Showing papers on "Receptor published in 1980"

The role of hormone receptors and GTP-regulatory proteins in membrane transduction

Abstract: Cell membrane receptors for hormones and neurotransmitters form oligomeric complexes with GTP-regulatory proteins and inhibit the latter from reacting with GTP. Hormones and neurotransmitters act by releasing the inhibitory constraints imposed by the receptors, thus allowing the GTP-regulatory proteins to interact with and control the activity of enzymes such as adenylate cyclase. This theory may apply generally to membrane signal transduction involving surface receptors.

Pirenzepine distinguishes between different subclasses of muscarinic receptors

Abstract: Some antagonists exhibit tissue selectivity in their pharmacological antagonism of muscarinic responses. However, the affinity constants for equilibrium binding of classical antagonists to muscarinic receptors in subcellular preparations have shown only small variations in different peripheral tissues and regions of the brain. The binding curves do not deviate significantly from the simple Langmuir isotherm, indicating apparent homogeneity of the receptor population in any given region. In contrast, heterogeneity has been detected by agonist binding studies but this may arise from different environmental or coupling restraints on the agonist-induced conformational change and cannot be taken as evidence for different receptor subtypes. We report here binding studies using a new anti-muscarinic drug, pirenzepine, in which we found heterogeneity of binding that correlates well with the pharmacological activity.

Subclasses of external adenosine receptors

Abstract: Cell surface adenosine receptors mediate either stimulation or inhibition of adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1], and the receptors that mediate these different responses can be discriminated with selected adenosine analogs. 5'-N-Ethylcarboxamide-adenosine is a more potent agonist at stimulatory receptors (Ra) than is N6-phenylisopropyladenosine, whereas the reverse potency order is seen with inhibitory receptors (Ri). The potency of adenosine is intermediate between the potencies of these two analogs. The relative potencies of adenosine receptor agonists are maintained in physiological responses in intact cells, such as steroidogenesis and inhibition of lipolysis. As with adrenergic receptors, subclasses of adenosine receptors differ functionally and pharmacologically.

Transforming growth factors produced by certain human tumor cells: polypeptides that interact with epidermal growth factor receptors

Abstract: Three different human tumor lines in culture, a rhabdomyosarcoma, a bronchogenic carcinoma and a metastatic melanoma, release proteins (transforming growth factors, TGFs) into the medium that confer the transformed phenotype on untransformed fibroblasts. These proteins are acid and heat-stable; produce profound morphologic changes in rat and human fibroblasts; and enable normal anchorage-dependent cells to grow in agar. Removal of the transforming protein results in a reversion of cell phenotype. The major activity interacts with epidermal growth factor (EGF) cell membrane receptors. The peptides from these tumor cells are similar in their action to the sarcoma growth factor (SGF) released by murine sarcoma virus-transformed rodent cells. The most anchorage-independent tumor cells released the most TGFs. EGF-related TGFs were not detectable in fluids from cultures of cells with high numbers of free EGF membrane receptors (normal human fibroblasts and human carcinomas).

Receptors for the age of anxiety: pharmacology of the benzodiazepines

Abstract: Investigation of the actions of the benzodiazepines has provided insights into the neurochemical mechanisms underlying anxiety, seizures, muscle relaxation, and sedation. Behavioral, electrophysical, pharmacological, and biochemical evidence indicates that the benzodiazepines exert their therapeutic effects by interacting with a high-affinity binding site (receptor) in the brain. The benzodiazepine receptor interacts with a receptor for gamma-aminobutyric acid, a major inhibitory neurotransmitter, and enhances its inhibitory effects. The benzodiazepine receptor may also interact with endogenous substances and several naturally occurring compounds, including the purines and nicotinamide, are candidates for this role. Both the purines and nicotinamide possess some benzodiazepine-like properties in vivo, although further work will be required to confirm their possible roles as endogenous benzodiazepines.

Chloroquine inhibits lysosomal enzyme pinocytosis and enhances lysosomal enzyme secretion by impairing receptor recycling.

