The insulin receptor is an age-dependent integral component of the human erythrocyte membrane

Insulin and wound healing

Skin is a dynamic and complex organ that relies on the interaction of different cell types, biomacromolecules and signaling molecules. Injury triggers a cascade of events designed to quickly restore skin integrity. Depending on the size and severity of the wound, extensive physiological and metabolic changes can occur, resulting in impaired wound healing and increased morbidity resulting in higher rates of death. While wound dressings provide a temporary barrier, they are inherently incapable of significantly restoring metabolic upsets, post-burn insulin resistance, and impaired wound healing in patients with extensive burns. Exogenous insulin application has therefore been investigated as a potential therapeutic intervention for nearly a century to improve wound recovery. This review will highlight the important achievements that demonstrate insulin's ability to stimulate cellular migration and burn wound recovery, as well as providing a perspective on future therapeutic applications and research directions.

Acetylcholinesterase activity in chronic renal failure

Twenty healthy subjects and 39 Chronic Renal Failure patients (CRF-patients) maintained on chronic hemodialysis were used in this investigation to study the changes in acetylcholinesterase (AChE) activity of red blood cells (RBCs). The CRF-patients were all undergoing hemodialysis treatment. AChE activity from the CRF-patients was determined before and after dialysis. An additional objective was to study the effect of chronic renal failure on human red blood cell aging. Blood samples were drawn from controls and CRF-patients in tubes containing EDTA or sodium heparin as an anticoagulant. Red blood cells were purified to avoid interference with monocytes, reticulocytes and leukocytes. The purified RBCs were subfractionated into young (y) (1.08-1.09), mid (m) (1.09-1.11) and old (o) (1.11-1.12) percoll density (g/mL) fractions using a discontinous percoll gradient. The mean +/- SD AChE per gram hemoglobin (U/g Hgb) activities in whole blood (WB), purified human red blood cells (PRBCs), young human red blood cells (y-RBCs), mid age human red blood cells (m-RBCs) and old human red blood cells (o-RBCs) in CRF-patients were 31.2+/-3.43, 29.3+/-3.26, 30.4+/-3.91, 25.1+/-5.25, 17.1+/-6.02 in females and 29.8+/-5.39, 28.8+/-5.29, 28.7+/-5.29, 23.7+/-5.39 and 16.0+/-5.60 in males. AChE activity from CRF-patients were higher than that found in the control subjects. The aging of human RBCs in both the controls and CRF-patients showed a progressive reduction in AChE activity. AChE activity of RBCs from female CRF-patients were significantly higher (p < 0.05) than that of the female control subjects. The RBCs isolated from male CRF-patients showed a higher AChE activity than control males, but a significant difference was only observed with the mid-age-cells. These studies further indicate that AChE activity remained insignificantly different in the various density based age subfractions of RBCs of both CRF-patients and controls.

The IGF-I/IGFBP system in congenital partial lipodystrophy

BACKGROUND AND OBJECTIVES

Insulin and IGF-I interact at many levels. Little is known about the insulin-like growth factor-I/insulin-like growth factor binding proteins (IGF-I/IGFBP) system in congenital partial lipodystrophy, a syndrome characterized by insulin resistance, hyperinsulinaemia and absence of truncal and limb fat. Some cases have acromegaloid features with thick skin and large hands and feet in association with normal levels of circulating growth hormone.

METHODS

In four females known with congenital partial lipodystrophy, hyperinsulinaemia with acromegaloid features, the number and affinity of the IGF-I receptors on peripheral blood mononuclear cells (PBMCs), and the concentration of circulating insulin, total and free IGF-I, IGFBP-1 and IGFBP-3 levels were measured in the fasting and the fed state. Cultures of PBMCs of the patients with lipodystrophy were also used to study the effect of IGF-I stimulation on thymidine uptake in vitro.

