Peptide hormone binding to receptors: a review of direct studies in vitro

Insulin action

Our understanding of the molecular basis of insulin actin remains incomplete, but important new insights have been achieved recently. All available evidence to date indicates that intracellular signalling by the hormone results from its initial interaction with specific cell surface receptors. Insulin receptors from all tissues studied to date appear to be minimally composed of two Mr 125,000 subunits denoted as alpha and two Mr 90,000 subunits denoted as beta. The beta subunit is extremely sensitive to proteolytic cleavage near the center of its amino acid chain. The four subunits are linked together by disulfide bonds to give a symmetrical configuration with a stoichiometry of (alpha-s-s-beta)-s-s-(alpha-s-s-beta). This structure is remarkably similar to the general subunit composition of immunoglobulin G molecules and provides a structural basis for the postulate that this minimum insulin receptor structure may be divalent for binding hormone. A second area of recent progress involves the successful generation of a soluble factor or factors by insulin that are capable of modulating the activity of insulin-sensitive enzymes such as pyruvate dehydrogenase, glycogen synthase and cyclic adenosine monophosphate (cyclic AMP)-dependent protein kinase in cell-free systems. Indirect evidence indicates that the putative mediator or mediators of insulin action exhibits properties expected of a low molecular weight peptide, including destruction by proteases. The data available are consistent with the hypothesis that insulin-receptor interaction leads to the activation of a membrane protease that catalyzes the release of a peptide mediator or mediators of insulin action.

Hypophosphatemia and glucose intolerance: evidence for tissue insensitivity to insulin

Although hypophosphatemia is commonly present in diabetics, little is known about its isolated effects on glucose and insulin metabolism. We therefore investigated glucose metabolism in six nondiabetic subjects with chronic hypophosphatemia. When glucose was infused to maintain a constant hyperglycemic level (125 mg per deciliter [6.9 mmol per liter] above basal levels), the glucose infusion rate was 36 per cent less in the hypophosphatemic group than in controls (4.90 +/- 0.34 mg per kilogram of body weight per minute vs. 7.64 +/- 0.37, P < 0.001), although responses to endogenous insulin were similar. When exogenous insulin was infused at a constant rate to maintain an insulin level about 100 microU per milliliter (718 pmol per liter) above basal levels and glucose was infused as necessary to maintain fasting glucose levels, the infusion rate of glucose was 43 per cent lower in the hypophosphatemic group than in controls (3.80 +/- 0.58 mg per kilogram per minute vs. 6.70 +/- 0.33, P < 0.001), although the clearance rate of insulin was similar in both groups. These results indicate that hypophosphatemia is associated with impaired glucose metabolism in both the hyperglycemic and euglycemic states, and that this associated primarily reflects decreased tissue sensitivity to insulin. (N Engl J Med. 1980; 303; 1259-63.).

Rapid regulation of plasma membrane insulin receptors

Injection IP of insulin at a dosage of 1 microgram/g body weight into normal rats produced a rapid rise in serum insulin levels from < 1 to 298 ng/ml, and a rapid decrease in specific 125I-insulin binding to its receptors in purified liver plasma membranes. A fall in binding was seen as early as 10 minutes after injection and binding remained decreased for up to 60 min. At 10 min, 125I-insulin binding had fallen to 59% of controls; in contrast, 125I-glucagon binding remained unchanged. Extraction of these plasma membrane followed by radioimmunoassay for insulin did not reveal appreciable amounts of exogenous insulin. The 125I-insulin dissociation rate from plasma membranes of control and insulin treated rats was the same, also indicating a lack of exogenous insulin. Scatchard analyses indicated that the decreased binding seen after insulin injection was due primarily to a change in the number of insulin receptors and not their affinity. These studies suggest, therefore, that high doses of insulin in vivo can rapidly regulate the number of plasma membrane insulin receptors in liver.

