Identification of concanavalin A receptors and galactose-binding proteins in purified plasma membranes of Dictyostelium discoideum.

Two techniques have been modified to provide simple means for the identification of molecules which bind concanavalin A (Con A). Crossed immunoelectrophoresis was altered by replacing antibody with Con A, and receptors were identified by the precipitin arcs which they produced. Con A, tagged with fluorescein isothiocyanate, was also diffused into prefixed sodium dodecyl sulfate (SDS)-polyacrylamide gels, and additional receptors identified by fluorescence. More than 35 molecules in the plasma membranes of the cellular slime mold Dictyostelium discoideum which bind Con A were identified with these techniques. At least 12 of these diminish and 12 increase in importance as receptors during differentiation of the cells from the vegetative to the preculmination stage of development. In the course of these experiments, it was possible to confirm the presence of the galactose-binding protein discoidin, in the plasma membrane, by electrophoresing membrane proteins into an agarose gel. This lectin regains its sugar-binding activity after denaturation and electrophoresis in SDS.

Distribution and mobility of lectin receptors on synaptic membranes of identified neurons in the central nervous system.

The distribution and mobility of concanavalin A (Con A) and Ricinus communis agglutinin (RCA) receptors (binding sites) on the external surfaces of Purkinje, hippocampal pyramidal, and granule cells and their attached boutons were studied using ferritin-lectin conjugates. Dendritic fields of these cells were isolated by microdissection and gently homogenized. Cell fragments and pre- and postsynaptic membranes were labeled with the ferritin-lectin conjugates at a variety of temperatures, and the distribution of lectin receptors was determined by electron microscopy. Both classes of these lectin receptors were concentrated at nearly all open and partially open postsynaptic junctional membranes of asymmetric-type synapses on all three neuron types. Con A receptors were most concentrated at the junctional membrane region, indicating that the mature neuron has a specialized nonrandom organization of carbohydrates on its outer surface. Lectin receptors located on postsynaptic junctional membranes appeared to be restricted in their mobility compared to similar classes of receptors on extrajunctional membrane regions. Labeling with ferritin-RCA and -Con A at 37 degrees C produced clustering of lectin receptors on nonjunctional surfaces; however, Con A and RCA receptors retained their nonrandom topographic distribution on the postsynaptic junctional surface. The restricted mobility of lectin receptors was an inherent property of the postsynaptic membrane since the presynaptic membrane was absent. It is proposed that structures in the postsynaptic density may be transmembrane-linked to postsynaptic receptors and thereby determine topographic distribution and limit diffusion of specialized synaptic molecules. Speicalized receptor displays may play an important role in the formation and maintenance of specific synaptic contacts.

Nonuniform distribution of concanavalin-A receptors and surface antigens on uropod-forming thymocytes.

Uropods can form spontaneously in a variable fraction of mouse thymocytes incubated for 30--60 min in vitro at temperatures between about 8 degrees and 37 degrees C. The majority of the cells with a typical uropod are medium and large thymocytes. The "normal" distribution of concanavalin-A receptors and antigens recognized by a rabbit anti-mouse thymocyte serum was studied on these cells by electron microscopy using ferritin-conjugated lectin or antibodies. The cells were fixed with glutaraldehyde or formaldehyde before labeling. The distribution was essentially uniform on spherical cells. On the contrary, on cells which had formed a uropod the labeled receptors and antigens appeared to be preferentially concentrated around the nucleus, and depleted over the uropod, and especially over the constriction at the base of the uropod. Uropod formation and inhomogeneous distribution were inhibited or reversed by cytochalasin B, but not by vinblastine or colchicine. When the same ligands were applied to unfixed cells, the labeled and cross-linked components capped normally towards the cytoplasmic pole of the cell. These observations are described in relation to the ability of receptors and antigens to interact with an intracellular mechanical structure, and to the mechanism of capping.

Distribution of cell surface saccharides on pancreatic cells. II. Lectin-labeling patterns on mature guinea pig and rat pancreatic cells.

