The Surface Morphology and Fine Structure of CHO (Chinese Hamster Ovary) Cells Following Enucleation

Chinese hamster ovary cells grown in monolayer culture and exposed to cytochalasin B were enucleated by centrifugation. Thereafter, the karyoplasts (the nucleated parts obtained from the bottoms of the centrifuge tubes) and the cytoplasts (the enucleated cytoplasmic parts attached to the coverslips) were allowed to recover and subsequently were examined by scanning and transmission electron microscopy. Microscopy of thin sections revealed that the karyoplasts, limited by an intact plasma membrane, contain an intact nucleus surrounded by a layer of cytoplasm that includes ribosomes, mitochondria, and fragments of the endoplasmic reticulum, but no centrioles or microtubules. The cytoplasts, similarly examined, appear to contain all cytoplasmic organelles and systems, including centrioles and microtubules. The karyoplasts, when replated in fresh medium adhere to the substrate but remain essentially spherical and are incapable of motility. They disintegrate in about 72 hr. The cytoplasts, under identical conditions, recover a shape similar to that of the whole Chinese hamster ovary cell and display some motility. They generally survive not more than 48 hr. It appears that this enucleation procedure consistently separates the nucleus and limited cytoplasm from the centrosphere and microtubule-containing cytoplasts and, furthermore, that the formdetermining and motility mechanisms reside in the cytoplast and function without nuclear participation for the short period of viability.     

Cytoplasts made from human blood polymorphonuclear leukocytes with or without heat: preservation of both motile function and respiratory burst oxidase activity.

Anucleate fragments (cytoplasts) from polymorphonuclear leukocytes (PMN) are simplified systems that can be used to elucidate specific pathways by which cell function is altered. PMN cytoplasts in current use are defective either in activatable respiratory burst oxidase activity or in motile function. By centrifugation of PMN on discontinuous gradients of Ficoll without cytochalasin B, we have created granule-poor cytoplasts in which both these capacities are preserved. Specifically, they generate superoxide anion (O2-.) and reduce nitroblue tetrazolium dye on appropriate stimulation; they respond chemotactically to erythrocytes destroyed by laser microirradiation or to the specific chemoattractants fMet-Leu-Phe (10 nM) and C5a (zymosan-activated serum); and they ingest and kill staphylococci. We can improve the yield of these fragments progressively by preheating (45 degrees C) the cells in suspension for increasing periods of time, but those treatments are attended by a decreasing percentage of cytoplasts with activatable oxidase activity, and a progressive inability of the cytoplasts to ingest and to kill staphylococci. These easily made and multipotent cytoplasts readily lend themselves to studies of PMN physiology.     

Possible role of nucleus-membrane interaction in capping of surface membrane receptors.

Interaction of multivalent ligands and cell surface receptors can induce redistribution of these receptors to form patches and caps. In this study, we have investigated the role of nucleus-membrane interaction in the capping of membrane components. Mouse L cells and leukemia EL4 cells were enucleated with the aid of cytochalasin B, yielding cytoplasts and karyoplasts. Capping of surface receptors was induced by allo- and hetero-immune sera followed by fluorescein-conjugated antiglobulin serum, or by the plant lectin concanavalin A. Capping could easily be induced in intact cells, but virtually no capping was detected in the nucleus-free cytoplasts. Interestingly, karyoplasts, which posses cell-membrane components but very little cytoplasm, could be easily induced to cap their surface antigens. Hence, cap formation of membrane components seems not to be an autonomous membrane process. The data suggest that interaction of surface membranes and inner cell components associated with the nucleus is involved in the movement of surface membrane receptors.     