Insulin antibodies prevent insulin-receptor interactions

The study was carried out to investigate whether insulin bound to antibody is able to bind the insulin receptor of target tissues. Three specific rabbit anti-insulin sera as well as sera from eight diabetic patients with insulin antibodies were incubated, free of insulin, with labelled insulin for 48 h at 4 degrees C; following incubation labelled insulin was employed in binding experiments on monocytes, erythrocytes and placenta membranes. Using rabbit sera, receptor binding was absent when insulin was totally combined with antibody, and appeared in increasing amounts as the percentage of free insulin increased to reach a maximum when no insulin was combined with antibody. The same experiment using sera from diabetic patients showed a close negative relationship (r = 0.95) between the amount of insulin bound to the antibody and the amount bound to receptors. The influence of the insulin-antibody complex on the insulin receptor interaction was evaluated by exposing the insulin-antibody complex to the receptor in pH, temperature and competition-inhibition curve experiments. The complex had no effect on receptor affinity or on the pH and temperature relationship influence with insulin-receptor interaction. The findings suggest that insulin resistance in the presence of insulin antibodies is due only to an alteration occurring before the interaction of insulin with its receptor, and demonstrate that the insulin-antibody complex does not influence the insulin receptor interaction.

Characteristics of insulin receptors in the heart muscle: binding of insulin to isolated muscle cells from adult rat heart

Adult rat heart muscle cells obtained by perfusion of the heart with collagenase have been used to characterize the insulin receptors by equilibrium binding and kinetic measurements. Binding of 125I-labelled insulin to heart cells exhibited a high degree of specificity; it was dependent on pH and temperature, binding at steady state increased with decreasing temperatures. Above 70% of the radioactivity bound at equilibrium at 25 degrees C could be dissociated by addition of an excess of unlabelled insulin. 54 and 40% of 125I-labelled insulin was degraded by isolated heart cells after 2 h at 37 degrees C and 4 h at 25 degrees C, respectively. This degrading activity was effectively inhibited by high concentrations of albumin. Equilibrium binding studies were conducted at 25 degrees C using insulin concentrations ranging from 2.5 x 10(-11) mol/l to 10(-6) mol/l. Scatchard analysis of the binding data resulted in a curvilinear plot (concave upward), which was further analyzed using the average affinity profile. The empty site affinity constant was calculated to be 9.5 x 10(7) l/mol with a total receptor concentration of 3.4 x 10(6) sites per cell. The presence of site-site interactions of the negative cooperative types among the insulin receptors has been confirmed by kinetic experiments. The rate of dilution induced dissociation was enhanced in the presence of native insulin (5 x 10(-9) mol/l), both, under conditions of low and high fractional saturation of receptors. These studies demonstrate the presence of specific insulin receptors in isolated muscle cells from adult rat heart and provide a useful model for the study of insulin action on the heart.

Regulation of catecholamine-responsive adenylate cyclase activity in rat reticulocyte membranes by endogenous factors: general characteristics and resolution into protein and nucleotide components

A 100 000 X g soluble, supernatant fraction obtained from the hemolysate of rat reticulocytes was studied for its effect upon catecholamine-sensitive adenylate cyclase activity in reticulocyte membranes. The supernatant material, devoid of adenylate cyclase activity itself, amplified isoproterenol-dependent activity in responsive membranes and was an essential requirement for the expression of hormone sensitivity in membranes rendered unresponsive to isoproterenol alone. The increment in catecholamine-associated activity conferred upon reticulocyte membranes by the supernatant material was beta-adrenergic because it did not affect basal or fluoride-related activity and was completely inhibited by propranolol. Guanine nucleotides were present in the supernatant but could account for only a fraction of the total activity because the supernatant was able to cause greater stimulation than maximal concentrations of GTP and when specified concentrations of exogenous GTP were compared with equivalent nucleotide concentrations in the supernatant, the supernatant always led to greater activity. The supernatant was resolved into protein-and nucleotide-containing components by ion-exchange chromatography. Each component was approximately one-half as active in amplifying catecholamine-dependent adenylate cyclase as the unresolved, crude supernatant material. The activity eluted in the first peak of the DEAE chromatogram was resistant to alkaline phosphatase, sensitive to trypsin, not dialyzable and contained no detectable concentrations of GTP or GDP. In contrast, the activity eluted the second peak of the DEAE chromatogram was sensitive to alkaline phophatase, resistant to trypsin, completely dialyzable and contained both GTP (30 microM) and GDP (10 microM) in significant concentrations. Neither the crude supernatant nor its two active components affected the binding of [125I]-iodohydroxybenzylpindolol to reticulocyte membranes. These observations establish in rat reticulocytes the presence of protein and guanine nucleotide constituents which have independent influences upon the catecholamine-responsive adenylate cyclase of reticulocyte membranes.

