Thymic Lymphocytes in Obese (ob/ob) Mice

[The insulin receptor discovery is 50 years old - A review of achieved progress]

The isolation of insulin from the pancreas and its purification to a degree permitting its safe administration to type 1 diabetic patients were accomplished 100 years ago at the University of Toronto by Banting, Best, Collip and McLeod and constitute undeniably one of the major medical therapeutic revolutions, recognized by the attribution of the 1923 Nobel Prize in Physiology or Medicine to Banting and McLeod. The clinical spin off was immediate as well as the internationalization of insulin's commercial production. The outcomes regarding basic research were much slower, in particular regarding the molecular mechanisms of insulin action on its target cells. It took almost a half-century before the determination of the tri-dimensional structure of insulin in 1969 and the characterization of its cell receptor in 1970-1971. The demonstration that the insulin receptor is in fact an enzyme named tyrosine kinase came in the years 1982-1985, and the crystal structure of the intracellular kinase domain 10 years later. The crystal structure of the first intracellular kinase substrate (IRS-1) in 1991 paved the way for the elucidation of the intracellular signalling pathways but it took 15 more years to obtain the complete crystal structure of the extracellular receptor domain (without insulin) in 2006. Since then, the determination of the structure of the whole insulin-receptor complex in both the inactive and activated states has made considerable progress, not least due to recent improvement in the resolution power of cryo-electron microscopy. I will here review the steps in the development of the concept of hormone receptor, and of our knowledge of the structure and molecular mechanism of activation of the insulin receptor.

LAPS Insulin115: A novel ultra-long-acting basal insulin with a unique action profile

AIMS

To conduct a comprehensive pre-clinical study of the novel ultra-long acting insulin analogue ^LAPS Insulin115.

METHODS

Pharmacokinetic/pharmacodynamic studies comparing ^LAPS Insulin115 with other basal insulins were conducted in genetically diabetic (db/db) mice. Insulin signalling in the major target organs was analysed using Western blot after single subcutaneous injection in wild-type male Wistar rats. Using in vitro assays we analysed transendothelial transport, insulin receptor (IR) interaction, and the mitogenic and metabolic properties of ^LAPS Insulin115. Furthermore, IR downregulation after long-term exposure to high concentrations of ^LAPS Insulin115 was analysed using an in vitro desensitization/resensitization model.

RESULTS

The novel Fc-conjugated insulin derivative ^LAPS Insulin115 showed an extensively prolonged pharmacokinetic and pharmacodynamic profile in rodents. Despite its size of 59 kDa, ^LAPS Insulin115 passes the vascular endothelial barrier and induces insulin signalling in all major target tissues in rats. In vitro, ^LAPS Insulin115 showed a very slow onset of action because of its reduced IR affinity; however, after long-term stimulation it was equipotent in respect to its metabolic potency and showed no increased mitogenic action when compared with regular insulin. Remarkably, under conditions of chronic exposure, ^LAPS Insulin115 does not induce irreversible desensitization of target cells, which is probably attributable to much less prominent IR downregulation.

CONCLUSION

Thus, ^LAPS Insulin115 exhibits a unique in vivo and in vitro profile and thereby represents an excellent candidate for a once-weekly insulin analogue.

Impaired Glucose Metabolism in Mice Lacking the Tas1r3 Taste Receptor Gene

The G-protein-coupled sweet taste receptor dimer T1R2/T1R3 is expressed in taste bud cells in the oral cavity. In recent years, its involvement in membrane glucose sensing was discovered in endocrine cells regulating glucose homeostasis. We investigated importance of extraorally expressed T1R3 taste receptor protein in age-dependent control of blood glucose homeostasis in vivo, using nonfasted mice with a targeted mutation of the Tas1r3 gene that encodes the T1R3 protein. Glucose and insulin tolerance tests, as well as behavioral tests measuring taste responses to sucrose solutions, were performed with C57BL/6ByJ (Tas1r3+/+) inbred mice bearing the wild-type allele and C57BL/6J-Tas1r3^tm1Rfm mice lacking the entire Tas1r3 coding region and devoid of the T1R3 protein (Tas1r3-/-). Compared with Tas1r3+/+ mice, Tas1r3-/- mice lacked attraction to sucrose in brief-access licking tests, had diminished taste preferences for sucrose solutions in the two-bottle tests, and had reduced insulin sensitivity and tolerance to glucose administered intraperitoneally or intragastrically, which suggests that these effects are due to absence of T1R3. Impairment of glucose clearance in Tas1r3-/- mice was exacerbated with age after intraperitoneal but not intragastric administration of glucose, pointing to a compensatory role of extraoral T1R3-dependent mechanisms in offsetting age-dependent decline in regulation of glucose homeostasis. Incretin effects were similar in Tas1r3+/+ and Tas1r3-/- mice, which suggests that control of blood glucose clearance is associated with effects of extraoral T1R3 in tissues other than the gastrointestinal tract. Collectively, the obtained data demonstrate that the T1R3 receptor protein plays an important role in control of glucose homeostasis not only by regulating sugar intake but also via its extraoral function, probably in the pancreas and brain.