Abstract: Adsorptive pinocytosis of acid hydrolases by fibroblasts depends on phosphomannosyl recognition markers on the enzymes and high-affinity pinocytosis receptors on the cell surface. In this study, beta-glucuronidase binding to the cell surface of attached fibroblasts was found to be saturable and inhibitable by mannose-6-phosphate (Man-6-P). Dissociation of cell-bound beta-glucuronidase occurred very slowly at neutral pH, but was greatly accelerated by lowering the pH below 6.0, or by exposure to Man-6-P. Comparison of the maximal cell surface binding and the observed rate of enzyme pinocytosis suggests that the pinocytosis receptors are replaced or reused about every 5 min. Enzyme pinocytosis was not affected by inhibition of new protein synthesis for several hours, suggesting a large pool of internal receptors and/or reuse of internalized receptors. Chloroquine treatment of normal human fibroblasts had three effects: (a) greatly enhanced secretion of newly synthesized acid hydrolases bearing the recognition marker for uptake, (b) depletion of enzyme-binding sites from the cell surface, and (c) inhibition of pinocytosis of exogenous enzyme. Only the third effect was seen in I-cell disease fibroblasts, which were also less sensitive than control cells to this effect. These observations are consistent with a model for transport of acid hydrolases that proposes that delivery of newly synthesized acid hydrolases to lysosomes requires the phosphomannosyl recognition marker on the enzymes, and intracellular receptors that segregate receptor-bound enzymes into vesicles for transport to lysosomes. This model explains how chloroquine, which raises intralysosomal pH, can disrupt both the intracellular pathway for newly synthesized acid hydrolases, and the one for uptake of exogenous enzyme by cell surface pinocytosis receptors.

OKT3: a monoclonal anti-human T lymphocyte antibody with potent mitogenic properties.

Abstract: OKT3, a monoclonal anti-human T cell antibody (IgG2), was found to induce DNA synthesis in human peripheral lymphocyte cultures. OKT3 induced maximal mitogenesis at a concentration of 10 to 20 ng/ml and was about 20-fold more potent than PHA as a mitogen. No high-dose inhibition of thymidine incorporation was noticed at concentrations up to 2.5 microgram OKT3/ml. The monovalent Fab fragment of OKT3 was also mitogenic but about 100 times less potent than the parent IgG. OKT3 appeared to be a T lymphocyte mitogen as only sheep red blood cell rosetting lymphocytes were responsive. Quantitative studies on the binding of 125I-labeled Fab fragment of OKT3 to human lymphocytes showed an average of 5.1 x 10(4) receptor sites/cell with an association of about 10(8) M-1 at 37 degrees C, with no heterogeneity of the cell binding sites. These data suggest a strong interaction of the monoclonal OKT3 with a limited number of identical T cell membrane receptors. As this interaction can trigger mitogenesis, the cell membrane determinant recognized by OKT3 could be described as a "T cell stimulation receptor." The mitogenecity of the lymphocytes is not solely dependent on cross-linking of these receptors.

Mechanisms of insulin resistance in human obesity: evidence for receptor and postreceptor defects.