MEASUREMENTS

In the subjects with lipodystrophy the affinity and the number of the IGF-I receptors on peripheral mononuclear cells (PBMCs) and erythrocytes did not differ significantly from controls in the fasting state. Insulin levels were significantly higher in subjects with lipodystrophy both in the fasting as well in the fed state. Total IGF-I, free IGF-I and IGFBP-3 levels did not differ but serum IGFBP-1 levels were lower in lipodystrophy subjects than in healthy controls. The free IGF-I/IGFBP-1 ratio was increased in lipodystrophy subjects both in the fasting and the fed states. The effects of IGF-I stimulation on thymidine uptake by PBMCs of lipodystrophy subjects in the absence of IGFBP-1 were not different from healthy controls cultures in vitro. When a combination of IGFBP-1 (in a concentration comparable to the fasting serum IGFBP-1 levels in lipodystrophy patients found in our study) and IGF-I was added to PBMC cultures from lipodystrophy patients no decrease in thymidine uptake by PBMCs was found.

CONCLUSIONS

In the four subjects with lipodystrophy hyperinsulinaemia, lowered free IGF-I and IGFBP-1 levels, but increased free IGF-I/IGBP-1 ratios were observed. Low IGFBP-1 concentrations in culture media did not reduce the stimulating IGF-I effect on thymidine uptake by PBMCs from lipodystrophy patients. Our data suggest that the observed increased IGF-I/IGFBP-1 ratio in lipodystrophy patients contributes to an unopposed biological effect of IGF-I on IGF-I receptors, thereby inducing the development of acromegaloid features, acanthosis nigricans and polycystic ovaries in some patients with congenital partial lipodystrophy.

Insulin-like growth factor I receptors on blood cells: their relationship to circulating total and "free" IGF-I, IGFBP-1, IGFBP-3 and insulin levels in healthy subjects

The relationships between the insulin-like growth factor I/insulin-like growth factor binding protein (IGF-I/IGFBP) system and the IGF-I receptor characteristics on erythrocytes and PBMCs in healthy subjects in the fasting state were studied to establish whether this would be a valid way of examining IGF-I receptors in vivo. The K(d) of the IGF-I receptor on erythrocytes was positively related to circulating "free" IGF-I levels. For the IGF-I receptor on PBMCs no relationship was observed with "free" IGF-I levels. IGFBP-3 levels were inversely related to the number of IGF-I binding sites on erythrocytes and to the K(d) of the IGF-I receptor on PBMCs. Total IGF-I, insulin and IGFBP-1 levels showed no relation to the IGF-I receptor on erythrocytes and PBMCs in the fasting state. This report suggests that studies of IGF-I receptor characteristics on erythrocytes and PBMCs in the fasting state are cell-specific and cannot be extrapolated to other cell types, which may be more relevant target tissues for IGF-I action in vivo.

Insulin receptor binding to erythrocytes in the first half of pregnancy is increased in healthy pregnant women as compared with non-pregnant or gestational diabetic women

Insulin binding to erythrocytes was measured longitudinally by a competitive radioreceptor assay in 21 healthy pregnant (HP) and 20 well-controlled gestational diabetic women (GD) in 4-week intervals throughout pregnancy and at day 4 post-partum. Maximum insulin binding (maxbdg) at weeks 8-14 was increased (P < 0.001) in HP (median: 6.0%) but not in GD (median: 2.7%) as compared with non-pregnant control subjects (C) (median: 3.6%; previously reported: Clin. Chim. Acta 1992;207:57-71) due to an increased number of high-affinity insulin receptors. Throughout gestation the binding decreased continuously, to reach at term the levels found in C. In GD maxbdg remained close to the level of C throughout pregnancy. Binding differences between HP and GD were independent of the body mass index. Maxbdg did not differ between diet- and insulin-treated patients. It was higher in women whose offspring had low umbilical cord insulin levels (< 10 mu units/ml). The findings suggest that (a) higher insulin binding in HP could contribute to the improved glucose tolerance in early pregnancy and (b) the lack of increase in insulin binding during early pregnancy in gestational diabetes might be one factor leading to the manifestation of the disease in late pregnancy. However, it must be kept in mind that insulin receptors on erythrocytes do not necessarily resemble those on the major target tissues of insulin.