Specific receptors for the serum thymic factor (FTS) in lymphoblastoid cultured cell lines

The interaction of the synthetic serum thymic factor (FTS, "facteur thymique sérique") with human T lymphocytes from two established T cell lines (1301 and HSB2) was studied by using [3H]FTS. At 30 degrees C, in the presence of 10 nM labeled FTS, a steady state was observed within 120 min. Bound labeled FTS was dissociated by simultaneous addition of an excess of unlabeled FTS (dissociation constant Kd = 3 nM). The binding of [3H]FTS was reduced to 10-20% when unlabeled FTS was added at 20 microgram/ml (20 micro M), but not when unrelated peptides or FTS analogues differing from FTS by only one amino acid residue were added. No FTS binding was detectable on B and null cell lines or on two T cell lines other than 1301 and HSB2. These data indicate that only certain T cell subsets possess receptors with high affinity and specificity for FTS.

The mechanism of adaptive hyperlipogenesis: insulin receptor binding and glucokinase activity in rat liver during fasting and refeeding

In the present study, rats were fasted for 3 days and subsequently refed for 1, 3, or 5 days. Measurements of insulin binding to its receptors on liver plasma membranes were carried out in conjunction with measurements of the activity of an insulin-regulated enzyme from liver cytosol, glucokinase. In response to the 3-day fast (chronic hypoinsulinemia), the insulin receptor number almost tripled, whereas the glucokinase activity was halved. The insulin receptor number slowly fell to control values during the 5 days of refeeding. In contrast, glucokinase activity rose to levels 2.5 times higher than control (5 times higher than the fasting values) after 1 day of refeeding. Altough the activity fell off somewhat during refeeding it was still dobule control values after 5 days refeeding. It was concluded that in the fasted rat there was a dissociation between insulin receptor concentration and the activity of the insulin-regulated enzyme glucokinase. However, the fasting-induced increase in receptor concentration appeared to play a permissive role in the rapid overshoot of glucokinase activity observed in the early stages of refeeding. Such a scheme would explain the metabolic changes occurring in the fasted-refed rat.

Proteins iodinated by the chloramine-T method appear to be degraded at an abnormally rapid rate after endocytosis

Proteins labeled with either (3)H by reductive methylation or (125)I by the chloramine-T method were incubated with Xenopus laevis oocytes; the incorporation and acid precipitability of the proteins were then studied. The uptake rates of both specifically incorporated (vitellogenin) and nonspecifically incorporated proteins (bovine serum albumin and X. laevis serum proteins lacking albumin) were not influenced by the method of labeling. However, (125)I-labeled proteins were apparently degraded at rates far exceeding their (3)H-labeled counterparts, based on the generation of acid-soluble radioactivity. Thus, after a 3-hr incubation, 3-5 times more (125)I-labeled bovine serum albumin and X. laevis serum proteins lacking albumin were degraded than the corresponding (3)H-labeled proteins (95% compared to 30% and 75% compared to 15%, respectively), whereas after a 24-hr incubation, the degradation of (125)I-labeled vitellogenin was 15 times greater than that of [(3)H]vitellogenin labeled in vivo (60% compared to 4%). Moreover, examination of the relative amounts of (3)H- compared to (125)I-labeled bovine serum albumin deposited into the exogenously derived yolk platelet compartment of the oocyte revealed 7 times more acid-precipitable (3)H-labeled protein, indicating that the observed discrepancies were not due to reincorporation of the (3)H-labeled ligands. Passage of dissolved oocytes previously exposed to (125)I-labeled bovine serum albumin (chloramine-T method) over a column of Bio-Gel P-10 revealed some breakdown of bovine serum albumin to intermediate molecular weight components and the presence of a large amount ( approximately 90%) of labeled low molecular weight compounds, which analysis showed to be 72% free iodine. The evolution of either iodotyrosine or free iodine would nevertheless be perceived as protein degradation by most analytical procedures (e.g., acid precipitation or autoradiography). We conclude, therefore, that apparent degradation rates observed for endocytotically incorporated proteins may vary depending on the method used to label the protein and caution should be exercised when interpreting results obtained with labeled, particularly chloramine-T labeled, proteins.