Alterations in hepatocyte insulin binding in chronic pancreatitis: effects of pancreatic polypeptide

BACKGROUND

Hepatic insulin resistance has previously been demonstrated in chronic pancreatitis, and has been shown to be ameliorated by pancreatic polypeptide administration. Insulin binding was investigated in chronic pancreatitis induced by infusion of oleic acid into the pancreatic duct of rats.

METHODS

Acute pancreatitis was induced in 12 200 to 225 g 8-week-old male Sprague-Dawley rats by intubation of the main bile duct at its junction with the duodenum through a small midline abdominal incision, and infusion of 99% oleic acid 0.015 mL/min for 4 minutes, with an additional 4 minutes dwell-time after infusion. Sham-operated animals served as controls. After 6 weeks, chronic pancreatitic and sham-operated animals received either intraperitoneal bovine pancreatic polypeptide or saline vehicle for 5 days. Intraduodenal glucose tolerance tests (GTT) were performed in fasted animals, after which tissues were procured. Insulin receptors were isolated from solubilized hepatocyte and rectus abdominus membranes and competitive-binding studies were performed by incubation with 125I-insulin. Dissociation coefficients (Kd) and maximum binding capacities (Bmax) for high-affinity receptors were derived from Scatchard analyses.

RESULTS

Bmax and Kd in muscle were not altered in animals with chronic pancreatitis. In liver, Bmax was significantly less in rats with chronic pancreatitis given saline than in sham-operated rats given saline (17.0 +/- 6.3 versus 47.6 +/- 13.1 fmol/mg protein; data are mean +/- SEM). Pancreatic polypeptide administration increased hepatic Bmax in rats with chronic pancreatitis (to 47.2 +/- 9.8 fmol/mg protein), but had no significant effect in sham-operated rats. Receptor affinity was not significantly different in rats with chronic pancreatitis or rats who underwent sham operations and was unaltered by the administration of pancreatic polypeptide. The integrated plasma glucose response during the GTT was reduced by pancreatic polypeptide administration in rats with chronic pancreatitis (29.5 +/- 15.0 mg/dL per minute versus 69.0 +/- 21.8 in chronic pancreatitis without pancreatic polypeptide), but was not significantly altered in sham-operated animals.

CONCLUSION

Diminished expression of high-affinity receptors on the hepatocyte membrane may contribute to hepatic insulin resistance in chronic pancreatitis. In this model, pancreatic polypeptide improved glucose tolerance and increased receptor capacity to the level observed in livers from nonpancreatitic animals.

The binding of FITC-insulin to ANAE-positive cells in rat thymus

To determine if thymic macrophages have insulin receptors, alternate sections of rat thymus were stained with FITC-insulin and examined for nonspecific esterase (ANAE) activity. Cells showing a diffuse ANAE staining pattern also bound FITC-insulin. These cells were concentrated in the cortico-medullary border and increased in number following administration of cortisol. Thymic macrophages may be insulin-dependent and therefore could be malfunctional in diabetes.