The surface saccharide composition of collagenase-dispersed pancreatic cells from adult guinea pig and rat glands was examined by using eight lectins and their ferritin conjugates: Concanavalin A (ConA); Lens culinaris (LCL); Lotus tetragonolobus (LTL); Ricinus communis agglutinins I and II (RCA I, RCA II); Soybean agglutinin (SBA); Ulex europeus lectin (UEL); and wheat germ agglutinin (WGA). Binding studies of iodinated lectins and lectin-ferritin conjugates both revealed one population of saturable, high-affinity receptor sites on the total cell population (approximately 95% acinar cells). Electron microscopy, however, revealed differences in lectin-ferritin binding to the plasmalemma of acinar, centroacinar, and endocrine cells. Whereas acinar cells bound heavily all lectin conjugates, endocrine and centroacinar cells were densely labeled only by ConA, LCL, WGA, and RCA I, and possessed few receptors for LTL, UEL, and SBA. Endocrine and centroacinar cells could be differentiated from each other by using RCA II, which binds to centroacinar cells but not to endocrine cells. Some RCA II receptors appeared to be glycolipids because they were extracted by ethanol and chloroform-methanol in contrast to WGA receptors which resisted solvent treatment but were partly removed by papain digestion. RCA I receptors were affected by neither treatment. The apparent absence of receptors for SBA on endocrine and centroacinar cells, and for RCA II on endocrine cells, was reversed by neuraminidase digestion, which suggested masking of lectin receptors by sialic acid. The absence of LTL and UEL receptors on endocrine and centroacinar cells was not reversed by neuraminidase. We suggest that the differential lectin-binding patterns observed on acinar, centroacinar, and endocrine cells from the adult pancreas surface-carbohydrate-developmental programs expressed during morphogenesis and cytodifferentiation of the gland.

Domains of receptor mobility and endocytosis in the membranes of neonatal human erythrocytes and reticulocytes are deficient in spectrin.

It has previously shown (Schekman, R., and S.J. Singer, Proc. Natl. Acad. Sci. U.S.A. 73:4075-4079) that receptors in the membranes of neonatal human erythrocytes show a restricted degree of lateral mobility, whereas in adult human erythrocytes the receptors are essentially immobile. This restricted mobility is exhibited, for example, when concanavalin A (Con A) induces a limited clustering of its receptors in the neonatal erythrocyte membrane, resulting in the formation of invaginations and endocytic vesicles. This does not happen with adult cells. By the use of indirect immunoferritin labeling of ultrathin frozen sections of Con A-treated neonatal blood cells, we now show that the invaginations and endocytotic vesicles do not stain for spectrin, whereas the adjacent unperturbed membrane is heavily stained. The reticulocytes in the neonatal cell population undergo substantially more Con A-induced invagination and endocytosis than do the erythrocytes. These results lend strong support to the hypothesis that specialized discrete domains exist, or are induced, in the membranes of these neonatal cells, in which receptors are laterally mobile, whereas in the remaining (and predominant) part of the membrane the receptors are immobile. Such mobile domains are characterized by an absence of spectrin. During the maturation of the neonatal reticulocyte to erythrocyte, it is proposed that these domains are in large part, but not completely, eliminated.

A synthetic pentapeptide with biological activity characteristic of the thymic hormone thymopoietin.

The pentapeptide arginyl-lysyl-aspartyl-valyl-tyrosine, corresponding to amino acid residues 32--36 in thymopoietin, was synthesized. In vitro, this pentapeptide induced the differentiation of murine prothymocytes to thymocytes and inhibited differentiative induction of cells of the B lineage. This combination of actions is presently unique to the parent molecule thymopoietin. In vivo, the pentapeptide reduced the high numbers of autologous rosette-forming cells normally present in the spleens of athymic mice; this also is a property of thymopoietin. These results suggest that this readily synthesized pentapeptide corresponds to an active site of thymopoietin and might serve as a therapeutic substitute for thymopoietin.

Plasma membrane vesiculation: a new technique for isolation of plasma membranes.

Monolayer cell cultures of macrophages, monocytes, myoblasts, and density-inhibited and transformed fibroblasts form and release cell surface membrane vesicles following exposure to formaldehyde, related low-molecular-weight aldehydes, and disulfide blocking agents. Vesicles have a unique composition of proteins and lipids. They show enrichment of cholesterol and sphingomyelin content and a seven-to tenfold enrichment of 5'-nucleotidase activity. Vesicles also contain intramembranous particles and show a trilamellar unit membrane and no ultrastructural evidence of contamination with other cytoplasmic organelles. The technique is proposed as a novel method for isolating plasma membrane vesicles from cells in culture.