Abstract: To assess the mechanisms of the insulin resistance in human obesity, we have determined, using a modification of the euglycemic glucose clamp technique, the shape of the in vivo insulin-glucose disposal dose-response curves in 7 control and 13 obese human subjects. Each subject had at least three euglycemic studies performed at insulin infusion rates of 15, 40, 120, 240, or 1,200 mU/M2/min. The glucose disposal rate was decreased in all obese subjects compared with controls (101 +/- 16 vs. 186 +/- 16 mg/M2/min) during the 40 mU/M2/min insulin infusion. The mean dose-response curve for the obese subjects was displaced to the right, i.e., the half-maximally effective insulin concentration was 270 +/- 27 microU/ml for the obese compared with 130 +/- 10 microU/ml for controls. In nine of the obese subjects, the dose-response curves were shifted to the right, and maximal glucose disposal rates (at a maximally effective insulin concentration) were markedly decreased, indicating both a receptor and a postreceptor defect. On the other hand, four obese patients had right-shifted dose-response curves but reached normal maximal glucose disposal rates, consistent with decreased insulin receptors as the only abnormality. When the individual data were analyzed, it was found that the lease hyperinsulinemic, least insulin-resistant patients displayed only the receptor defect, whereas those with the greatest hyperinsulinemia exhibited the largest post-receptor defect, suggesting a continuous spectrum of defects as one advances from mild to severe insulin resistance. When insulin's ability to suppress hepatic glucose output was assessed, hyperinsulinemia produced total suppresssion in all subjects. The dose-response curve for the obese subjects was shifted to the right, indicating a defect in insulin receptors. Insulin binding to isolated adipocytes obtained from the obese subjects was decreased, and a highly significant inverse linear relationship was demonstrated between insulin binding and the serum insulin concentration required for halfmaximal stimulation of glucose disposal. In conclusion: (a) decreased cellular insulin receptors contribute to the insulin resistance associated with human obesity in all subjects; (b) in the least hyperinsulinemic, insulin-resistant patients, decreased insulin receptors are the sole defect, whereas in the more hyperinsulinemic, insulin-resistant patients, the insulin resistance is the result of a combination of receptor and postreceptor abnormalities; (c) all obese patients were insensitive to insulin's suppressive effects on hepatic glucose output; this was entirely the result of decreased insulin receptors; no postreceptor defect in this insulin effect was demonstrated.

Comparative airway and vascular activities of leukotrienes C-1 and D in vivo and in vitro

Abstract: The pharmacologic activities of leukotrienes C-1 and D(LTC-1 and LTD), constituents of slow reacting substance of anaphylaxis (SRS-A), were evaluated in vitro on airway contractile tissues and in vivo on pulmonary mechanical function, mean systemic arterial pressure, and cutaneous microcirculation. In vitro both LTC-1 and LTD were potent and selective peripheral airway agonists, being more active than histamine; furthermore, LTD was active on peripheral airways at concentrations 1/100th those of LTC-1. The concentration-effect relationship for LTD and the profile of antagonism by FPL 55712 are consistent with the activity of this molecule at two separate peripheral airway receptors. In vivo, LTC-1 and LTD were nearly equally active in their effects on pulmonary mechanics, and the pattern of alterations was consistent with the predominant site of action being in the lung periphery. Furthermore, both agents had a direct systemic arterial hypotensive effect and were vasoactive on the cutaneous microcirculation. Thus, these compounds are likely to be major mediators of the pathologic alterations in immediate type hypersensitivity reactions in which peripheral airway constriction and hypotension are prominent features.

Identification of the membrane glycoprotein that is the C3b receptor of the human erythrocyte, polymorphonuclear leukocyte, B lymphocyte, and monocyte.

Abstract: A human erythrocyte membrane glycoprotein of 205,000 mol wt (gp205) has been identified as the C3b receptor of the erythrocyte, polymorphonuclear leukocyte (PMN), B lymphocyte, and monocyte. Initially, gp205 was sought and characterized as a constituent of the human erythrocyte membrane that can impair activation of the alternative complement pathway by inducing loss of function of the properdin-stabilized amplification C3 convertase (C3b,Bb,P) through displacement of Bb from C3b and by promoting cleavage-inactivation of C3b by C3b inactivator. These inhibitory activities of gp205 suggested that this membrane glyeoprotein had an affinity for C3b and prompted an analysis of its possible identity as the C3b receptor of human peripheral blood cells. The F(ab')2 fragment of rabbit IgG anti-gp205 inhibited the formation of rosettes with sheep EC3b of human erythroeytes, B lymphocytes, monocytes and PMN in a dose-response manner; the 50 percent inhibitory doses were 0.13/mug/ml, 0.90 mug/ml, 1.25 mug/ml, and 1.20 mug/ml of F(ab')2, respectively. Anti-gp205 did not impair the formation of rosettes by monocytes and B lymphocytes with sheep EC3bi or with EC3d. Scatchard analysis of the number of specific (125)I-F(ab')(2) anti-gp205 binding sites/cell revealed 950 sites/erythrocyte, 21,000 sites/cell of B lymphocyte preparation, 57,000 sites/PMN, and 48,000 sites/monocyte, indicating that the higher concentrations of antibody that had been required for inhibition of rosette formation by the nucleated cells reflected larger numbers of receptors on these cells. Direct evidence for the identity of gp205 as the C3b receptor of the four cell types was obtained when detergent-solubilized membrane proteins of the surface-radioiodinated cells were reacted with anti- gp205 and the immunoprecipitate was analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. In each instance, the antigenic material reacting with anti-gp205 represented a single protein with an apparent 205,000 mol wt. Thus, gp205 is the C3b receptor of human erythrocytes, PMN, B lymphocytes, and monocytes.