Insulin receptors on circulating blood cells from patients with pancreatogenic diabetes: a comparison with type I diabetes and normal subjects

We studied 125I-insulin binding to erythrocytes from 14 patients with diabetes secondary to chronic pancreatitis or pancreatectomy and compared the results with those found in 10 patients with type I diabetes and 25 normal controls. Patients with pancreatogenic diabetes had higher 125I-insulin binding and enhanced tissue sensitivity to exogenous insulin measured with the glucose clamp technique as compared with patients with type I diabetes. Similar binding data were obtained with monocytes from 3 patients with pancreatogenic diabetes. The increase in insulin binding seemed due mainly to an increase in receptor number. The increase in insulin binding to cells from patients with pancreatogenic diabetes in comparison with cells from normal subjects was also seen in young-erythrocyte-rich fractions and in old-erythrocyte-rich fractions obtained from the mixed population of circulating erythrocytes by centrifugation in density gradient of Percoll-Pielografin. These data, in the absence of any sign of major hematological disorders, suggest that the increase in insulin receptors seen in erythrocytes and in monocytes from patients with pancreatogenic diabetes, can mirror a general phenomenon on tissues throughout the body, including major target cells for insulin and correlate with the heightened sensitivity to insulin characteristic of these patients. In conclusion, patients with pancreatogenic diabetes have increased insulin binding as compared to controls and to patients with type I diabetes with chronic hypoinsulinemia of the same degree. Thus, in addition to insulin deficiency, other factor (s), such as glucagon deficiency, are responsible for the clinical and metabolic differences between these two conditions of insulin deficiency.

Increased insulin binding and glucose transport in white adipocytes isolated from C57B1/6 ob/ob mice treated with the thermogenic beta-adrenoceptor agonist BRL 26830

Obese (ob/ob) mice were treated with the thermogenic beta-adrenoceptor agonist BRL 26830 for 14 days. White adipocytes prepared from these animals showed significant increases in insulin receptor number, with no change in the affinity for these receptors. Increased receptor number was accompanied by increased glucose transport, as measured by 2-deoxyglucose uptake in vitro.

Characteristics of histamine receptors present on suppressor T cells in "healthy individuals"

Twelve to 30% histamine receptor bearing cells were detectable in the peripheral blood mononuclear cells of healthy tuberculin sensitive individuals. The number of binding sites per cell ranged from 2.1 X 10(4) to 5.08 X 10(4) (mean 2.5 X 10(4)) with an affinity ranging from 2.5 X 10(-6) M to 10.9 X 10(-6) M (mean 3.6 X 10(-6) M). The histamine receptors on these cells were found to be of H2 type as indicated by the abrogation of binding of 3H-histamine by cimetidine. It was further confirmed that histamine receptor bearing cells in the peripheral blood belonged to a T cell subset which formed rosettes with AET treated sheep erythrocytes and had receptors for Fc portion of IgG and phenotype markers of T3 and T8. Deletion of such cells by means of affinity chromatography on histamine bound Sepharose columns, led to enhanced antigen induced lymphoproliferation indicating the suppressor nature of these T cells.

Insulin receptors in normal and disease states

The binding of insulin to its receptor has been studied under various physiological and pathological conditions. Quantitative studies have involved human circulating cells such as monocytes and erythrocytes, adipocytes, placental cells, and cultured cells such as fibroblasts and transformed lymphocytes. In animals, other target tissues such as liver and muscle have been studied and correlated with the human studies. Various physiological conditions such as diurnal rhythm, diet, age, exercise and the menstrual cycle affect insulin binding; in addition, many drugs perturb the receptor interaction. Disease affecting the insulin receptor can be divided into five general categories: (1) Receptor regulation--this involves diseases characterized by hyper- or hypoinsulinaemia. Hyperinsulinaemia in the basal state usually leads to receptor 'down' regulation as seen in obesity, type II diabetes, acromegaly and islet cell tumours. Hypoinsulinaemia such as seen in anorexia nervosa or type I diabetes may lead to elevated binding. (2) Antireceptor antibodies--these immunoglobulins bind to the receptor and competitively inhibit insulin binding. They may act as agonists, antagonists or partial agonists. (3) Genetic diseases which produce fixed alterations in both freshly isolated and cultured cells. (4) Diseases of receptor specificity where insulin may bind with different affinity to its own receptor or related receptors such as receptors for insulin-like growth factors. (5) Disease of affinity modulation where physical factors such as pH, temperature, ions, etc. may modify binding. In this review, we have considered primarily abnormality in insulin receptor binding. There are numerous other functions of the receptor such as coupling and transmission of the biological signal. These mechanisms are frequently referred to as postreceptor events, but more properly should be referred to as postbinding events since the receptor subserves other functions in addition to recognition and binding of insulin.