Alternative method for identifying reconstituted cells

Enucleation techniques combining mild centrifugation in the presence of cytochalasin B permit cells to be separated into nuclear fragments (karyoplasts) and cytoplasmic fragments (cytoplasts). These fragments, though stable for a short time, will ultimately degenerate by the procedures described in this report. One can, however, fuse cytoplasts to karyoplasts by using polyethylene glycol and obtain viable reconstituted cells whose properties may be useful for understanding some aspects of the nuclear-cytoplasmic interactions associated with tumorigenicity and steroidogenesis. However, the presence of cybrids, hybrids, and parental whole cell contaminants along with the reconstituted cell population make it necessary to have genetic markers that reside in both the nucleus and cytoplasm in order to preferentially identify reconstituted cells derived from a karyoplast fused to a cytoplast. By utilizing the Y-1 cell line, which is tumorigenic and responds to corticotropin by secreting steroids, and the AMT-BU-A1 (AMT) cell line, which is nontumorigenic and does not respond to corticotropin but has a nuclear marker, BrdUrd^r, and a cytoplasmic marker, CAP^r, we have reconstituted cells containing Y-1 karyoplasts and AMT cytoplasts. In this report we extend our previous techniques by describing an identification procedure that allowed us to isolate cells reconstituted from AMT karyoplasts fused to Y-1 cytoplasts. The results of these experiments support the concept that with these cell lines the nucleus (karyoplast) is ultimately sufficient to control the phenotypic expression or suppression of tumorigenicity and steroidogenesis.     

Cellular cytotoxicity mediated by granule-depleted neutrophil cytoplasts

Summary Neutrophil-derived nucleus- and granule-free cytoplasts, consisting of cytosol enclosed by an intact plasma membrane, were able to destroy ⁵¹Cr-labelled ox red blood cells (ORBC) in the presence of phorbol myristate acetate (PMA). The slope of the target cell lysis vs the log of the cytoplast number was similar to that observed with neutrophils as effector cells. Nevertheless, a number of cytoplasts 60–80 times higher than that of neutrophils was required to obtain a common level of cytotoxicity. The ability of cytoplasts and neutrophils to lyse ORBC was completely abolished by catalase and unaffected by superoxide dismutase and mannitol, suggesting the involvement of hydrogen peroxide in the target cell damage. Addition of myeloperoxidase (MPO) to cytoplasts increased lysis. The MPO inhibitor azide significantly reduced the cytolysis by neutrophils, but not the cytolysis by cytoplasts, except when experiments were carried out in the presence of MPO. The results indicate that neutrophil cytosol and plasma membrane represent the basic requirement for the PMA-dependent cytolytic process, whereas MPO behaves as a device to amplify lysis.     

Regulation of polymorphonuclear leukocyte degranulation and oxidant production by ceramide through inhibition of phospholipase D

Exogenous C(2)-ceramide has been shown to inhibit polymorphonuclear leukocyte (PMN) phagocytosis through inhibition of phospholipase D (PLD) and downstream events, including activation of extracellular signal-regulated kinases 1 and 2, leading to the hyphothesis that the sphingomyelinase pathway is involved in termination of phagocytosis. Here it is postulated that increased PLD activity generating phosphatidic acid and diacylglycerol (DAG) is essential for superoxide release and degranulation and that ceramide, previously shown to be generated during PMN activation, inhibits PLD activation, thereby leading to inhibition of PMN function. When PMNs were primed with granulocyte colony-stimulating factor (G-CSF) and then activated with N-formyl-methionyl-leucyl-phenylalanine (FMLP), C(2)-ceramide (10 microM) completely inhibited release of superoxide, lactoferrin, and gelatinase; the DAG analog sn-1,2-didecanoylglycerol (DiC10) (10 microM) restored oxidase activation and degranulation in the ceramide-treated cells. Similarly, C(2)-ceramide inhibited oxidase activity and degranulation of PMNs treated with cytochalasin B followed by FMLP, and DiC10 restored function. In contrast, C(2)-ceramide did not inhibit phosphorylation of p47phox or p38 mitogen-activated protein kinase, or translocation of p47phox, PLD-containing organelles, adenosine diphosphate-ribosylation factor 1, RhoA, protein kinase C (PKC)-beta or PKC-alpha to the plasma membrane in G-CSF or cytochalasin B-treated, FMLP-activated PMNs. PLD activity increased by 3-fold in G-CSF-primed PMNs stimulated by FMLP and by 30-fold in cytochalasin B-treated PMNs stimulated by FMLP. Both PLD activities were completely inhibited by 10 microM C(2)-ceramide. In conclusion, superoxide, gelatinase, and lactoferrin release require activation of the PLD pathway in primed PMNs and cytochalasin B-treated PMNs. Ceramide may affect protein interactions with PLD in the plasma membrane, thereby attenuating PMN activation.     