Multiple opiate receptor sites on primary afferent fibres

Abstract: In addition to their actions at supraspinal sites, opiates can act directly at the spinal cord level to produce analgesia1–3. Opiate receptors and enkephalins are found in abundance in the dorsal horn of the spinal cord, in the region of termination of small-diameter primary afferents4–6. Furthermore, there is evidence that exogenously administered opiates or endogenous enkephalins act both on postsynaptic receptors in dorsal horn and presynaptically to block transmitter release from nociceptive primary afferent terminals7–10. The possiblity of these two distinct sites of opiate action in dorsal horn is of particular interest in view of recent evidence for the existence of multiple opiate receptors11–13. On the basis of differences in the rank orders of potency of opiate alkaloids and opoid peptides observed in different tissues, Lord et al.14 have proposed two categories of opiate receptor, μ and δ, and these can be differentiated in binding studies, provided that selective radioglands are used in the appropriate experimental conditions15–18. Here we have used the selective radioligands 3H-morphine (μ sites) and 3H-D-Ala2, D-Leu5-enkephalin (δ sites) to examine directly in rat the distribution and binding characteristics of opiate receptors on dorsal root and in various regions of the adjacent spinal cord. Primary afferent tissue (dorsal root) and dorsal horn were found to contain μ and δ opiate binding sites with a relatively high proportion of μ sites. Partial destruction of small-diameter primary afferents after cutting the sciatic nerve led to a significant reduction in both μ and δ binding sites on dorsal roots, suggesting that both types of opiate receptors may exist on small-diameter primary afferents.

A quantitative analysis of beta-adrenergic receptor interactions: resolution of high and low affinity states of the receptor by computer modeling of ligand binding data

Abstract: The properties of ligand binding to the beta-adrenergic receptor have been studied using a computer modeling technique to analyze data obtained by indirect binding methods. Antagonists are shown to bind to the receptor with one homogeneous state of uniform affinity, while agonists manifest heterogeneous binding. For agonists, two distinct binding states are apparent; one of high and one of low affinity. Affinity states of the receptor are characterized by specific macroscopic dissociation constants, and the proportion of total receptors in each state can be determined. The ability of an agonist to activate adenylate cyclase (intrinsic activity) correlates closely with the amount of high affinity state formed in the presence of the agonist (p

Distinct cholecystokinin receptors in brain and pancreas.

Abstract: 125I-Labeled (Bolton-Hunter) cholecystokinin triacontatriapeptide (CCK-33) binds saturably and reversibly to distinct receptors in brain and pancreatic membranes. The peptide specificity of pancreatic CCK binding is the same as that for pancreatic amylase release. In brain, gastrin and pentagastrin display nanomolar affinity for binding sites, whereas in pancreas these two peptides are virtually inactive. Though these differences indicate that brain and pancreas possess distinct CCK receptors, the two tissues show some similarities. Both pancreas and brain receptors show greater sensitivity to sulfated than to desulfated COOH-terminal octapeptide of CCK and display dissociation constants of 0.3-9.5 nM. The pancreas possesses about 300 times more binding sites than does brain. CCK binding in both brain and pancreas is enhanced by divalent cations and reduced by monovalent cations. Receptor binding in both tissues is regulated in a selective fashion by guanyl nucleotides.