Evidence for independent regulation of human erythrocyte and adipocyte insulin receptors

Specific binding of [125I]-insulin was measured in erythrocytes and fat obtained at the time of mastectomy in twenty-three subjects. There was a significant negative correlation between the level of specific binding to fat and fasting insulin concentration (r=0.55, p less than 0.05, n=23) suggesting down-regulation of fat insulin level by insulin. Insulin binding to erythrocytes did not correlate with insulin nor was there any relationship between insulin binding to erythrocytes and fat from the same subject. Thus while the erythrocyte insulin receptor may be of interest in its own right, it may not be subject to the normal regulatory influences seen in classical insulin target tissues. Insulin binding to erythrocytes may therefore be invalid as a model to study down-regulation of insulin receptors.

Aminoisobutyric acid transport in soleus muscles of lean and gold thioglucose-obese mice

The uptake of alpha-aminoisobutyric acid (AIB) was studied in soleus muscles isolated from lean and gold thioglucose-lesioned obese mice (GTG-obese), both in basal and in insulin-stimulated conditions. The basal (i.e., non-insulin-stimulated) influx of AIB was decreased in muscles of GTG-obese mice because of a decrease in the apparent Vmax of the transport system. In muscles from both types of mice, insulin stimulated AIB influx by increasing the apparent Vmax of transport; this effect was partially dependent on new protein synthesis. In muscles of GTG-obese mice, the effect of insulin was altered in two ways: the absolute value of the maximally insulin-stimulated AIB influx was lower than in muscles of lean controls, and the dose response of insulin-stimulated AIB influx was shifted to the right (half-maximally effective concentration, EC50 congruent to 1.7 nM) compared to that observed in muscles of lean animals (EC50 congruent to 0.7 nM). It is concluded that in muscles of obese mice a) amino acid transport per se is altered, and b) the sensitivity of this process to the stimulatory effect of insulin is markedly diminished.

Carbohydrate intake affects insulin binding to human erythrocytes in normal weight subjects but not in subjects with family obesity

The effects of different carbohydrate intakes on insulin binding to human erythrocytes were studied in thirty nine obese and twelve normal weight subjects belonging to twelve families with a strong penetrance of obesity("family experiment"), and in nine normal weight subjects with no family or personal history of obesity or diabetes ("diet experiment"). In the "family experiment," the mean insulin binding in obese subjects was significantly lower than in control normal weight siblings and parents. This difference cannot be related to an increase in carbohydrate or caloric intakes, since there was no difference in daily food intake between the obese and the control subjects. In the "diet experiment," the volunteers were studied four times: twice with their spontaneous diet, one with a normocaloric carbohydrate rich diet and once with a normocaloric fat rich diet. Both the carbohydrate and fat rich diets resulted in a significant lowering of binding when compared to the period of spontaneous diet. In the two experiments, the decrease in binding is due to a decrease in the number of receptors per erythrocyte. The possibility of a common underlying mechanism is discussed.

The relationship between the hyperplastic pancreatic islet and insulin insensitivity in obesity

In summary, the present review provides evidence in support of the proposition that pancreatic islet cell hyperplasia precedes the development of insulin insensitivity in the obese mouse and, it is likely, that similar events occur in obese humans. Moreover, the hyperplastic pancreatic islet appears to be responsible for the development of insulin insensitivity, since suppression of the hyperplastic islet, by either alloxan or streptozotocin administration to the obese mouse, results in amelioration of insulin insensitivity in vivo. Since no change occurred in the degree of obesity or in adipocyte cell size or number, it is evident that insulin sensitivity is independent of obesity per se. Hence, although obesity and insulin insensitivity frequently co-exist, insulin insensitivity is independent of obesity and is due rather to the presence of pancreatic islet cell hyperplasia. Light and electron microscopy of the hyperplastic pancreatic islets of the obese mouse reveal increased numbers of A- B- and D-cells. Islet suppression with alloxan or streptozotocin results in the selective reduction of B-cells with preservation of A- and D-cells. Therefore, restoration of insulin sensitivity in the obese mouse following pancreatic islet cell suppression appears to be directly related to suppression of B-cell hypersecretion. Biochemical studies of muscle and adipose tissues from the obese mouse reveal profound insulin unresponsiveness without clear cut improvement in vitro following pancreatic islet cell suppression and restoration of insulin sensitivity in vivo. These data are consistent with a relatively modest reduction in the number of available insulin receptors upon these tissues in relation to the marked insulin resistance and imply an impairment of insulin action beyond the insulin receptor interaction [either transport or intracellular action(s)] as the major site(s) of insulin resistance in the muscle and adipose tissues of obese mice. Conversely a reduction of insulin receptors upon hepatocytes of obese mice and their improvement following a reduction of B-cell hypersecretion support the proposition that the number of available insulin receptors may be the major site for the regulation of insulin action upon that tissue. Finally, evidence is presented which suggests that an inability of insulin to limit hepatic gluconeogenesis may be the predominant cause of insulin insensitivity in the obese mouse.