NMDA receptor losses in putamen from patients with Huntington's disease.

N-Methyl-D-aspartate (NMDA), phencyclidine (PCP), and quisqualate receptor binding were compared to benzodiazepine, gamma-aminobutyric acid (GABA), and muscarinic cholinergic receptor binding in the putamen and cerebral cortex of individuals with Huntington's disease (HD). NMDA receptor binding was reduced by 93 percent in putamen from HD brains compared to binding in normal brains. Quisqualate and PCP receptor binding were reduced by 67 percent, and the binding to other receptors was reduced by 55 percent or less. Binding to these receptors in the cerebral cortex was unchanged in HD brains. The results support the hypothesis that NMDA receptor-mediated neurotoxicity plays a role in the pathophysiology of Huntington's disease.

Drug and neurotransmitter receptors in the brain.

Biochemical investigation of receptors for neurotransmitters and drugs in the brain has been one of the most active areas of molecular neuroscience during the past decade. This work has permitted fundamental insights into how binding of neurotransmitters to their receptors excites or inhibits neuronal firing or changes cellular metabolism. The recognition of receptor subtypes has suggested subtle ways for neurotransmitters to modulate neuronal functioning. Finally, the ability to measure receptor sites in simple test tube systems and to distinguish readily between agonists and antagonists has provided useful probes for drug discovery programs.

Expression of thiazide-sensitive Na+-Cl- cotransporter in the rat endolymphatic sac.

The endolymphatic sac (ES) is a part of the membranous labyrinth and is believed to absorb endolymph. It has been well-established that the endolymph absorption is dependent on several ion transporters in a manner similar to that in the kidney, and the ES is regulated by hormones such as aldosterone and vasopressin that also affect on the kidney. The thiazide-sensitive Na(+), Cl(-) cotransporter (TSC) is an electroneutral cotransporter specific to the kidney that plays an important role in absorption of NaCl in renal tubules. In the inner ear, TSC expression has never been examined. The expression of TSC in the rat ES was examined by RT-PCR, in situ hybridization and immunohistochemistry. These analyses indicated that TSC genes and proteins were expressed in the rat ES. In contrast, it was not observed in the rat cochlea by RT-PCR. This is the first report confirming the expression of TSC in the ES.

Sodium-potassium pump, ion fluxes, and cellular dehydration in sickle cell anemia.

We studied the role of the sodium-potassium pump in erythrocytes of 12 patients with sickle cell anemia (SS). Ouabain-binding sites per cell and pump-mediated Rb/K uptake were significantly higher in SS patients than in white or black controls. Ouabain-resistant Rb/K influx was also greater than in normal controls or patients with sickle cell trait. Deoxygenation of SS erythrocytes increased ouabain-sensitive Rb/K influx without altering ouabain binding, presumably as the consequence of an increase in the passive influx of sodium. Deoxygenation increased mean corpuscular hemoglobin concentration (MCHC) by 5.5%, and studies of the density distribution of SS cells indicated an increase in highly dense fractions known to contain sickled erythrocytes. Ouabain prevented the rise in MCHC and reduced the percentage of dense cells. These findings indicate a magnified role for the sodium-potassium pump in the pathophysiology of SS erythrocytes and suggest that its inhibition might prove useful in therapy.

Effects of thyroid hormone on sodium pump sites, sodium content, and contractile responses to cardiac glycosides in cultured chick ventricular cells.

Sensitivity of cardiac muscle to digitalis glycosides depends on the thyroid state. The mechanism of this interaction was investigated at the cellular level using spontaneously beating monolayers of cultured chick embryo ventricular cells. Cells were grown for 48 h in serum-free medium containing concentrations of triiodothyronine (T3) from zero to 10(-7) M, and the total number of sodium pump sites, sodium content, and contractile amplitude in the presence and absence of various concentrations of ouabain were determined. T3 caused a concentration-dependent increase in the number of specific ouabain binding sites; the maximal increase to 160% of control was observed in response to 10(-8) M T3. T3 lowered steady-state cellular sodium content in a concentration-dependent manner, also. Ouabain (1 microM) exposure elevated cellular sodium content in all cells, but the increase was greatest in cells grown in T3-free medium and least in cells grown in 10(-8) M T3. The positive inotropic and toxic effects of ouabain in cells grown in 10(-8) M T3 were diminished at any given ouabain concentration, and thus, the dose-response curve was shifted to the right. These results indicate that T3 causes induction of additional sodium pump sites that are functional. The increased tolerance of hyperthyroid cells and reduced tolerance of hypothyroid cells to cardiac glycosides can be explained by these changes in the number of sodium pump sites and cellular sodium content, and consequently, calcium influx via sodium-calcium exchange.