Monoclonal antibodies to human estrogen receptor.

Abstract: Extranuclear estrogen receptor protein (estrophilin) of MCF-7 human breast cancer cells was purified by passage of the cytosol fraction of a cell homogenate through an affinity column of estradiol linked to Sepharose by a substituted di-n-propyl sulfide bridge in the 17 alpha position. Elution with 50 micro M [3H]estradiol in 10% (vol/vol) dimethyl formamide/0.5 M sodium thiocyanate gave 40% recovery of [3H]estradiol-estrophilin showing 14% of the specific radioactivity expected for the pure complex. Serum from a Lewis rat immunized with this partially purified estradiol-receptor complex contained antiestrophilin antibodies that reacted not only with nuclear and extranuclear estradiol-receptor complexes from MCF-7 cells but also with estrophilin from rat, calf, and monkey uterus, hen oviduct, and human breast cancers. Splenic lymphocytes from the immunized rat were fused with cells of two different mouse myeloma lines (P3-X63-Ag8 and Sp2/0-Ag14) to yield hybridoma cultures, 2% of which produced antibodies to estrophilin. After cloning by limiting dilution, three hybridoma lines secreting antiestrophilin were expanded in suspension culture and as ascites tumors in athymic mice to provide substantial quantities of monoclonal antibodies that recognize mammalian but not avian estrophilin and that show different degrees of reactivity with receptor from nonprimate sources. By growing the clone from Sp2/0 in the presence of [35S]methionine, radiolabeled monoclonal IgG has been prepared. These monoclonal antibodies should prove useful in the study of estrogen receptors of human reproductive tissues, in particular for the radioimmunochemical assay and immunocytochemical localization of receptors in breast cancers.

Relationship of neuroleptic drug effects at brain dopamine, serotonin, alpha-adrenergic, and histamine receptors to clinical potency.

Abstract: The authors examined the potencies of 22 neuroleptic drugs competing for binding sites associated with dopamine, serotonin, alpha-adrenergic, and histamine receptors in brain membranes. They found that although many neuroleptics are quite potent in competing at several of these receptor sites, the average antipsychotic clinical potency correlates closely only with the drug affinity for dopamine receptors labeled by 3H-spiroperidol At clinically effective doses, however, substantial occupancy of serotonin, alpha-adrenergic, and histamine receptors often occurs and may account for some of the auxiliary actions of neuroleptics.

Urinary and brain beta-carboline-3-carboxylates as potent inhibitors of brain benzodiazepine receptors

Abstract: Benzodiazepines probably exert their anxiolytic, hypnotic, and anticonvulsant effects by interacting with brain-specific high-affinity benzodiazepine receptors. In searching for possible endogenous ligands for these receptors we have purified a compound 10(7)-fold from human urine by extractions, treatment with hot ethanol, and column chromatography. The compound was identified as beta-carboline-3-carboxylic acid ethyl ester (IIc) by mass spectrometry, NMR spectrometry, and synthesis; IIc was also isolated from brain tissues (20 ng/g) by similar procedures. Very small concentrations of IIc displaced [3H]diazepam completely from specific cerebral receptors, but not from liver and kidney binding sites; the concentration causing 50% inhibition of specific [3H]diazepam binding (IC50) was 4-7 nM compared to ca. 5 nM for the potent benzodiazepine lorazepam. Specific binding sites for quinuclidinyl benzilate, naloxone, spiroperidol, serotonin, muscimol, and WB 4101 were not affected by IIc. In contrast to benzodiazepines, IIc exhibits "mixed type" competitive inhibition of forebrain benzodiazepine receptors (negative cooperativity). We surmise that an endogenous ligand for benzodiazepine receptors may be a derivative of beta-carboline-3-carboxylic acid.

