The BeWo choriocarcinoma cell line as a model of iodide transport by placenta

Cultured human choriocarcinoma cells of the BeWo line exhibited saturable accumulation of radioiodide. Inhibition by competing anions followed the affinity series perchlorate >> iodide > or = thiocyanate, consistent with uptake through the thyroid iodide transporter, NIS, whose messenger RNA was found in BeWo cells, and whose protein was distributed towards the apical pole of the cells. Efflux obeyed first order kinetics and was inhibited by DIDS, an antagonist of anion exchangers including pendrin, whose messenger RNA was also present. In cultures where iodide uptake through NIS was blocked with excess perchlorate, radioiodide accumulation was stimulated by exposure to medium in which physiological anions were replaced by 2-morpholinoethanesulfonic acid (MES), consistent with the operation of an anion exchange mechanism taking up iodide. Chloride in the medium was more effective than sulfate at inhibiting this uptake, matching the ionic specificity of pendrin. These studies provide evidence that the trophoblast accumulates iodide through NIS and releases it to the fetal compartment through pendrin.

Facilitation of renal autoregulation by angiotensin II is mediated through modulation of nitric oxide

AIMS

This study was designed to investigate the influence of angiotensin II (Ang II) and nitric oxide (NO) on autoregulation of renal perfusion.

METHODS

Autoregulation was investigated in isolated perfused kidneys (IPRK) from Sprague-Dawley rats during stepped increases in perfusion pressure.

RESULTS

Ang II (75-200 pM) produced dose-dependent enhancement of autoregulation whereas phenylephrine produced no enhancement and impaired autoregulation of GFR. Enhancement by Ang II was inhibited by the AT1 antagonist, Losartan, and the superoxide scavenger, Tempol. Under control conditions nitric oxide synthase (NOS) inhibition by 10 microm N-omega-nitro-L-arginine methyl ester (L-NAME) facilitated autoregulation in the presence of non-specific cyclooxygenase (COX) inhibition by 10 microm indomethacin. Both COX and combined NOS/COX inhibition reduced the autoregulatory threshold concentration of Ang II. Facilitation by 100 pm Ang II was inhibited by 100 microm frusemide. Methacholine (50 nm) antagonised Ang II-facilitated autoregulation in the presence and absence of NOS/COX inhibition. Infusion of the NO donor, 1 microm sodium nitroprusside, inhibited L-NAME enhancement of autoregulation under control conditions and during Ang II infusion.

CONCLUSIONS

The results suggest than an excess of NO impairs autoregulation under control conditions in the IPRK and that endogenous and exogenous NO, vasodilatory prostaglandins and endothelium-derived hyperpolarizing factor (EDHF) activity antagonise Ang II-facilitated autoregulation. Ang II also produced a counterregulatory vasodilatory response that included prostaglandin and NO release. We suggest that Ang II facilitates autoregulation by a tubuloglomerular feedback-dependent mechanism through AT1 receptor-mediated depletion of nitric oxide, probably by stimulating generation of superoxide.

Microtubule integrity is essential for apical polarization and epithelial morphogenesis in the thyroid

In this study, we examined the contribution of microtubules to epithelial morphogenesis in primary thyroid cell cultures. Thyroid follicles consist of a single layer of polarized epithelial cells surrounding a closed compartment, the follicular lumen. Freshly isolated porcine thyroid cells aggregate and reorganize to form follicles when grown in primary cultures. Follicular reorganization is principally a morphogenetic process that entails the assembly of biochemically distinct apical and basolateral membrane domains, delimited by tight junctions. The establishment of cell surface polarity during folliculogenesis coincided with the polarized redistribution of microtubules, predominantly in the developing apical poles of cells. Disruption of microtubule integrity using either colchicine or nocodazole caused loss of defined apical membrane domains, tight junctions and follicular lumina. Apical membrane and tight junction markers became randomly distributed at the outer surfaces of aggregates. In contrast, the basolateral surface markers, E-cadherin and Na(+),K(+)-ATPase, remained correctly localized at sites of cell-cell contact and at the free surfaces of cell aggregates. These findings demonstrate that microtubules play a necessary role in thyroid epithelial morphogenesis. Specifically, microtubules are essential to preserve the correct localization of apical membrane components within enclosed cellular aggregates, a situation that is also likely to pertain where lumina must be formed from solid aggregates of epithelial precursors.

Sodium iodide symporter (NIS) gene expression in human placenta

The placenta must allow the passage of iodide from the maternal to the fetal circulation for synthesis of thyroxine by the fetal thyroid. The thyroid sodium iodide symporter (NIS) was cloned in 1996 and, although widely distributed among epithelial tissues, early studies failed to detect it in placenta. We demonstrated NIS mRNA in human placenta and in the human choriocarcinoma cell line, JAr. NIS protein was localized to trophoblasts, with a tendency to apical distribution, in sections of human placenta immunostained with a monoclonal antibody against hNIS. We conclude that NIS is expressed in placenta and may mediate placental iodide transport.