Age-dependent development of insulin resistance of soleus muscle in genetically obese (ob/ob) mice

The relationship between insulin binding and its biological effects was studied in soleus muscle of 3- to 15-wk-old genetically obese (ob/ob) mice. At 3 and 4 wk of age, soleus muscle from lean and obese mice bound similar amounts of insulin under equilibrium binding conditions. However, by 6 wk of age, insulin binding and total receptor concentration (Ro) were significantly decreased in soleus muscle from obese compared to lean mice. In addition lean and obese mice demonstrated an age-dependent decrease in insulin binding, Ro, and receptor affinity. At 4 wk of age, insulin-stimulated 2-deoxyglucose transport and glucose utilization were significantly lower in soleus muscle from obese mice and preceded alterations in insulin binding. The postmembrane decrease in insulin sensitivity was dissimilar for various pathways of glucose metabolism. Glucose conversion of glycogen, but not the glycolytic rate or glucose oxidation, remained sensitive to insulin stimulation. These data indicate that glucose transport and utilization rather than insulin binding may play the primary role in the development of insulin resistance of muscle in the obese diabetic syndrome.

Role of insulin receptors in insulin-resistant states

The interaction of insulin with its receptor represents one of the key intermediate steps between secretion of insulin and its final biologic effects. Alterations in this interaction have been found in a number of disease states, including obesity, non-insulin-dependent diabetes mellitus (NIDDM), glucocorticoid excess, and acromegaly, as well as several rare forms of severe insulin resistance. The major factor regulating the receptor in obesity and NIDDM appears to be insulin. In obesity this alteration in normal regulation occurs secondary to overeating, whereas in the diabetic state the nature of the primary defect is uncertain. The role of the receptor in insulin resistance and methods for its evaluation are discussed.

Insulin binding and glucose uptake of adipocytes in rats adapted to hypergravitational force

Rats were exposed to 4.15 g for 1 yr and weight and age matched, and lean noncentrifuged rats were used as control groups. Rats exposed to chronic hypergravity (hypergravic rats) were found to show lower ambient insulin levels, greater food intake with smaller body weight gain, and decreased size of isolated adipocytes. The ability of adipocytes from the hypergravic rats to bind insulin was increased. With Scatchard analysis, both number and affinity of receptors were increased. In contrast to the increased binding, glucose transport was found to be decreased in adipocytes from these animals. However, when the data were expressed as a percentage of maximal effect, the half maximal insulin effect for both the hypergravic and lean control groups was produced at an insulin concentration of 0.23 +/- 0.02 ng/ml, which was lower than the insulin concentration of 0.31 +/- 0.02 ng/ml for the weight-matched control group (P less than 0.05). This increased insulin sensitivity in the hypergravic group was accounted for by an increased number of receptors.

Alterations in the responsiveness of diabetic fibroblasts to insulin

Fibroblastic cultures from the skin of nondiabetic and diabetic (db/db) mice have been used to investigate alterations in the biological responses of diabetic cells to insulin. Confluent cultures from the skin of both nondiabetic and diabetic animals possess specific receptors for insulin. Diabetic fibroblasts exhibit only 36% as mich specific binding of insulin as nondiabetic fibroblasts, because of a decrease in the total number of binding sites, without a change in binding affinity. Insulin caused a time- and dose-dependent increase in the rate of 2-deoxy D-glucose (dGlc) uptake and in ornithine decarboxylase (ODC) activity of both nondiabetic and diabetic fibroblasts. In nondiabetic cells, half-maximal increase in dGlc uptake was obtained with 0.3 nM insulin, and a maximum increase of 120% was obtained with 4.1 nM insulin. In contrast, diabetic cultures required 0.8 nM insulin for a half-maximal increase in dGlc uptake, and maximum stimulation with 4.1 nM insulin was only 50% above control levels. With 4-fold higher insulin concentrations, ODC activity of diabetic cells was only 40% that of non-diabetic cells. In nondiabetic cells, doing regulation of insulin receptors by insulin abolished the ability of insulin to stimulate dGlc uptake. These results demonstrate that cells cultured from diabetic animals, which possess a decreased number of insulin receptors, also exhibit decreased stimulated of deoxy D-glucose uptake and ornithine decarboxylase activity by insulin.