Endogenous digitalis-like substance in plasma of volume-expanded dogs

Abstract: Considerable evidence supports the hypothesis that renal sodium excretion is partly regulated by a humoral agent released or activated by extracellular fluid volume (ECFY) expansion1, including the demonstration of biological activities in extracts of plasma and urine which support the natriuretic hormone (NH) hypothesis2–4. NH is a low molecular weight peptide formed from a precursor in plasma5, which inhibits sodium transport (anti-natriferic activity) in isolated toad bladder, an analogue of the distal renal tubule and collecting duct. Other evidence indicates that NH can inhibit renal (Na++ K+) ATPase6, which suggests that it might be an endogenous digitalis-like substance. Although an immunoreactive ouabain-like substance has been demonstrated in amphibian plasma7, none has been reported in mammals. To identify an endogenous digitalis-like substance, we used an approach suggested by the work of Spector and his collaborators, who demonstrated that biological substances which bind to the same receptors as drugs, such as endogenous opioids8 and benzodiazepines9, may compete with antibodies specific for the drugs. Hough and Edwardson10 showed that antibodies to thaumatin, a sweet-tasting plant protein, also bind non-protein sweet substances in a linearly related fashion to the sweetness of the compound, suggesting that the antibody binding site may be similar to the sweet taste receptor. Based on these observations, we postulated that if NH were an endogenous digitalis-like substance, it might bind to antibodies specific for digitalis glycosides. We report here the preliminary isolation of a substance in plasma of dogs which competes with digoxin for two specific digoxin antibodies and is an inhibitor of (Na+ + K+) ATPase activity. Increased amounts of this factor are found in plasma of volume-expanded dogs, suggesting it is the putative NH.

Differentiation of delta and mu opiate receptor localizations by light microscopic autoradiography

Abstract: We have observed two discrete populations of opiate receptors that are differently localized in rat brain. Morphine-like (mu) receptors, labeled by 125I-labeled [D-Ala-2MePhe4Met(O)5-ol]enkephalin, are concentrated selectively in lamina IV of the cerebral cortex, certain thalamic nuclei, and the periaqueductal grey, while delta receptors, labeled by 125I-labeled [D-Ala2-D-Leu5]enkephalin, are more diffused, having high densities in cerebral cortex, corpus striatum, amygdala, and olfactory tubercle. Because of similarities in their localizations, we propose that mu and delta receptors are respectively the physiologic receptors for [Met]- and [Leu]enkephalin neurons. These distributions reflect the different physiological functions attributed to mu and delta receptors and thus represent discrete functions of [Met]- and [Leu]enkephalin neurons.

Plasma Retinol-Binding Protein

Abstract: Vitamin A is mobilized from liver stores and transported in plasma in the form of the lipid alcohol retinol, bound to a specific transport protein, retinol-binding protein (RBP). A great deal is known about the chemical structure, metabolism, and biological roles of RBP. RBP is a single polypeptide chain with molecular weight close to 20,000. RBP interacts strongly with plasma prealbumin, and normally circulates in plasma as a 1:1 molar RBP-prealbumin complex. Both the primary and the tertiary structure of prealbumin are known, and the primary structure of RBP has recently been reported. Much information is available about the protein-protein and protein-ligand interactions that are involved in this transport system. Many clinical studies have examined the effects of a variety of diseases on the plasma levels of RBP and prealbumin in humans. Plasma RBP levels are low in patients with liver disease and are high in patients with chronic renal disease. These findings reflect the facts that RBP is produced in the liver and mainly catabolized in the kidneys. Delivery of retinol to extra-hepatic tissues appears to involve specific cell surface receptors for RBP. Vitamin A mobilization from the liver, and delivery to peripheral tissues, is highly regulated by factors that control the rates of RBP production and secretion. Retinol deficiency specifically blocks the secretion of RBP, so that plasma RBP levels fall and liver RBP levels rise. Injection of retinol into vitamin A-deficient rats stimulates the rapid secretion of RBP from the liver into the plasma. The cellular and molecular mechanisms that mediate these phenomena are under investigation. Elucidation of these mechanisms should help define the basic mechanisms that control the mobilization, transport, and delivery of vitamin A.