Human neutrophils contain an intracellular pool of putative receptors for the chemoattractant N-formyl-methionyl-leucyl-phenylalanine

Purified human peripheral blood neutrophils were disrupted by nitrogen cavitation or sonication and fractionated on sucrose density gradients in order to separate the plasma membranes and granule fractions. Quantitatively, the fractions containing the specific granules by marker enzyme/protein enrichment contained the most tritiated N-formyl- methionyl-leucyl-phenylalanine (fmet-leu-[3H]phe)-binding activity. Competitive binding experiments using unlabeled formyl peptide analogues indicated that the intracellular binding sites display the same structure-function specificity as formyl peptide receptors on intact polymorphonuclear leukocytes (PMN) or isolated plasma membranes. Analysis of the fractions for membrane, primary, and secondary granule markers, as well as the distribution of 125I-labeled plasma membranes in sucrose density gradients, indicated that the specific fmet-leu- [3H]phe binding to granule-containing fractions was not due to contamination by plasma membranes. In addition, membranes isolated from PMN previously stimulated with phorbol myristate acetate (PMA) demonstrated increased binding sites, while isolated membranes exposed to PMA under the same conditions failed to show such increases. The data lend direct support to the concept that there is an intracellular pool of fmet-leu-phe receptors that serves as a source of new surface membrane constituents and receptor material that may allow PMN to maintain functional responsiveness during chemotaxis.     

Identification of the stereospecific hexose transporter from starved and fed chicken embryo fibroblasts.

When deprived of D-glucose for 24 hr, chicken embryo fibroblasts exhibit a marked increase in hexose transport activity compared with that of control cells. Scatchard analysis of [3H]cytochalasin B binding to starved cell plasma membranes (46 pmol/mg) indicated a six-fold increase compared with fed cell plasma membranes (7.5 pmol/mg). Irradiation of starved cell plasma membranes with high-intensity UV light in the presence of 0.5 microM [3H]cytochalasin B resulted in covalent labeling of polypeptides of Mr 52,000 and 46,000. In fed cell plasma membranes irradiated under the same conditions, both polypeptides were labeled but at greatly decreased levels. In fact, labeling of the Mr 52,000 polypeptide was barely detectable. The amount of D-glucose-sensitive [3H]cytochalasin B covalent insertion into these membrane components was increased 11 +/- 2 (n = 4)-fold in starved versus fed cell plasma membranes. Photoaffinity labeling of both polypeptides in starved cell plasma membranes was inhibited by D-glucose, 3-O-methylglucose, 2-deoxyglucose, cytochalasin B, and cytochalasin A but not by D-sorbitol, L-glucose, or cytochalasin E. Half-maximal inhibition of labeling of the Mr 52,000 polypeptide occurred at 8 mM D-glucose whereas, for the Mr 46,000 polypeptide, half-maximal inhibition occurred at 40 mM D-glucose. It is concluded that (i) two hexose transport proteins, one of Mr 46,000 and one of Mr 52,000, have been identified in chicken embryo fibroblasts and (ii) the increased affinity labeling of these transporter components after cell starvation may reflect increased numbers of transporters in the plasma membrane.     