Mitochondrial dysfunction induced by fatty acid ethyl esters, myocardial metabolites of ethanol.

Mechanisms responsible for alcohol-induced heart muscle disease have been difficult to elucidate partly because of previously obscure, demonstrable cardiac metabolism of ethanol. Recently, fatty acid ethyl esters were identified in our laboratory and found to be myocardial metabolites of ethanol. In the present study, they have been shown to induce mitochondrial dysfunction. Incubation of isolated myocardial mitochondria with fatty acid ethyl esters led to a concentration-dependent reduction of the respiratory control ratio index of coupling of oxidative phosphorylation and decrement of maximal rate of oxygen consumption. Furthermore, fatty acid ethyl esters were demonstrated to bind to mitochondria in vitro, and, importantly, 72% of intracellularly synthesized ethyl esters were found to bind to mitochondria isolated from intact tissue incubated with ethanol. Protein binding of fatty acid ethyl esters was markedly less than that of fatty acids. Because uncoupling of mitochondrial oxidative phosphorylation correlated with the cleavage of fatty acid ethyl ester shown to be initially bound to mitochondria, with resultant generation of fatty acid, a potent uncoupler, in a locus in or near the mitochondrial membrane, fatty acid ethyl esters may contribute to a potentially toxic shuttle for fatty acid with transport from physiological intracellular binding sites to the mitochondrial membrane; direct effects of fatty acid ethyl esters may also be deleterious. Operation of this shuttle as a result of ethanol ingestion and subsequent accumulation of fatty acid ethyl esters may account for the impaired mitochondrial function and inefficient energy production associated with toxic effects of ethanol on the heart.

In vitro and ex vivo distribution of [3H]harmane, an endogenous beta-carboline, in rat brain.

The endogenous beta-carboline, harmane, has been shown to bind to monoamine oxidase A (MAO-A) and a separate, high affinity, non-MAO site. Research in our laboratory has shown that harmane is an active component of clonidine-displacing substance (CDS), the proposed endogenous ligand for imidazoline binding sites (IBS). In the present study we have investigated the distribution of [3H]harmane in rat brain, and related the binding profile to the distribution of the MAO-A selective ligand [3H]Ro41-1049 and the I2BS ligand [3H]2-BFI. The in vivo distribution of [3H]harmane following intravenous administration was also investigated. Receptor autoradiography revealed a highly significant correlation for the distribution of [3H]harmane and [3H]Ro41-1049, and a significant correlation for [3H]harmane and the I2BS ligand [3H]2-BFI. The in vivo distribution of [3H]harmane suggests that the ligand accumulates in the adrenal gland and throughout the brain with the primary route of excretion occurring via the duodenum. In conclusion, these studies have shown that [3H]harmane labels a population of binding sites that reflect the distribution of MAO-A. Further evidence for a non-MAO, IBS [3H]harmane population has not been shown but the high level of expression of the MAO-A site is likely to have masked the much smaller population of I2BS.

Actions of ZD0947, a novel ATP-sensitive K+ channel opener, on membrane currents in human detrusor myocytes.