Solubilization of insulin binding and degrading activity from guinea pig kidneys

Insulin binding and degrading activities were solubilized by a nonionic detergent. Triton X-100, from guinea pig kidney particulate fractions (100,000 x g pellet). The solubilized insulin binding activity appeared as a single peak on Sepharose 6B gel filtration with a Stokes radius of 73 A. The pI of the solubilized insulin binding activity determined by flat-bed isoelectric focusing was 5.6. On the other hand, the Stokes radius of the solubilized moelcule with insulin degrading activity was 54 A by the same column with a pI of 5.2. More than 98% of the insulin binding activity could be adsorbed to a column of concanavalin A-agarose, while about 94% of the insulin degrading activity could not be adsorbed to this column. These results strongly suggest that the macromolecule for the insulin binding activity is not identical to that for the insulin degrading activity.

Radioimmunoassay of the insulin receptor: a new probe of receptor structure and function

A sensitive and specific radioimmunoassay for the insulin receptor has been developed employing receptor autoantibodies from the serum of a patient with insulin-resistant diabetes. The assay detects insulin binding sites at concentrations as low as 0.1 nanomolar; distinguishes between receptors originating from human placental membranes, human lymphoblastoid cells, and mouse liver membranes; and measures the receptor independently of its binding function. Down-regulation, or loss of binding after exposure to insulin, is associated with loss of immunoreactive receptor.

Effects in adipocytes of diamide on GSH levels, glucose uptake and cell integrity

Concentrations of insulin and chemical agents (H2O2, vitamine K-5) which stimulate hexose transport in fat cells do not alter the cellular levels of glutathione (reduced form; GSH). Diamide, another agent used in studies of insulin action, markedly reduces GSH levels and increases the movement of sugar into the cell. However, unlike insulin, H2O2 or vitamin K-5, diamide causes a change in the permeability of fat cells that allows entry of compounds (insulin, sucrose, L-glucose) which are normally excluded by the plasma membrane. Moreover, the accelerated rate of methylglucose uptake produced by diamide treatment is not inhibited by cytochalasin B, an agent that blocks basal and insulin-stimulated methylglucose transport. These results indicate that diamide does not cause a stimulation of the glucose transport system and should not be used (or used with caution) in transport studies. Furthermore, oxidation of GSH does not appear to be necessary for the stimulation of hexose transport in adipocytes by insulin, H2O2 or vitamin K-5.

Insulin binding and glucose metabolism in adipocytes of streptozotocin-diabetic rats

To investigate the mechanism of the cellular insulin insensitivity of diabetic rats, insulin binding, glucose transport, and glucose oxidation were studied in adipocytes from streptozotocin-diabetic rats. Increased insulin binding was found in cells from diabetic rats, and this was due to an increased number of insulin receptors rather than a change in receptor affinity. Basal and insulin-stimulated glucose oxidation was decreased in adipocytes from diabetic rats when the data are expressed in absolute terms or as percent increased above basal. Although the absolute rate of basal and insulin-stimulated glucose transport was decreased in adipocytes from diabetic rats, the percent increase above basal of insulin-stimulated glucose transport was not decreased. In conclusion, although the cellular insulin insensitivity exists in adipocytes from diabetic rats, the number of insulin receptors was increased, coupling between insulin receptors and the glucose transport system is intact in adipocytes from diabetic rats, and a defect in intracellular glucose metabolism rather than glucose transport plays a major role in the insulin insensitivity of adipocytes from diabetic rats.

Prolactin receptors: a comparison between chloramine-T and lactoperoxidase iodination

The binding properties of chloramine-T iodinated oPRL (using a modified iodination procedure) to prolactin receptors of female rat liver membrane particles were compared with those of lactoperoxidase iodinated oPRL. The results indicated that both iodination methods provided 125I-oPRLs which were suitable for receptor binding studies. Our results suggested that chloramine-T 125I-oPRL was even better than lactoperoxidase 125I-oPRL in terms of lower nonspecific binding. The chloramine-T iodinated oPRL was used to study the prolactin receptors in rat ovaries and DMBA induced rat mammary tumors. The results showed that the amount of prolactin receptors in rat ovaries was related to the plasma level of prolactin as it had been reported in liver by other investigators. The study of prolactin receptors in rat mammary tumors indicated that the prolactin receptor content of hormone dependent mammary tumors was much higher than that of hormone independent mammary tumors.