Centrosomal control of microtubule dynamics

In many animal cells, minus ends of microtubules (MTs) are thought to be capped by the centrosome whereas plus ends are free and display dynamic instability. We tested the role of the centrosome by examining MT behavior in cytoplasts from which the centrosome was removed. Cells were injected with Cy3–tubulin to fluorescently label MTs and were enucleated by using a centrifugation procedure. Enucleation resulted in a mixture of cytoplasts containing or lacking the centrosome. Fibroblast (CHO-K1) and epithelial (BSC-1) cells were investigated. In fibroblast cytoplasts containing the centrosome, MTs showed dynamic instability indistinguishable from that in intact cells. In contrast, in cytoplasts lacking the centrosome, MTs treadmilled—shortened at the minus end at about 12 μm/min while growing at the plus end at the same rate. The change in behavior of the plus end from dynamic instability to persistent growth correlated with an elevated level of free tubulin subunits (78% in centrosome-free cytoplasts vs. 44% in intact cells) generated by minus-end depolymerization. In contrast to fibroblast cells, in centrosome-free cytoplasts prepared from epithelial cells, MTs displayed dynamic instability at plus ends and relative stability at minus ends presumably because of specific minus-end stability factors distributed throughout the cytoplasm. We suggest that, in fibroblast cells, a minus-end depolymerization mechanism functions to eliminate errors in MT organization and that dynamic instability of MT plus ends is a result of capping of minus ends by the centrosome.     

Anomalous neutrophil granule distribution in a patient with lactoferrin deficiency: pertinence to the respiratory burst

Neutrophils from a patient with lactoferrin deficiency were examined and the quantity and subcellular localization of protein markers were determined on Percoll density gradients. Distribution of azurophilic and specific granule markers was abnormal in that azurophilic granules were lighter than normal and appeared in the fraction of the gradient where normally the specific granules sediment. The specific granule membrane markers, cytochrome b-235 and its associated flavoprotein, were abnormally distributed in the gamma fraction, the site of the plasma membrane marker alkaline phosphatase. Thus, the b-cytochrome-flavoprotein complex had either been incorporated into the plasma membrane or was still present in the membranes of granules that were abnormally light and cosedimented with the plasma membranes. This is of particular interest in regard to the patient's respiratory burst oxidase function, since the b-cytochrome/flavoprotein complex normally translocates from the specific granules to the plasma membrane to constitute the active respiratory burst oxidase. The functional consequences of this abnormal distribution are discussed, as is the importance of characterizing both intragranular enzymatic markers and granule membrane proteins to define granular disorders.     

Nuclear localization of unoccupied receptors for glucocorticoids, estrogens, and progesterone in GH3 cells

We have previously shown that the unoccupied receptor for estrogen is not present in enucleated cells (cytoplasts) and probably is nuclear in cells that have been enucleated using cytochalasin B and centrifugation. We now demonstrate, using enucleation without cytochalasin, that the unoccupied receptors for glucocorticoids and progesterone also appear to be nuclear, and that enucleation without using cytochalasin results in the same distribution of estrogen receptor as that seen when this drug is used. Enucleated cells (cytoplasts) contained only 5-10% of the concentration of unoccupied receptors found in the whole cells. The unoccupied receptors for all three hormones were recovered instead in the nucleus-containing cell fragments (nucleoplasts). The cytosolic marker lactate dehydrogenase was present in the cytoplasts at the same or higher concentration than in the whole cells or nucleoplasts. Cytoplasts were formed from approximately 90% of the cells. When the nucleoplasts were homogenized, the unoccupied receptors for all three hormones were extracted into the cytosol, as is usually seen in homogenized cells and tissues. These results are, therefore, consistent with the possibility that the unoccupied glucocorticoid and progesterone receptors, like the unoccupied estrogen receptor, are nuclear rather than cytoplasmic proteins. In addition, localization of these unfilled receptors does not appear to be an artifact of cytochalasin treatment.     