Red blood cell insulin receptors in health and disease

CONTENTS

Structure and characteristics of erythrocyte insulin receptor. Red blood cell age and insulin receptors. Insulin receptors in human disease states. Obesity. Chronic renal failure. Acanthosis nigricans. Miscellaneous disease states. Insulin receptors in children. Insulin receptors in women during pregnancy. Insulin binding and other hormones. Comparison of biosynthetic insulin, pancreatic human insulin and porcine insulin binding to erythrocytes. Effect of exercise on insulin binding to red blood cells of normal human volunteers. Miscellaneous insulin binding studies. Insulin internalization and degradation. Insulin and erythrocyte metabolism. Summary and conclusion.

Interaction of receptors for prostaglandin E1/prostacyclin and insulin in human erythrocytes and platelets

Prostaglandin E1/I2 and insulin receptors of human erythrocyte and platelet are capable of modulating each other's activity. This modulation of the receptor activity and number in one system by a second receptor system in human platelet and erythrocyte seems to be beneficial. Insulin increases the PGE1 binding to platelets and thereby enhances the platelet antiaggregatory action of prostaglandin by increasing cyclic AMP levels. Similarly, PGE1 increases insulin binding to human erythrocyte, and thereby reduces the optimum concentration of insulin for a maximal reduction in membrane microviscosity. During ischemia the reduced response of platelets to the inhibitory effect of PGE1 or PGI2 relates to the impaired PGE1/I2 receptor activity. Treatment of these platelets with insulin at physiological concentrations can normalise the PGE1/I2 receptor activity. This review focuses on the relationship between the two receptor systems in human blood cells.

Erythrocyte rheology

Two main subjects of erythrocyte rheology, deformation and aggregation, are discussed in detail, on the basis of biochemical structure. The close relationship between the life span (or cell aging) and the rheology of individual erythrocytes is also briefly described. A currently important problem is emphasized, that is, the molecular aspect of the dynamic cytoskeletal structure and the mechanism of its regulation. This concerns not only the rheological function and the survival of circulating erythrocytes, but also the pathophysiology of abnormal erythrocytes.

Insulin resistance in acromegaly: evaluation by studies of insulin binding to erythrocytes

Insulin binding to erythrocytes (RBC) was evaluated in 10 acromegalic patients (6 females and 4 males) in comparison to 22 normal subjects (12 females and 10 males) in an attempt to study the insulin resistance of acromegaly. Basal glucose from all acromegalic patients were within the normal range but incremental glucose and insulin curves, respectively, on oGTT were significantly increased in the acromegalic patients suggesting an insulin resistant state. Basal growth hormone concentrations were elevated in all acromegalic patients, but no correlation was observed between insulin and GH levels. The insulin binding studies in the acromegalic patients showed a decreased binding due to a reduction in the receptor number per cell but with no alterations in the affinity state. Correction of data for creatine, as a procedure of normalization of binding data for a standardized RBC cell age, enhanced the reduction of insulin binding of the acromegalic patients in comparison to controls in consequence to the younger population of acromegalic RBC as indicated by their increased creatine concentrations. In conclusion, the insulin resistant state in acromegalics observed in our study is accompanied by a decrease in the insulin binding to the RBC receptor due to a reduction of the receptor concentration as mediated by the compensatory hyperinsulinemia.