Noradrenergic alpha 1 and alpha 2 receptors: light microscopic autoradiographic localization

Abstract: [3H]WB-4101 and p-[3H]aminoclonidine were used for light microscopic autoradiographic localization of alpha 1 and alpha 2 adrenergic receptors, respectively, in the rat brain. The binding of these ligands to slide-mounted tissue sections had all of the characteristics associated with alpha 1 and alpha 2 receptors. It was saturable with appropriate kinetic constants and was blocked only by other alpha-adrenergic drugs with the appropriate potency. Autoradiographic studies revealed a distribution of alpha-adrenergic receptors throughout the nervous system. Certain areas had elevated levels. These included parts of the olfactory bulb and nucleus, parts of the cerebral cortex and dentate gyrus, the more medial portions of the hypothalamus and thalamus, the locus ceruleus and nucleus tractus solitarii, and parts of the spinal cord. In certain areas, the distribution of alpha 1 and alpha 2 receptors was markedly different. These results provide some rational basis for the observed actions of alpha-adrenergic drugs on the central nervous system. For example, the finding of high densities of alpha 2 receptors in the nucleus tractus solitarii is most likely related to its antihypertensive action. The observed codistribution of alpha 2 receptors with opiate receptors would provide an explanation of the observation that alpha 2 agonists block opiate withdrawal. The results are also discussed in relationship to the anatomy of catecholamine systems in the brain.

2,3,7,8,-Tetrachlorodibenzo-p-dioxin: segregation of toxocity with the Ah locus.

Abstract: 2,3,7,8-Tetrachlorodibenzo- p -dioxin (TCDD) and related halogenated aromatic hydrocarbons (a) produce a similar pattern of toxic responses; (b) induce a battery of enzymes, including aryl hydrocarbon hydroxylase (AHH) activity; and (c) bind reversibly and with a high affinity to a cytosol protein that is the receptor for this enzyme induction response. To test the hypothesis that the toxic effects produced by these compounds are mediated by their binding to the induction receptor, we examined the genetic segregation of two toxic responses, with the Ah locus, which determines this receptor. The dose-response relationship for thymic atrophy produced by TCDD was examined in C57BL/6J mice, which have a high affinity receptor and are sensitive to enzyme induction, DBA/2J mice, which have a lower affinity receptor and are less sensitive to induction of AHH activity, and hybrid B6D2F1/J mice. C57BL/6J mice were approximately 10-fold more sensitive to thymic involution than DBA/2J mice and the hybrid B6D2F1/J were intermediate between the two parental strains. The progeny of a B6D2F1/J x DBA/2J mating were phenotyped (as Aa or aa), and the mice heterozygous for the high affinity receptor were more sensitive to thymic atrophy. The capacity of other halogenated aromatic hydrocarbons to produce thymic atrophy corresponded to their capacity to bind to the induction receptor, and nonhalogenated compounds which are agonists for the receptor (3-methylcholanthrene, β-naphthoflavone) also produced this toxic response. TCDD produced a dose-related incidence of cleft palate formation in the fetuses of C57BL/6J mice. A single dose of 30 µg/kg of TCDD on day 10 of pregnancy produced an incidence of cleft palates of 54% in C57BL/6J fetuses, 13% in B6D2F1/J fetuses and only 2% in DBA/2J fetuses. Five inbred strains of mice with a low affinity receptor developed only a 0-3% incidence of cleft palates from TCDD (30 µg/kg), while four of five inbred strains with a high affinity receptor developed a 50% or greater incidence. For TCDD and congeners, (a) the correspondence of the structure-activity relationship for receptor binding and for toxic potency and (b) the segregation of thymic involution and probably teratogenesis with the Ah locus indicate that binding of these compounds to the receptor is an essential step in their mechanism of toxicity.