Abnormal adherence-related functions of neutrophils, monocytes, and Epstein-Barr virus-transformed B cells in a patient with C3bi receptor deficiency

We evaluated a 3-year-old female patient with leukocytosis, recurrent infections, severe periodontal disease, and a history of delayed separation of the umbilical stump. This patient's polymorphonuclear leukocytes (PMNs) had normal membrane depolarization responses, normal oxygen metabolism, normal granule secretion responses, normal bactericidal activity, and normal C3b rosetting. However, by fluorescent cell analysis and C3bi rosetting, it was determined that her cells lacked the C3bi receptor. In addition, the patient's PMNs showed markedly abnormal chemotaxis, adherence, and aggregation responses, and partial abnormalities were detected in PMN spreading and phagocytosis. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that the subject's neutrophil cytoplasts were missing a 180,000-dalton moiety. Her monocytes also had defective chemotaxis and failed to adhere and grow normally in culture. Epstein-Barr virus- transformed B cells from the patient lacked an aggregation response to phorbol myristate acetate. Laboratory and clinical evaluations of this patient's mother showed no abnormalities. These studies demonstrate that C3bi receptor deficiency can be associated with functional abnormalities in multiple myeloid cells and that the absence of C3bi receptor is associated with abnormal adherence-related functions of these cells.     

Regulation of intramitochondrial cholesterol transfer to side-chain cleavage cytochrome P-450 in rat adrenal gland.

Rat adrenal mitochondria accumulated cholesterol during ether stress in vivo when side-chain cleavage was inhibited by aminoglutethimide (control = 14.6 vs. aminoglutethimide = 26.5 micrograms of cholesterol per mg of protein). This accumulation was insensitive to simultaneous administration of cycloheximide (24.2 micrograms/mg), but side chain cleavage in the mitochondria was greatly decreased. Outer and inner mitochondrial membrane fractions were separated by discontinuous Ficoll gradient centrifugation. Quantitation of marker enzymes for inner, outer, and microsomal enzymes indicated that outer membranes contained less than 5% inner membranes. The inner membrane fraction contained less than 7% outer membrane and included 90% of mitochondrial cytochrome P-450. Electron microscopy revealed outer membranes as circular intact ghosts, whereas inner membranes were largely intact and retained vesicular structure typical of intact adrenal cortex mitochondria. Administration of aminoglutethimide effected a 2-fold increase in inner membrane cholesterol (9.4 vs. 20.1 micrograms/mg) but simultaneous administration of cycloheximide completely blocked this increase (10.9 micrograms/mg). We conclude that: (i) in the presence of aminoglutethimide, stress stimulates accumulation of cholesterol in the inner membrane of adrenal mitochondria; and (ii) transfer of cholesterol from outer to inner membranes requires a cycloheximide-sensitive agent.     

Cytochalasin B: Effect on Lysosomal Enzyme Release from Human Leukocytes

The morphological and biochemical consequences of treatment of human peripheral blood leukocytes with cytochalasin B were studied. Incubation of human polymorphs with cytochalasin B resulted in nuclear and cytoplasmic spreading, but not in spontaneous release of lysosomal enzymes. Cytochalasin B inhibited particle uptake. Consequently, phagocytic vacuoles were not observed; instead, granule contents were discharged directly into the surrounding medium when cytochalasin B-treated cells were challenged with zymosan particles. Cytochalasin B enhanced the release of lysosomal enzymes from human polymorphonuclear leukocytes whether these encountered zymosan particles or immune complexes on a nonphagocytosable Millipore filter. Cytochalasin B-treated leukocytes thus constitute a model system for quantitative study of lysosome fusion. Augmented enzyme release was blocked by prior treatment of cells with pharmacological doses of agents that influence the accumulation of cyclic nucleotides (cyclic nucleotides themselves, prostaglandin E₁) or by compounds that interfere with microtubule function (e.g., colchicine, vinblastine). These observations suggest that one action of cytochalasin B on phagocytic cells is to remove the normal constraints to merger of granules, either with each other or with the plasma membrane, and that intact microtubule function is required for translocation of lysosomes.     