BACKGROUND AND PURPOSE: ATP-sensitive K+ channels (K(ATP) channels) play important roles in regulating the resting membrane potential of detrusor smooth muscle. Actions of ZD0947, a novel KATP channel opener, on both carbachol (CCh)-induced detrusor contractions and membrane currents in human urinary bladder myocytes were investigated. EXPERIMENTAL APPROACH: Tension measurements and patch-clamp techniques were utilized to study the effects of ZD0947 in segments of human urinary bladder. Immunohistochemistry was also performed to detect the expression of the sulphonylurea receptor 1 (SUR1) and the SUR2B antigens in human detrusor muscle. KEY RESULTS: ZD0947 (> or = 0.1 microM) caused a concentration-dependent relaxation of the CCh-induced contraction of human detrusor, which was reversed by glibenclamide. The rank order of the potency to relax the CCh-induced contraction was pinacidil > ZD0947 > diazoxide. In conventional whole-cell configuration, ZD0947 (> or = 1 microM) caused a concentration-dependent inward K+ current which was suppressed by glibenclamide at -60 mV. When 1 mM ATP was included in the pipette solution, application of pinacidil or ZD0947 caused no inward K+ current at -60 mV. Gliclazide (< or =1 microM), a selective SUR1 blocker, inhibited the ZD0947-induced currents (Ki = 4.0 microM) and the diazoxide-induced currents (high-affinity site, Ki1 = 42.4 nM; low-affinity site, Ki2 = 84.5 microM) at -60 mV. Immunohistochemical studies indicated the presence of SUR1 and SUR2B proteins, which are constituents of KATP channels, in the bundles of human detrusor smooth muscle. CONCLUSIONS AND IMPLICATIONS: These results suggest that ZD0947 caused a glibenclamide-sensitive detrusor relaxation through activation of glibenclamide-sensitive KATP channels in human urinary bladder.

Preparation of monoclonal antibody against celangulin V and immunolocalization of receptor in the oriental armyworm, Mythimna separata walker (Lepidoptera: Noctuidae).

The botanical insecticide celangulin V (CA-V) is an insect digestive poison acting on midgut tissue of the target insect larvae. With the aim of localizing the receptor enacted by CA-V, monoclonal antibodies (MAbs) specific to the compound were developed. A hapten was synthesized by introducing a succinoyl into the CA-V structure and conjugated with three carrier proteins. From mice immunized with one conjugate, three MAbs were obtained with a potential capacity of detecting protein-bound residue forms of CA-V in the biological tissues. The oriental armyworm larvae ingested CA-V were examined by the technique of immuno-electron-microscopy (IEM) using the anti-CA-V MAb as the primary antibody and goat anti-mouse/IgG labeled with colloidal gold as the secondary antibody. Electron micrographs of the armyworm midgut tissues showed that the CA-V was associated with the midgut epithelia of the insects. These results demonstrated the existence of a receptor enacted by CA-V on the midgut cells of the oriental armyworm larvae.

[Imidazoline I(2) receptors as a possible marker for malignancy in human glial cell tumours]. / Los receptores para imidazolinas I(2) como posible marcador de malignidad en tumores gliales humanos.

AIM: To present the experimental data that support the hypothesis that the imidazoline I(2) receptors may be assessed as a biological marker to establish diagnosis and grade of human gliomas. DEVELOPMENT: Gliomas constitute the most important group of brain neoplasm in humans. In these tumours accurate histopathologic diagnosis is a first crucial prerequisite for patient treatment. However, current grading schemes are still limited by subjective histologic criteria. Therefore, the search for new molecular and biological markers of gliomas represents a crucial step. In this context, it has been reported a significant increase in I(2) density in human gliomas when compared with normal brain tissue and other intracranial non-glial tumours. Moreover, this increase seems to fit well with the degree of malignancy in human gliomas. Thus, in glioblastomas multiformes the I(2) density is 1.4 times higher than in anaplastic astrocytomas and 2.2 higher than in low-grade astrocytomas. CONCLUSIONS: The present results demonstrate that the measurement of the I(2) density by positron emission tomography techniques could be used in the future for grading and prognosis of human gliomas. This could avoid the current need for tumour biopsies in order to obtain a histopathologic diagnosis.

Structure-activity study of the actions of camptothecin derivatives on mammalian topoisomerase I: evidence for a specific receptor site and a relation to antitumor activity.

Twenty-two compounds related to camptothecin, a known inhibitor of eukaryotic topoisomerase I, were studied. The following effects on the actions of topoisomerase I were observed and were well correlated among most of the compounds studied: (a) inhibition of the first-order rate of relaxation of supercoiled DNA; (b) conversion of supercoiled DNA to nicked circles; and (c) single-strand cleavage of linear DNA at specific sites. The locations of the stimulated cleavage sites were the same for all of the active derivatives. Stereochemistry and the positions of substituents were found to be crucial for the presence or absence of effects on topoisomerase I, indicating that the compounds interact with an asymmetrical receptor site on the enzyme or enzyme-DNA complex. From the structure-activity relations, the regions of interaction between the camptothecin ring system and the receptor site were inferred. Striking correlations were observed between activity against topoisomerase I and reported activity against murine leukemias, indicating that an action on topoisomerase I is responsible for the antitumor activity of the camptothecins.

