced a 72% increase (P < 0.005) in the maximal rate of insulin-stimulated glucose disposal, 321 ± 32 mg/M2/min compared with 187 ± 32 mg/M2/min before treatment, indicating that the postreceptor defect in insulin action was significantly ameliorated by insulin treatment. The dose-response curve for insulin-mediated suppression of hepatic glucose output was rightshifted, consistent with the decrease in insulin binding, with no decrease in the maximal effect before treatment and not significantly changed following insulin treatment. In conclusion, the postreceptor defect in insulin-stimulated glucose disposal is largely ameliorated by exogenous insulin treatment, suggesting that this defect in insulin action is an acquired abnormality which is secondary to some aspect of the insulin-deficient state.

Factitious hypoglycemia. Diagnosis by measurement of serum C-peptide immunoreactivity and insulin-binding antibodies.

In seven patients with factitious hypoglycemia due to the surreptitious injection of insulin, we made the diagnosis by measurements of plasma insulin and C-peptide immunoreactivity (in seven patients), facilitated by the finding of circulating insulin-binding antibodies (in two patients). The simultaneous demonstration of low plasma glucose, high immunoreactive insulin and suppressed C-peptide immunoreactivity represents a triad of results pathognomonic of exogenous insulin administration. Determination of plasma free C-peptide and free insulin permitted patients with high titers of insulin antibodies, including those with a history of insulin-treated diabetes, to be studied and diagnosed in a way similar to that in subjects who had no circulating insulin antibodies.

Role of Glucose Transport in the Postreceptor Defect of Non-insulin-dependent Diabetes Mellitus

In an attempt to elucidate the cellular mechanism(s) of the insulin resistance associated with impaired glucose tolerance (IGT) and non-insulin-dependent diabetes (NIDDM), insulin-sensitive glucose transport was studied employing isolated adipocytes obtained from these subjects using the nonmetabolized glucose analogue, 3-O-methyl glucose. In the subjects with IGT, basal and maximal rates of 3-O-methyl glucose uptake were normal while the responses at submaximal insulin concentrations were decreased, i.e., the dose-response curves were shifted rightward, indicative of decreased insulin sensitivity. In contrast, the dose-response curves for insulin-stimulated 3-O-methyl glucose uptake in adipocytes obtained from subjects with NIDDM were right-shifted and, in addition, there was a marked decrease in both the basal and maximally stimulated rates of glucose transport (i.e., decreased insulin responsiveness). Thus, the adipocytes from subjects with IGT show decreased insulin sensitivity consistent with decreased insulin binding, whereas the adipocytes from subjects with NIDDM exhibit both decreased insulin sensitivity and decreased insulin responsiveness consistent with a combined receptor and postreceptor defect in cellular insulin action. In the groups as a whole, the magnitude of the rightward shift in the dose-response curve for insulin-stimulated glucose transport correlated with the reduction in adipocyte insulin binding (r = 0.47, P < 0.02). In these subjects, the level of fasting hyperglycemia was correlated (P < 0.01) to the magnitude of the decrease in maximal glucose transport and a highly significant correlation was found between the maximal insulinstimulated rate of glucose transport and the maximal in vivo rate of insulin-stimulated glucose disposal (r = 0.49, P < 0.01). Therefore, we conclude that the insulin resistance in subjects with IGT is due solely to a decrease in insulin binding, whereas subjects with NIDDM exhibit both decreased insulin binding and decreased maximal rates of insulin-stimulated adipocyte glucose transport due to a postreceptor defect at this site in the insulin action pathway. If adipocytes are reflective of changes in glucose transport in other insulin target tissues, then these findings suggest that the cellular lesion responsible for the postreceptor defect in insulin action, previously demonstrated in vivo in subjects with NIDDM, resides in part at the level of the glucose transport system.

In Vivo Deactivation of Peripheral, Hepatic, and Pancreatic Insulin Action in Man

The in vivo deactivation of insulin action has been studied in 10 lean, nondiabetic subjects using a modification of the euglycemic glucose clamp technique. Following cessation of 40- and 120-mU/m2/min insulin infusions, the serum insulin levels fell to one-half their initial values (mean ± SE) of 126 ± 7 and 350 ± 14 μU/ml in 7 ± 1 and 8 ± 1 min, respectively. The mean incremental glucose disposal rates (IGDR) fell more slowly following discontinuation of the 40- and 120-mU/m2/min insulin infusions, so that the time required for the IGDRs to fall to one-half their initial values (D50 IGDR) were were 42 ± 5 and 78 ± 5 min, respectively. Mean hepatic glucose output was totally suppressed during the 40- and 120-mU/m2/min insulin infusions, remained completely suppressed following cessation of the infusions for 50 and 80 min, and subsequently returned to basal levels. The times required for the HGOs to return to one-half their basal levels (R50 HGO) were 59 ± 8 and 119 ± 6 min, respectively. The times required for insulin action to decrease to one-half the initial values in the periphery (D50 IGDR) and in the liver (R50 HGO) were correlated with the preceding steady-state glucose disposal rates in individual subjects (r = 0.75, P < 0.001 and r = 0.58, P < 0.05, respectively). The suppression of endogenous insulin secretion by exogenous insulin infusions was also studied in 4 subjects during a total of 5 euglycemic glucose clamps; the mean basal serum C-peptide level was 0.67 ± 0.24 pmol/ml before administration of the exogenous insulin, fell to 0.34 ± 0.17 pmol/ml during the steady-state phase of the study, and remained suppressed throughout the duration of the deactivation phase of the glucose clamp. Residual pancreatic insulin secretory capacity was demonstrated by a rise in the serum C-peptide level to 1.77 ± 0.50 pmol/ml at 120 min following a standardized meal given at the conclusion of the deactivation phase of the glucose clamp. These results demonstrate that the deactivation of insulin action in the periphery, liver, and pancreas lags behind the disappearance of insulin from the plasma. The mechanisms responsible for this lag in in vivo deactivation are not known for certain, but may include slower clearance of insulin form tissue compartments than form the plasma, the necessity for the target tissues to generate specific deactivation signals, or a slow rate of decay of saturable steps in the cellular activation process.