Pathophysiology of diabetes mellitus

Techniques have been developed for examining the binding of insulin to its target cells and for evaluating the in vivo action of insulin, rekindling interest in the possible role of insulin resistance in adult-onset diabetes. A host of new data have accumulated regarding the contribution of glucagon to the syndrome.

Angiotensin receptive neurones in the subfornical organ. Structure-activity relations

A microiontophoretic study was performed of the actions of angiotensin II and angiotensin fragments on neurones of the subfornical organ (SFO). Adult cats were anaesthetized and the SFO exposed for penetration by a multibarrelled micropipette. We found that angiotensin II-[2--8]-heptapeptide shows a significantly higher stimulation of firing rate compared to angiotensin II. Angiotensin II-[5--8]-tetrapeptide still produced an excitatory action on a single units. Both the action of the heptapeptide and the tetrapeptide were blocked by [sar1, Ala8]-angiotensin II (P 113). In contrast, angiotensin II-[6--8]-tripeptide failed to enhance the firing rate of the same neurones. Our data indicate that angiotensin II and some shorter chain peptide fragments can directly affect neurones of the SFO. The study may give new insight in structure-activity relations for angiotensin II. The results support the hypothesis that the subfornical organ is a receptor site which is available to this peptide.

Intracellular translocation of iodine-125-labeled insulin: direct demonstration in isolated hepatocytes

Insulin labeled with iodine-125 binds to receptors on isolated rat hepatocytes. At low temperatures initial binding is restricted to the plasma membrane as detected by direct quantitative autoradiographic analysis with the electron microscope. With increasing time and temperature of incubation there is a systematic and progressive translocation of autoradiographic grains to a highly limited area of the cell periphery representing no more than 15% of the radius of the cell.

Demonstration of the insulin receptor in vivo in rabbits and its possible role as a reservoir for the plasma hormone

Based on studies of the interaction of insulin with its receptors in vitro, we calculated that a receptor compartment should be measurable directly in vivo. For this purpose, rabbits were injected intravenously with a labeled insulin that has low affinity for receptors in combination with a radioiodinated insulin that has high affinity for receptors. Plasma concentrations of labeled insulins were measured at selected intervals after injection. Apparent volumes of distribution were calculated by extrapolation of plasma distribution were calculated by extrapolation of plasma disappearance curves; high affinity insulins consistently distributed into spaces that were two-three times greater than those of the low affinity insulins. Injections of unlabeled pork insulin before tracer insulins decreased the distribution space of the high affinity insulin in a dose-dependent manner while having little or no effect on the distribution space of the low affinity labeled insulin. When unlabeled insulin was injected after the tracer insulins, there was an immediate rise in the plasma concentration of the high affinity insulin with only a slight change in the plasma concentration of the low affinity insulin. These results demonstrate that high affinity insulins distribute into a body compartment which has many properties of the insulin receptor previously studied in vitro. This receptor compartment: (a) recognizes insulins based on their biological potencies; (b) is saturated by elevated concentrations of insulin; and (c) insulin bound to receptors is in equilibrium with free hormone in plasma. Further, the bound to free ratios for hormone, calculated from these data, suggest that in vivo greater than 50% of the extrapancreatic insulin is bound to receptors during normal physiological states.