Distribution of insulin receptors among mouse liver endomembranes

Specific binding of insulin to highly purified preparations of rough endoplasmic reticulum, Golgi apparatus, and plasma membrane of mouse liver was determined. 125I-labeled insulin bound maximally to the plasma membrane in radio-receptor assays. Golgi apparatus fractions exhibited binding 10--20% that of plasma membrane and rough endoplasmic reticulum exhibited only 1--2% of plasma membrane binding. Binding was proportional to membrane concentration and dose vs. response curves were very similar for the different fractions. Scatchard analysis of the insulin binding data for the plasma membrane and Golgi apparatus fractions showed curvilinear plots yielding similar apparent binding affinities (0.9 and 3.0-10(8) M-1, respectively). Purity of the isolated endomembranes was analyzed by morphometry and (Na+ + K+ + Mg2+)-ATPase and these preparations displayed less than 1% contamination by plasma membrane. These findings provide important confirmation of the presence of insulin receptors in Golgi apparatus membranes comparable to those located on the plasma membrane. Finally, the present study did not allow us to verify the existence of insulin receptors in the endoplasmic reticulum.

Evaluation of insulin receptor in myotonic dystrophy

Only 2 of 6 patients with myotonic dystrophy had glucose intolerance and hyperinsulinemia. All, however, had markedly reduced insulin binding to specific receptors on circulating monocytes. A significant difference in receptor affinity for insulin was observed, but the difference in affinity was not as marked as that in the amount of 125I-insulin bound per cell, which was sixfold greater in control cells. No evidence for a circulating factor competing with insulin for binding sites could be demonstrated. These alterations in binding did not directly correlate with glucose intolerance or hyperinsulinemia and are consistent with a postulated generalized membrane defect. Study of the insulin receptor in this disorder may uncover regulatory mechanisms in normal and pathological conditions, including myotonic dystrophy.

Improved insulin receptor assay: effects of an antidiabetic sulphonylurea on liver membrane insulin receptors from obese hyperglycaemic mice

1 A simple assay of liver membrane insulin receptors is described, under conditions which apparently stabilized the insulin-receptor complex. The receptor population was measured in the presence of physiological doses of radioactive hormone under conditions which minimized negative co-operativity effects. 2 Equilibrium between receptor and hormone was attained in vitro within 60 min of incubation time at 13 degrees C with no apparent loss of binding sites after 3 h of incubation. 3 Liver membranes from lean mice possessed 8.8 times the receptor number per mg membrane protein measured in membranes from obese hyperglycaemic (ob/ob) littermates. After treatment of ob/ob mice with ARDF-26 (Gliquidon; Boehringer Ingelheim Ltd) the concentration of liver membrane insulin receptors was increased 5 fold.

Hormonal regulation of membrane receptors and cell responsiveness: a review

Hormone receptors are those components of target-cells that specifically bind hormones and convey the hormonal message to the intracellular machinery. Such receptors can be localized inside the cell, such as the nuclear receptors of thyroid hormones and the nuclear and cytoplasmic receptors of steroid hormones, or on the outer surface of the plasma membrane, such as the membrane-bound receptors of polypeptide hormones and neurotransmitters. Extensive studies during recent years have shown that the interaction between hormone and membrane-bound receptor can affect the receptor characteristics in at least two ways. Firstly, receptor occupancy can modify, by way of cooperativity, the affinity of homologue receptors for the given hormone. Secondly, the binding capacity of a target cell appears to vary as a function of the preexposure of the cell to the hormone. The latter phenomenon has been related to the so-called states of subsensitivity, desensitization, or refractoriness, and might be responsible for the physiologic regulation of the target cell sensitivity and for the hormone resistance which accompanies various metabolic disorders. In this review we attempt to describe the major findings related to hormone desensitization or resistance of these hormones that have plasma-membrane-bound receptors. Data from the literature are presented independently for each hormone and when applicable, conflicting results are discussed in each section. The various theories which might explain hormone desensitization are outlined in the last section of this paper.