Monoclonal antibodies to a particulate superoxide-forming system stimulate a respiratory burst in intact guinea pig neutrophils

Monoclonal rat antibodies were produced against a subcellular preparation of phorbol 12-myristate 13-acetate (PMA)-stimulated guinea pig neutrophils that retains NADPH-oxidase activity. Two antibodies, 1A10.4 and IG4, were isolated that bind to a surface antigen restricted to guinea pig neutrophils from bone marrow and peritoneal exudate and to macrophages and that trigger a respiratory burst in neutrophils in the presence of cytochalasin B. Intact antibody 1A10.4, subclass IgG2c, can trigger superoxide anion release directly; F(ab')2 fragments of 1A10.4 and intact IG4 require further cross-linking by F(ab')2 fragments of anti-rat immunoglobulin antibody. Both antibodies recognize the same antigen, a proteolipid of apparent molecular mass 10 kDa. Immunoprecipitation of solubilized oxidase activity with 1A10.4 brings down this activity as part of a macromolecular complex. Surface expression of the antigen is increased on treatment of cells with both PMA and cytochalasin B. 1A10.4 also triggers release of the granule enzyme beta-glucuronidase. Triggering of a respiratory burst by the antibodies appears distinct from the PMA and fMet-Leu-Phe signalling systems. These studies indicate that the antigen defined by antibodies 1A10.4 and IG4 becomes associated with the superoxide anion-generating system of neutrophils but may play a more general role in signal transduction in phagocytic cells.     

Zymogen granule exocytosis is characterized by long fusion pore openings and preservation of vesicle lipid identity

The dynamics of the fusion pore that forms between a secretory vesicle and the plasma membrane are important in the regulation of both exocytosis and endocytosis. Here, we describe characteristics of fusion during zymogen granule exocytosis in exocrine pancreatic acinar cells. By using fluorescence recovery after photobleaching techniques, we show that the fusion pore remains open to allow free aqueous exchange with the vesicle lumen. There is no lipid interchange between the plasma and granule membranes during this time, and at the end of its life, the intact granule shrinks in situ, probably by a gradual pinching off of membrane patches. We propose that the protracted fusion pore lifetime is adapted to permit compound exocytosis, whereby the lingering primary granule acts as a conduit through which the contents of a secondary granule can be released. The lack of lipid intermixing may then facilitate selective recycling of granule membrane and preservation of apical membrane integrity.     

Evidence that insulin causes translocation of glucose transport activity to the plasma membrane from an intracellular storage site.

The glucose transport activity of fat cells was assayed in a cell-free system. The activity was solubilized and incorporated into egg-lecithin liposomes. The carrier-mediated glucose transport activity was estimated by subtracting the cytochalasin B-insensitive component from the total glucose uptake activity of the modified liposomes. When a crude microsomal preparation from fat cells was fractionated by sucrose density gradient centrifugation, two transport activities (peaks A and B) were separated. Peak A coincided with the peak of 5'-nucleotidase, a marker of the plasma membrane. Peak B appeared to coincide with the peak of UDPGal:N-acetylglucosamine galactosyltransferase, a marker of the Golgi apparatus. Peak A was considerably smaller than peak B under basal conditions. When cells were exposed to 1 nM insulin for 5 min before homogenization, the height of peak A increased whereas that of peak B decreased. Insulin had no significant effect on the galactosyltransferase activity. The Km values of glucose transport facilitated by the activities in peaks A and B were both approximately 10-15 mM. These results imply that insulin facilitates translocation of the transport activity from an intracellular storage site to the plasma membrane.     

Intracellular localization of DNA polymerase alpha.