125I-insulin binding to cultured human lymphocytes. Initial localization and fate of hormone determined by quantitative electron microscopic autoradiography

Morphologic and biochemical studies indicate that the initial action of insulin is binding to a cell surface receptor. Whether further translocation of the hormone, or a product of the hormone, occurs is unclear and has not been investigated by direct means. To determine the fate of 125I-insulin bound to its receptor, we have examined the distribution of radioactivity by quantitative electron microscopic autoradiography. Cultured lymphocytes of the IM-9 cell line were incubated with 0.1 nM 125I-insulin at 15 degrees and 37 degreesC for incubation periods extending from 2 to 90 min. At 15 degreesC, grains localize to the plasma membane and there is no translocation as a function of time. At 37 degreesC, grains predominantly localize to the plasma membrane but there is a small shift in distribution to a distance of 300-700 nm from the plasma membrane. This small additional band component of irradiation extends to approximately to10--15% of the cell radius. When a morphometric analysis is applied to grains extending 300 nm and beyond from the plasma membrane, we find no preferential localization to any intracellular organelle. We interpret these data to indicate that in the cultured lymphocyte, labeled insulin initially localizes to the plasma membrane but as fuanction of time and increasing temperature there is a small but definite translocation of the hormone or a product of the hormone to a hihgly limited aea of the cell periphery.

Insulin receptors in the newborn. Increase in receptor affinity and number

Insulin hs been implicated as a growth factor in fetal life. To clarify the role of insulin in the fetus, we studied insulin receptors on mononuclear leukocytes in placental-cord blood from 12 normal newborns. Eight healthy young adults served as controls. Specific binding (mean +/- S.E.M.) in the absence of unlabeled insulin per 10(7) monocytes was, respectively, 24.3+/-3.5 and 4.7+/-0.9 per cent in newborns and adults. This increase in binding is due to a rise in number of receptor sites per cell (newborns, 44,600, and adults, 7100) as well as an increase in receptor affinity (Ke for newborns = 5.9 X 10(8) M(-1), and that for adults = 2.9 X 10(8) M(-1). This finding of markedly high concentrations of high-affinity receptors for insulin on fetal cells supports the importance of insulin in intrauterine growth and development.

Effect of age and weight on plasma glucose and insulin responses in the rat

Plasma glucose and insulin responses to orally administered glucose were determined in rats of varying ages and weights. The results indicated that glucose tolerance tends to deteriorate as rats grow from 1 month to 9 months of age. This change in glucose tolerance was associated with an increase in the response of plasma insulin to glucose, consistent with the hypothesis that the decline in glucose tolerance is due to a loss of normal insulin sensitivity. The changes in glucose and insulin responses could not be explained entirely on the basis of the associated weight gain. They seemed to be at least partially secondary to either age or an age-related factor other than obesity.

Iodinated neurohypophyseal hormones as potential ligands for receptor binding and intermediates in synthesis of tritiated hormones

[3-Iodo-Tyr2]oxytocin (MIOT), [3,5-diiodo-Tyr2]oxytocin (DIOT), [3-iodo-Tyr2,Lys8]vasopressin (MILVP), [3,5-diiodo-Tyr2,Lys8]vasopressin (DILVP), [3-iodo-Tyr2,Arg8]vasopressin (MIAVP), and [3,5-diiodo-Tyr2,Arg8]vasopressin (DIAVP) were synthesized by iodination of the respective hormones, pruified, and characterized. All the monoiodo hormones had to be freshly prepared prior to bioassays, since on storage they gave rise to hormonal-like biological activity. The biological activities of these iodo analogues were measured in an adenylate cyclase assay employing neurohypophyseal hormone (NHH) sensitive bovine renal medullary membranes, and/or the rat oxytocic assay. In the cyclase assay, DIOT, DILVP, and DIAVP were inactive as agonists or antagonists. MIOT shows no agonistic activity in the renal cyclase system and uterus, but is a weak reversible inhibitor of oxytocin (OT) in both systems. When MIOT (10(-4) M) was preincubated with renal membranes for 10 min at 37 degrees C before addition of OT, it behaved as a noncompetitive inhibitor of NHH-stimulated adenylate cyclase. MILVP and MIAVP appear to be partial agonists with Km (half maximal response) 3 X 10(-6) and 3 X 10(-7) M, respectively, as determined in the cyclase assay. Upon preincubation with renal medullary membranes, MILVP (10(-6) M) behaves as a more potent noncompetitive inhibitor of OT than MIOT. Accordingly, iodo derivatives of NHH do not exhibit sufficient affinity to serve an specific ligands to measure OT, LVP, or AVP receptors in the uterus and kidney. Study of the specificity of inhibition produced by MIOT revealed that this analogue does not act selectively upon NHH receptors. Thus, MIOT modified adenylate cyclase systems which do not have NHH receptors, e.g., the PTH-sensitive adenylate cyclase in bovine renal cortex and the glucagon-sensitive adenylate cyclase in rat liver. DIOT, DILVP, and DIAVP were subjected to catalytic tritiation (employing carrier free tritium) and were converted to [3H]OT (25, 31, and 25 Ci/mmol), [3H]LVP (26 and 23 Ci/mmol), and [3H]AVP (17 Ci/mmol), respectively. These tritiated ligands have been successfully used to measure NHH receptor sites both in kidney and uterine membranes as described in other studies.