Amino acid transport in diaphragms from newborn rats: evidence for insulin resistance

Diaphragms from rats under 24-h-old did not show the well-known increased transport of alpha-aminoisobutyrate found in older tissues in respone to insulin in vitro. A small effect was apparent by 3 days, and stimulation increased as donor rats aged (up to 4--5 wk). One-day diaphragms also had greater uptake than older tissues, due to both decreased Km and elevated Vmax. The change in insulin sensitivity did not result from alteration in the transport system used by alpha-aminoisobutyrate because uptake showed characteristics of the A system at both 1 day and older. Results suggest instead that the 1-day tissues had been made insulin-resistant by high insulin levels in donor animals. Plasma insulin levels of 1-day-old rats were 5 times those of 5-day animals. Elevating the plasma insulin levels of 5-day or 25- to 35-day rats led to a decreased effectiveness of insulin in vitro in stimulating alpha-aminoisobutyrate transport into their diaphragms. In the older animals, the stimulation was inversely proportional to the plasma insulin level 2 h after insulin injection.

Laboratory animals exhibiting obesity and diabetes syndromes

Spontaneous hyperglycemia, hyperinsulinemia and obesity are common features for at least one period of the lifetime in some strains of mice. Both genetic and environmental factors are involved in the pathogenesis of the diabetes-like syndrome, making these strains excellent models for studies in both obesity and diabetes-like states. The metabolic peculiarities can be due to a dominant gene, as for the yellow obese, or a single recessive gene, as in the obese and the diabetes mouse; or they can be of polygenic origin, as for the KK and the NZO mouse. However, the severity of the metabolic disorder is due to the interaction of the mutant genes iwth modifiers in the bat genes themselves. Studies on the pathophysiology and biochemistry of these animals have revealed interstrain differences, different patterns of development of the metabolic disorder, and different degrees of severity of the diabetes-like syndrome. Although the primary causes of the syndrome remain unclear in some strains, an involvement of hypothalamic feeding centers has been implicated.

Insulin receptor sites as membrane markers during embryonic development. I. Data obtained with unfertilized and fertilized sea urchin eggs

Binding of insulin to sea urchin egg plasma membrane has been studied by biochemical and immunocytochemical methods. Unfertilized and fertilized eggs as well as embryos during the first cell division have been used. 1. Competition experiments between 125I-insulin (1 nM) and an excess of native insulin (30 muM) indicate a specific hormone fixation to membrane crude extracts from unfertilized and fertilized eggs. The magnitude of "specific binding" is comparable to values recorded for mammalian cells. 2. Inhibition of insulin fixation by concanavalin A (100 mug/ml) suggests the glycoprotein composition of plasma membrane receptors. 3. An 30-min incubation of unfertilized and fertilized eggs in the presence of insulin leads to a significant increase in cyclic AMP content. 4. An immunocytochemical method demonstrates that insulin is selectively and specifically bound to the plasma membrane of eggs incubated in the presence of insulin before fixation. It can be concluded that insulin receptor sites are components of sea urchin eggs plasma membrane. Insulin binding which leads to cyclic AMP accumulation is not deeply modified by fertilization and does not include visible morphological changes in the eggs.

The syndromes of insulin resistance and acanthosis nigricans. Insulin-receptor disorders in man

In six patients with acanthosis nigricans variable degrees of glucose intolerance, hyperinsulinemia and marked resistance to exogenous insulin were found. Studies of insulin receptors on circulating monocytes suggest that the insulin resistance in these patients was due to a marked decrease in insulin binding to its membrane receptors. When these patients were fasted, there was a fall in plasma insulin but no increase in insulin binding, suggesting that the receptor defect was not secondary to the hyperinsulinemia. The clinical features shared by these cases and several similar ones previously reported may be divided into two unique clinical syndromes: Type A, a syndrome in younger females with signs of virilization or accelerated growth, in whom the receptor defect may be primary, and Type B, a syndrome in older females with signs of an immunologic disease, in whom circulating antibodies to the insulin receptor are found.