Immunoglobulin (IgG) and the F(ab')2 fragment of IgG were prepared from serum of a rabbit immunized with purified calf thymus DNA polymerase alpha (deoxynucleosidetriphosphate:DNA deoxynucleotidyltransferase, EC 2.7.7.7). An indirect immunofluorescent method based on these reagents was used to detect the intracellular localization of DNA polymerase alpha in primary fetal bovine fibroblasts. The results show that the bulk of DNA polymerase alpha is located in the perinuclear region of the cytoplasm. Immunofluorescent staining of cytoplast and Ficoll-Paque gradient-purified karyoplast fragments resulting from cytochalasin enucleation show the presence of DNA polymerase alpha in cytoplasts and the virtual absence of the enzyme in the nucleus of the karyoplast itself. The implication of this unusual intracellular location for DNA polymerase alpha is discussed.     

Eosinophil granules function extracellularly as receptor-mediated secretory organelles

Intracellular granules in several types of leukocytes contain preformed proteins whose secretions contribute to immune and inflammatory functions of leukocytes, including eosinophils, cells notably associated with asthma, allergic inflammation, and helminthic infections. Cytokines and chemokines typically elicit extracellular secretion of granule proteins by engaging receptors expressed externally on the plasma membranes of cells, including eosinophils. Eosinophil granules, in addition to being intracellular organelles, are found as intact membrane-bound structures extracellularly in tissue sites of eosinophil-associated diseases. Neither the secretory capacities of cell-free eosinophil granules nor the presence of functional cytokine and chemokine receptors on membranes of leukocyte granules have been recognized. Here, we show that granules of human eosinophils express membrane receptors for a cytokine, IFN-γ, and G protein–coupled membrane receptors for a chemokine, eotaxin, and that these receptors function by activating signal-transducing pathways within granules to elicit secretion from within granules. Capacities of intracellular granule organelles to function autonomously outside of eosinophils as independent, ligand-responsive, secretion-competent structures constitute a novel postcytolytic mechanism for regulated secretion of eosinophil granule proteins that may contribute to eosinophil-mediated inflammation and immunomodulation.     

3,5,3'-tri-iodothyronine enhances sugar transport in rat thymocytes by increasing the intrinsic activity of the plasma membrane sugar transporter

We have shown that 3,5,3'-tri-iodothyronine (T3) produces a prompt increase in sugar transport in rat thymocytes by increasing the maximal velocity without changing the Michaelis-Menten constant of the plasma membrane sugar transport system. To elucidate further the mechanism of this effect, we have now assessed the influence of T3 on the number and affinity of sugar transporters in thymocytes, measured as the sugar (2-deoxyglucose; dGlc)-displaceable binding of cytochalasin B. Cytochalasin B inhibited in a dose-related manner the uptake of dGlc by rat thymocytes with inhibition constant values of 0.19 and 0.22 mumol/l in the presence and absence of T3 respectively. Binding of cytochalasin B by the sugar-displaceable sites was rapid and saturable, demonstrating a single class of sites having an apparent dissociation constant of 0.33 +/- 0.02 (S.D.) mumol/l and maximal binding capacity of 3.73 +/- 0.48 pmol/20 x 10(6) cells (11.2 +/- 1.4 x 10(4) sites/thymocyte). In the rat thymocyte, sugar transporters were found to be located in two major subcellular pools, the plasma membrane and microsomes, the latter being about twice the size of the former. In these subcellular compartments, as well as in the intact cell, binding of [3H]cytochalasin B by the sugar-displaceable sites constituted about 40% of total cytochalasin B binding. 3,5,3'-Tri-iodothyronine in concentrations that stimulated uptake of dGlc by thymocytes had no effect on [3H]cytochalasin B binding (total and sugar-displaceable) in the intact cell and in the plasma membrane and microsomal compartments, nor did it influence the affinity and number of sugar transporters.(ABSTRACT TRUNCATED AT 250 WORDS)