Erythrocyte insulin receptor: normalization of binding data for the average cell age by the red cell creatine determination in obese, diabetic and acromegalic patients

Insulin binding studies performed in erythrocytes (RBC) have been employed in clinical studies assessing the status of insulin receptors at target cell tissues. However, some authors challenged this assumption on the basis of some discrepancies described in comparative studies of other cell types, probably related to populations of different cell age affecting insulin binding to RBC. We evaluated insulin binding to RBC in normal males (n = 10), non-obese diabetic males (n = 13), normal females (n = 15), obese (n = 11) and acromegalic females (n = 5), before and after correction of insulin binding data for creatine concentration in the RBC as a procedure of correction for age, since a negative correlation was described between creatine content and RBC age which also correlates inversely with % insulin binding. Insulin binding in all three groups of patients was not statistically different from corresponding values for normal males and females respectively before correction of data for creatine, but significantly reduced values were found after adjustment for creatine in accordance with published data concerning monocytes. In conclusions, the procedure of correcting insulin binding in erythrocytes by the creatine content in RBC is potentially useful for clinical investigations, since the influence of RBC age is excluded.

Insulin sensitivity and beta-cell responsivity are not decreased in elderly subjects with normal OGTT

Glucose intolerance has been observed often in elderly subjects, but it is not yet clear whether this impaired metabolic state is due to the aging process itself or is secondary to the appearance of other age-related variables. This study attempts to elucidate the effect of age in itself on factors controlling glucose tolerance. Several metabolic parameters were measured in 10 young male controls (23-29 yr) and 17 nonhospitalized, healthy, nonobese, old (60-80 yr) male subjects. Insulin binding to circulating cells was performed along with the intravenous glucose tolerance test, and the data were analyzed by the minimal model method. This approach yields the following measures: tissue insulin sensitivity (SI), fractional glucose disappearance at basal insulin (glucose effectiveness, SG), and first (phi 1) and second (phi 2) phase beta-cell responsiveness to glucose. Insulin-binding capacity to monocytes and erythrocytes was respectively 6.03% +/- 0.57% and 5.96% +/- 0.53% (elderly), 5.97% +/- 0.39% and 5.36% +/- 0.57% (young); SI was 6.20 +/- 0.59 X 10(4) min-1/(microU/mL) (elderly) and 6.35 +/- 0.30 (young); SG was 0.016 +/- 0.002 min-1 (elderly) and 0.019 +/- 0.003 (young); phi 1 was 1.84 +/- 0.29 min-1 (microU/mL)/(mg/dL) (elderly) and 3.37 +/- 0.84 (young); phi 2 was 13.80 +/- 1.78 X 10(4) min-2 (microU/mL)/(mg/dL) (elderly) and 9.59 +/- 2.65 (young). These results show no change with aging of tissue insulin sensitivity and an intact beta-cell activity, suggesting that age per se does not contribute to the deterioration of glucose tolerance when the effect of other age-related variables, eg, obesity and physical inactivity, is precluded.

Characterization of differences in insulin receptors from young and old red blood cells

It has been demonstrated that young RBCs (reticulocytes and early mature erythrocytes) possess more insulin receptors than old RBCs (late mature erythrocytes) but it is not yet known whether insulin receptors on young and old RBCs are regulated similarly. In the present investigation insulin receptors on young and old RBCs have, therefore, been studied in five normal male subjects before and after 2 days dexamethasone ingestion (0.5 mg tablet every 6 h) and, in the same subjects, before and 5 h after ingestion of 75 g glucose. The results obtained clearly demonstrate that dexamethasone increases insulin receptor concentration while glucose ingestion increases both insulin receptor affinity and concentration on young RBCs. By contrast, neither stimuli modify insulin receptors on old RBCs. Studies on RBCs are usually performed on the whole RBC population not taking into account this differential responsiveness of receptors on young versus old RBCs; consequently, this phenomenon might be responsible of the fact that some data reported on RBCs are not in agreement with those reported on monocytes or adipocytes and it should be taken into consideration when using RBCs to evaluate insulin receptor regulation.