Pegvisomant in the treatment of acromegaly

Epidemiological studies have highlighted the need for tight control of growth hormone (GH) and insulin-like growth factor I (IGF-I) in patients with acromegaly. Studies highlighting the events involved in GH receptor signaling have allowed the development of a pegylated GH receptor antagonist (pegvisomant) for use in humans, which has been designed to outcompete GH for the GH receptor, but which contains a position 120 amino acid substitution that prevents recruitment of a second GH receptor. This process of receptor dimerisation is crucial for signal transduction and IGF-I generation. Clinical trials of pegvisomant suggest it is the most effective treatment for acromegaly to date, as this therapy is capable of normalising serum IGF-I in up to 97% of patients when doses of 40 mg per day are used. This paper reviews the development of pegvisomant and the clinical experience in patients with acromegaly to date.

Insulin Resistance in Non-Insulin Dependent (Type II) and Insulin Dependent (Type I) Diabetes Mellitus

Insulin resistance is a characteristic feature of non-insulin dependent diabetes mellitus (NIDDM) due to target tissue defects in insulin action. Abnormalities of cellular insulin action can be divided into receptor and post-receptor defects. Patients with impaired glucose tolerance are insulin resistant due to decreased insulin receptors resulting in decreased insulin sensitivity and rightward shifted in vivo dose response curves. Patients with NIDDM are insulin resistant due to a combination of receptor and post-receptor defects. The greater the severity of the diabetes (greater fasting hyperglycemia) the greater the post-receptor defect, and in those patients with more significant fasting hyperglycemia the post-receptor defect is the predominant abnormality leading to the insulin resistant state. At least one of the abnormalities underlying this post-receptor defect involves a decrease in glucose transport system activity in freshly isolated adipocytes. This defect in glucose transport, is not expressed in cultured fibro-blasts, indicating that the abnormality in glucose disposal seen in vivo and in glucose transport seen in freshly isolated cells is an acquired phenomenon. Consistent with this, the post-receptor defect is partially reversible by insulin therapy, which leads to a 50-70% reversal of the reduced rates of in vivo glucose disposal and in vitro glucose transport. Insulin resistance also exists in poorly controlled IDDM patients, due to a postreceptor defect in insulin action. This insulin resistance is not present in well controlled IDDM patients, and is completely reversible when poorly controlled patients are treated with intensive insulin therapy. Insulin is produced in the pancreatic beta cell as the primary biosynthetic product preproinsulin. This peptide is rapidly converted to proinsulin (MW approximately 9000). Proinsulin is converted to insulin (MW approximately 6000) plus C-peptide in the secretory granule with a small amount (approximately 5 percent) of the proinsulin remaining unconverted. After a brief time in the peripheral circulation (half-life six to 10 minutes), insulin interacts with target tissues to exert its biologic effects. One of insulins major biologic effects is the promotion of overall glucose metabolism, and abnormalities of this aspect of insulin action can lead to a number of important clinical and pathophysiologic states including Type II diabetes, also known as non-insulin-dependent diabetes mellitus (NIDDM). Since insulin travels from the beta cell through the circulation to the target tissues, abnormalities at any of these loci can influence the ultimate action of the hormone. These abnormalities, all

The Effects of Oral Fructose, Sucrose, and Glucose in Subjects with Reactive Hypoglycemia

We have evaluated the acute effects of orally administered 100-g loads of fructose, sucrose, or glucose given as drinks and of 100-g loads of fructose and sucrose given in cakes on the postprandial serum glucose, insulin, and cortisol responses in seven subjects with reactive hypoglycemia. We defined reactive hypoglycemia as a serum glucose nadir of 65 mg/dl or less, symptoms compatible with hypoglycemia occurring at or after the serum glucose nadir, a hypoglycemic index of greater than 1.0, and a rise in serum cortisol to greater than 20 μg/dl after the serum glucose nadir. The data demonstrated that (1) pure fructose given as a drink resulted in relatively flat serum glucose and insulin responses and did not cause a hypoglycemic reaction in any of the subjects, compared with the glucose drink, which caused a hypoglycemic reaction in all subjects; (2) ingestion of pure sucrose as a drink elicited significantly flatter serum glucose and insulin responses than did the glucose drink and was associated with some episodes of chemical hypoglycemia and symptoms, but did not result in a hypoglycemic reaction by our definition in any patient; and (3) ingestion of fructose cake led to serum glucose and insulin responses that were lower than those caused by ingestion of sucrose cake, but ingestion of neither fructose nor sucrose cake led to a hypoglycemic reaction by our definition in any patient. In conclusion, the use of fructose as a sweetening agent given either alone, in a drink, or with other nutrients in a cake resulted in markedly flatter serum glucose and insulin responses in subjects with reactive hypoglycemia. Fructose may thus prove useful as a sweetening agent in the dietary treatment of selected patients with reactive hypoglycemia.