Identification of Vinca alkaloid acceptors in P388 murine leukemia cells with a photoactive analogue of vinblastine.

The cytotoxic, antimitotic, and growth inhibition properties of a photoactive analogue of vinblastine, N-(p-azidobenzoyl)-N'-beta-aminoethylvindesine (NABV), and vinblastine on P388 murine leukemia cells were compared. After 72-h exposure, the 50% drug-inhibitory concentrations for exponentially growing P388 leukemic cells were 1.2 nM for NABV and 0.6 nM for vinblastine. The ultrastructural effects of NABV and vinblastine on P388 cells were similar: formation of tubulin paracrystals; mitotic arrest (C-mitosis); increased post-C-mitotic multinucleated cells; increased number of annulate lamellae; and the appearance of intracytoplasmic paired cisternae. [3H]NABV was used to identify Vinca alkaloid binding sites in P388 cells by photoaffinity labeling. After irradiation at 302 nm, radioactive Vinca alkaloid binding components were resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and identified in 1-mm gel slices. The most prominent photolabeled species were Mr 44,000, 54,000, and 75,000 polypeptides located in the 100,000 X g supernatant fraction. The Mr 54,000 component was also observed in the membrane fraction. Specific photolabeling of Mr 54,000 and 44,000 polypeptides was blocked in the presence of 20 microM excess of vinblastine and was saturable with half-maximal saturation concentrations of 0.18 and 0.4 microM [3H]NABV, respectively. The Mr 54,000 component was identified as a tubulin subunit by immunoprecipitation with antitubulin monoclonal antibodies. Since NABV and vinblastine have similar pharmacological and biological properties, this photoactive analogue may be useful for identifying important Vinca alkaloid cellular acceptors which may be responsible for drug cytotoxic and antineoplastic activities.

Ah receptor in human placenta: stabilization by molybdate and characterization of binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin, 3-methylcholanthrene, and benzo(a)pyrene.

Aryl hydrocarbon hydroxylase (AHH, cytochrome P1-450) is highly inducible in several human cells and tissues exposed to specific halogenated and nonhalogenated aromatic chemicals of the "3-methylcholanthrene-type." In laboratory animals AHH induction is known to be regulated by binding of inducers to the Ah receptor, a soluble intracellular protein. However, the induction mechanism in the human species is incompletely understood largely because the Ah receptor, which seems to be essential to the induction process, has not previously been detectable in certain human cells and tissues (including placenta) that are highly responsive to AHH induction. We found that human placenta contains high concentrations of Ah receptor (comparable to the receptor concentrations in rat and mouse liver) but that special modifications were necessary in the assay techniques in order to detect and accurately quantitate receptor binding. Receptor was detected at concentrations approximately equal to 100 fmol/mg cytosol protein using [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as the radioligand. This high concentration of specific binding sites was present only if the placental tissue was initially homogenized in a buffer containing sodium molybdate (10 or 20 mM). Without molybdate in the homogenizing buffer, specific [3H]TCDD binding was only about 35 fmol/mg. Specific Ah receptor binding also was detectable with [3H]-3-methylcholanthrene and, to a lesser extent, with [3H]-benzo(alpha)pyrene. The receptor sedimented near 9S on sucrose gradients whether molybdate was present or not. About 80% of specific binding was lost if excessive charcoal was used to adsorb "nonspecifically bound" ligand from cytosol prior to gradient analyses. The apparent affinity with which [3H]TCDD bound to Ah receptor in human placental cytosol was relatively low (apparent Kd approximately equal to 5 to 8 nM) when compared with the affinity of [3H]TCDD binding in rat or mouse hepatic cytosols (Kd approximately equal to 1 to 3 nM). These data suggest that while molybdate has very little effect on the quantity or molecular size of the rodent Ah receptor assay, it is very important in stabilizing the human Ah receptor. Our experiments demonstrate that human placenta contains a high concentration of Ah receptor and suggest that AHH induction in placenta is mediated through a receptor mechanism analogous to that previously established in tissues and cells from laboratory animals.