Aged erythrocytes exhibit decreased anion exchange

The rate of transport of [32P] phosphate into human and bovine erythrocytes and of a spin-label analogue of phosphate (Tempo-phosphate) into human erythrocytes was found to decrease with increasing erythrocyte age by 15-20% when comparing 20% most dense cells with 20% of lightest cells. The activation energy of Tempo-phosphate transport did not show significant changes upon erythrocyte aging.

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.

The effect of continuous subcutaneous insulin infusion on insulin binding to erythrocytes in diabetes mellitus

The effect of continuous subcutaneous insulin infusion (CSII) on glycaemic control and insulin binding to erythrocytes was studied in six diabetic patients. A marked improvement in blood glucose control during CSII was observed in these patients previously on conventional therapy. Specific 125I-insulin binding to erythrocytes of the diabetics before the institution of CSII was significantly lower than that of age-, weight- and sex-matched nondiabetic subjects, 6.5 +/- 0.1% vs. 9.8 +/- 0.3% (P less than 0.05). After 4-6 months of CSII, insulin binding was restored to normal levels. This normalization in insulin binding to erythrocytes of patients on CSII was due to a 35% increase in active binding sites, together with a small change in binding affinity.

Comparative effects of several simple carbohydrates on erythrocyte insulin receptors in obese subjects

The effects of simple carbohydrates on erythrocyte insulin receptors, plasma insulin and plasma glucose were studied during four hypocaloric, hyperproteic, diets. One diet contained no carbohydrate; the other three contained 36 g of either glucose, galactose or fructose. These diets were given for a 14-day period to groups of moderately obese subjects. The hypocaloric carbohydrate-free diet produced a decrease in plasma insulin and glucose concentrations concomitant with an increase in the number of insulin receptors. A similar increase in insulin receptor number was found when the diet was supplemented with glucose or galactose, but not with fructose. The presence of fructose in the diet prevented any increase in insulin receptor number.

Regulation of insulin receptor activity of human erythrocyte membrane by prostaglandin E1

Incubation of human erythrocyte membrane with low concentration of prostaglandin E1 or prostacyclin increased the binding of 125I-labeled insulin to the membrane. The binding of the radioiodinated hormone was maximally stimulated at 3 nM prostaglandin E1 and the use of higher concentrations (above 8 nM) of the autacoid tended to reverse its own effect at lower concentrations. While prostaglandins A1, A2, B1, B2, D2, F1 alpha, F2 alpha or 6-keto-prostaglandin F1 alpha had no effect on the binding of insulin to the erythrocyte membrane, prostaglandin E2 at similar concentrations decreased the binding of the hormone. The effect of prostaglandin E1 on the increased binding of the insulin was found to be reversible and depended on the occupancy of the autacoid molecules on the membrane and showed positive cooperativity. Scatchard analysis of the binding of 125I-labeled insulin to the erythrocyte ghosts indicated that in the presence of the autacoid, the binding capacity of the insulin receptor increased 2-fold (from 207 to 424 fmol/mg protein) without any change in the ghosts affinity for the ligand (Kd 2.4 X 10(-9) versus 2.49 X 10(-9) M). As a consequence of increased binding of insulin to the erythrocyte membrane in the presence of prostaglandin E1 (3.0 nM), the optimal concentration of the peptide hormone for the maximal reduction of the membrane microviscosity decreased from approx. 1.6 to approx. 0.4 nM. Addition of prostaglandin E1 alone at the above concentration to the assay mixture had no effect on the membrane microviscosity.

Differentiation of U-937 human monocyte-like cell line by 1 alpha,25-dihydroxyvitamin D3 or by retinoic acid. Opposite effects on insulin receptors

The monocyte-like human cell line U-937 has been differentiated in vitro by incubation with either 1 alpha,25-dihydroxyvitamin D3 or retinoic acid (RA) plus dibutyryl cyclic AMP (db-cAMP). Both methods were effective in inducing the appearance of maturation markers. Their actions on insulin receptors were the opposite, however; 1 alpha,25-dihydroxyvitamin D3 increased the binding of the hormone, while RA plus db-cAMP decreased the binding. These effects were specific for insulin, since the transferrin receptors were reduced by both methods of differentiation. Thus, the changes in insulin receptors during maturation in vitro depend on the inducing agent and are not causally related to the differentiation process.