Insulin, S-sulphonate A and S-sulphonate B-chain receptor binding to human cultured lymphocytes

The ability of insulin and the S-sulphonate A and B-chain derivatives to bind to a receptor on cultured human lymphocytes was evaluated. A receptor site for the S-sulphonate A-chain was identified and was strongly influenced by the intact insulin molecule. S-sulphonate A-chain weakly interfered with insulin binding. S-sulphonate B-chain showed no evidence of significant binding and did not interfere with insulin or S-sulphonate A chain binding. 14CO2 production from 14C-1-glucose was stimulated by insulin in cultured lymphocytes and this effect was blunted by S-sulphonate A-chain. The sulphhydryl blocking agent used in the production of insulin A-chain appears to be of critical importance.

Insulin receptors in adipocytes of non-diabetic and diabetic subjects. Preliminary report

We have measured insulin binding to human adipocytes isolated from subcutaneous adipose tissue removed during surgery in normal and insulin-independent diabetics. Collagenase digestion, 125I-monoiodoinsulin and Scatchard's plot were employed to analyze the results. Different kinetic patterns emerged together with differences in the dissociation constant and receptor numbers: in normal subjects K1 was 4 X 10(-9) moles/1 and K20.5 X 10(-8) moles/1, and in diabetic subjects K1 was 2.24 X 10(-9) moles/1 and K2 0.52 X 10 10(-8) moles/1; the two classes of receptors were 100,000 and 300,000 per cell in normals and 50,000 and 180,000 in diabetics. It was clear that even slight diabetes leads to receptor deficiency in adipocytes, though it could not be determined whether this a primary, perhaps genetic, defect or secondary to antibody damage, as suggested by some workers. Feed-back between circulating insulin and specific receptor availability in cells is another possibility.

Serum somatomedin-C in hypopituitarism and in other disorders of growth

Serum levels of somatomedin-C have been measured by a competitive membrane binding assay in normal individuals, in hypopituitary dwarfs, and in children with growth retardation from causes other than growth hormone deficiency. The mean level in untreated hypopituitary children is about 40% of that of age-matched normal children. Treatment of hypopituitary dwarfs with human growth hormone results in prompt increases in the serum concentration of somatomedin-C. Normalization of somatomedin is associated with the return of pituitary function in hospitalized psychosocial dwarfs. Supporting the possibility that somatomedin-C plays a pivotal role in skeletal growth is the finding that serum levels in hypopituitary children correlate with growth rate both on and off growth hormone therapy. In our hands, this assay has proved to be an efficient, reliable method for the diagnosis of growth hormone deficiency.

Distribution of angiotensin II receptors in rat brain

Angiotensin II binding activity of rat brain particles was examined using [125I]-angiotensin II (0.1-0.3 nM) in the presence and absence of excess unlabelled angiotensin II. Certain features of the binding suggested that physiological receptors were involved. The binding activity was temperature dependent and was increased 3-fold by the addition of 0.5 M EDTA. The binding appeared specific as judged by inhibition with angiotensin II agonists and antagonists. The "specific" binding was saturable, two-thirds reversible and occurred with high affinity. The equilibrium dissociation constant (Kd) of the "specific" binding was 0.9 nM. Subcellular fractionation studies indicated that over 90% of the binding was associated with particulate matter and was concentrated in the crude microsomal fraction. Binding was localized to the midbrain, thalamus, septum, hypothalamus and medulla; Very low levels of binding were found in the cortex, hippocampus and striatum; The lateral septum had the highest binding activity of all the tissues examined. Subdivision of the medulla showed that the highest binding activity was associated with the area postrema and medullary regions ventral to this organ.