Kinetics of cell age-dependent decline of insulin receptors in human red cells

Insulin receptors are present in human erythrocytes and correlate negatively with cellular age. Little is known about the function of these receptors, about the precise kinetics of their decline during cell aging or about their fate after disappearance from the cells. To elucidate some of these questions, we have prepared red blood cell populations of widely varying cellular ages (ranging from the erythroblast stage to senescent mature erythrocytes) by isopycnic centrifugation on isosmolar density gradients. In addition, young red cells were cultured for 4 days in vitro to permit observation of short-term changes. In mature erythrocytes, insulin receptors decreased as an exponential function of cell age with an estimated half time of 40 days. A more rapid decline of insulin receptors occurred coincident with reticulocyte maturation. Loss of receptors from cultured cells was accompanied by appearance of a soluble insulin receptor in the medium. The effect of insulin on glucose utilization in erythroblast and reticulocyte preparations was negligible, as assessed by CO2 and lactate production. We conclude that 1) insulin receptors are progressively lost from the red blood cell after the erythroblast stage; 2) receptor loss is particularly rapid during reticulocyte maturation; 3) shedding of receptors into the extracellular environment is one reason for their depletion from cells; and 4) in basophilic erythroblasts and reticulocytes, insulin exhibits little metabolic action despite the relatively high receptor complement present in these cells.

Distribution of insulin receptors in human erythrocyte membranes. Insulin binding to sealed right-side-out and inside-out human erythrocyte vesicles

Analyses of insulin binding to human erythrocytes and to resealed right-side-out and inside-out erythrocyte membrane vesicles have revealed that high affinity insulin binding receptors are present on both sides of the erythrocyte membranes. Insulin binding to human erythrocytes was examined with the use of a binding assay designed to minimize the potential errors arising from the low binding capacity of this cell type and from non-specific binding in the assay. Scatchard analysis of equilibrium binding to the cells revealed a class of high affinity sites with a dissociation constant (Kd) of (1.5 +/- 0.5) X 10(-8) M and a maximum binding capacity of 50 +/- 5 sites per cell. Interestingly, both resealed right-side-out and inside-out membrane vesicles exhibited nearly identical specific sites for insulin binding. At the high affinity binding sites, for both right-side-out and inside-out vesicles, the dissociation constant (Kd) was (1.5 +/- 0.5) X 10(-8) M, and the maximum binding capacity was 17 +/- 3 sites per cell equivalent. These findings suggest that insulin receptors are present on both sides of the plasma membrane and are consistent with the participation of the erythrocyte insulin receptors in an endocytic/recycling pathway which mediates receptor-ligand internalization/externalization.

Nonuniform loss of membrane glycoconjugates during in vivo aging of human erythrocytes: studies of normal and diabetic red cell saccharides

Changes occurring in membrane saccharides during the in vivo aging of normal human erythrocytes have been evaluated after the fractionation of the red cells into five age groups by density gradient centrifugation. The glycoconjugate fractions studied included sialoglycoproteins, macroglycolipids, low-molecular-weight glycolipids, and Band 3 glycoproteins. All of the carbohydrate constituents of the membrane were found to decrease relative to the total ghost protein as a function of cell age, with the most substantial losses occurring in the macroglycolipids (50%) and Band 3 glycoprotein (30%); the smallest changes were observed in the sialoglycoproteins (13%). No preferential loss of sialic acid or other peripheral sugars was found, making unlikely the importance of glycosidase action in the removal of sugars from the membrane. It is suggested that the changes observed in the composition of the ghosts during aging are best explained by a loss of membrane segments enriched in glycoproteins and glycolipids and deficient in internally located molecules such as spectrin. Analyses were also performed on the glycoconjugate fractions from diabetic erythrocytes separated according to cell age. These erythrocytes, which had glycosylated hemoglobin values twice those of normals, had somewhat smaller amounts of membrane-bound carbohydrate. The difference between diabetic and normal erythrocytes was greatest when young cells were examined (diabetic to normal = 0.93), suggesting that the known increased turnover of red cells in diabetes leads to an early loss of membrane constituents.

Developmental changes in insulin receptors of pig red blood cells

Scatchard analysis of the insulin binding to pig reticulocytes, fetal red cells, and adult erythrocytes showed the maximum number of high-affinity binding sites per cell to be 274, 147, and 29, respectively. All three cell types displayed a practically identical dissociation constant of approximately 1.22 X 10(-8) M at the high-affinity region. A long-term in vitro incubation of the fetal red cells and reticulocytes under tissue culture conditions was accompanied by a significant loss of insulin-binding capacity without any appreciable alteration of the dissociation constant. The isolation and characterization of insulin-receptor complexes from these cell types were carried out to establish whether the difference in insulin-binding capacity was due to the difference in the amount of the same species or due to different species of insulin receptors. Membrane proteins were extracted with Triton X-102 and fractionated by DEAE-Sephacel ion-exchange column chromatography. Each peak sample was complexed with 125I-insulin, and the complexes were covalently crosslinked and then applied to a Sepharose CL-6B column. A 95,000-Da complex was obtained from adult pig erythrocyte membranes; 220,000- and and 95,000-Da complex was obtained from adult pig erythrocyte membranes; 220,000- and 95,000-Da complexes from pig reticulocyte membranes; and greater than 600,000-, 220,000-, and 95,000-Da complexes from pig fetal cell membranes. Upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis under a nonreducing condition, the 95,000-Da complex was dissociated into a 53,000-Da component; the greater than 600,000-Da complex into greater than 320,000-, 130,000-, and 53,000-Da components; and the 220,000-Da complex was dissociated into 220,000-, 130,000-, and 53,000-Da components. These findings strongly suggest that the decrease in insulin binding during the developmental changes of red blood cells is due to a disappearance of high-molecular-weight insulin receptors rather than a decrease in the amount of the smaller receptor molecules.

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.

Changes in the mechanisms of hormone and neurotransmitter action during aging: current status of the role of receptor and post-receptor alterations. A review

Alterations in responsiveness to hormones and neurotransmitters during aging appear to be due to changes at both the receptor and post-receptor levels. Although many such observations have now been independently confirmed, disagreement over the extent and/or importance of receptor alterations exists in a number of cases. Receptors do not appear to change with age in certain systems, but only a few reports have actually been able to localize particular post-receptor alterations responsible for changes in response. This review attempts to catalogue studies in these areas which have been carried out to date, and discusses possible reasons for discrepancies as well as future research directions.

The effect of insulin on glucose transport in rabbit erythrocytes and reticulocytes

Insulin binding and 3-0-Methylglucose transport have been studied in erythrocyte- and reticulocyte-enriched fractions of blood cells in order to determine if the increased number of insulin binding sites in reticulocytes is associated with a glucose transport response to insulin. In these experiments rabbit reticulocytes demonstrate an eightfold increase in total insulin receptors when compared to erythrocytes. Glucose transport activity in the erythrocyte has Km of 3.2 mM. Reticulocytes demonstrate a saturable glucose transport activity of lower affinity, Km 18.9 mM. Neither the erythrocyte, nor the the reticulocyte glucose transport activity, was capable of an increased response to insulin. the low affinity glucose transport activity in reticulocytes could allow a fourfold increase in facilitated glucose transport at supraphysiological glucose concentrations that might occur in poorly controlled diabetes mellitus.