Different transporters for tri-iodothyronine (T(3)) and thyroxine (T(4)) in the human choriocarcinoma cell line, JAR

We investigated transport systems for tri-iodothyronine (T(3)) and thyroxine (T(4)) in the human choriocarcinoma cell line, JAR, using a range of structurally similar compounds to determine whether these thyroid hormones are transported by common or different mechanisms. Saturable T(3) but not saturable T(4) uptake was inhibited by a wide range of aromatic compounds (nitrendipine, nifedipine, verapamil, meclofenamic acid, mefenamic acid, diazepam, phenytoin). Nitrendipine and diazepam were the most effective inhibitors of saturable thyroid hormone uptake. Nitrendipine decreased the K(m) for T(4) uptake from a control value of around 500 nM to around 300 nM (n=6). In contrast, the K(m) for T(3) uptake was increased from a control value of around 300 nM to around 750 nM (n=4). Diazepam had similar effects. This divergent shift in affinity for the uptake of T(3) and T(4) suggested that separate uptake systems exist for these two thyroid hormones. This provides evidence for at least two transporters mediating uptake of T(3) and T(4) in JAR cells: a specific T(4) transporter that does not interact with T(3) or structurally similar compounds; and a shared iodothyronine transporter that interacts with T(3), T(4), nitrendipine and diazepam.

Lack of membrane transport of l-thyroxine sulphate in the human choriocarcinoma cell line, JAr

We examined uptake of l -thyroxine sulphate (T(4)S) and possible interactions between T(4)S and thyroxine (T(4)) uptake in the choriocarcinoma cell line JAr. Cells were incubated with 50 p m(125)I-T(4)S in the absence (total uptake) and in the presence (non-specific uptake) of 10 microm T(4)S. Cells were also incubated at 37 degrees C for 2 min with 50 p m(125)I-T(4)in the presence of an increasing amount of unlabelled T(4)(0-10 microm) or T(4)S (0-30 microm). There was negligible total uptake of(125)I-T(4)S (1.14+/-0. 05 fmol/mg cellular protein, mean+/-sem) and no specific uptake after 120 min incubation. Minor inhibition of(125)I-T(4)uptake by T(4)S could be explained entirely by a low level of residual T(4)(0. 2 per cent) in the T(4)S preparation. These findings indicate that T(4)S does not share the T(4)membrane transporter.

Comparison of mechanisms mediating uptake and efflux of thyroid hormones in the human choriocarcinoma cell line, JAR

We compared the specificities of transport mechanisms for uptake and efflux of thyroid hormones in cells of the human choriocarcinoma cell line, JAR, to determine whether triiodothyronine (T3), thyroxine (T4) and reverse T3 (rT3) are carried by the same transport mechanism. Uptake of 125I-T3, 125I-T4 and 125I-rT3 was saturable and stereospecific, but not specific for T3, T4 and rT3, as unlabelled L-stereoisomers of the thyroid hormones inhibited uptake of each of the radiolabelled hormones. Efflux of 125I-T3 was also saturable and stereospecific and was inhibited by T4 and rT3. Efflux of 125I-T4 or 125I-rT3 was, in contrast, not significantly inhibited by any of the unlabelled thyroid hormones tested. A range of compounds known to interfere with receptor-mediated thyroid hormone uptake in cells inhibited uptake of 125I-T3 and 125I-rT3, but not 125I-T4. We conclude that in JAR cells uptake and efflux of 125I-T3 are mediated by saturable and stereospecific membrane transport processes. In contrast, the uptake, but not the efflux, of 125I-T4 and 125I-rT3 is saturable and stereospecific, indicating that uptake and efflux of T4 and rT3 in JAR cells occur by different mechanisms. These results suggest that in JAR cells thyroid hormones may be transported by at least two types of transporters: a low affinity iodothyronine transporter (Michaelis constant, Km, around 1 microM) which interacts with T3, T4 and rT3, but not amino acids, and an amino acid transporter which takes up T3, but not T4 or rT3. Efflux of T4 and rT3 appears to occur by passive diffusion in these cells.

Uptake of L-triiodothyronine sulphate by human choriocarcinoma cell line, JAr

This study investigated uptake of triiodothyronine sulphate (T3S) and interactions between uptake of T3S and triiodothyronine (T3) using the human choriocarcinoma cell line (JAr) as a model of placental transport. Cells were incubated at 37 degrees C with 30 pM 125I-T3 for 2 min with unlabelled T3 (0-30 microM) or T3S (0-1 mM). Addition of an excess unlabelled T3 (30 microM) or T3S (1 mM) reduced the initial rate of 125I-T3 uptake by 69.3+/-3.6 per cent (P<0.0001) and 52.9+/-7.8 per cent (P<0.0001), respectively. The calculated Michaelis constant (Km) for T3 uptake was 0.378+/-0.133 microM (n = 3) with a corresponding maximum velocity (Vmax) of 15.4+/-6.9 pmol/min/mg protein. Uptake of 125I-T3 was inhibited in a dose-dependent way by the addition of unlabelled T3S (0-1 mM). The calculated inhibition constant (Ki) for the inhibition of 125I-T3 uptake by T3S was 121.8+/-35.2 microM (n = 6). Saturable uptake of 125I-T3S by JAr cells was negligible. The T3S preparation incubated with the cells contained about 0.1 per cent T3, sufficient to explain the apparent inhibition of 125I-T3 uptake by unlabelled T3S. These results suggest that, in contrast to T3 uptake in these cells, JAr cells do not have a saturable uptake mechanism for T3S, and that T3S does not interact with the T3 transporter in these cells.

Uptake of reverse T3 in the human choriocarcinoma cell line, JAr

The uptake and efflux of reverse triiodothyronine (rT3) in JAr cells were investigated. Uptake of 125I-rT3 was time dependent and reversible with a saturable component of around 70 per cent of total uptake after 30 min of incubation. Efflux was not saturable. Kinetic analysis of the initial specific uptake rates revealed an uptake process with a Michaelis constant of 3.04+/-0.53 microM (mean+/-SEM, n=15) and a corresponding maximum velocity of 9.65+/-2.49 pmol/min/mg protein (n=15). Uptake of rT3 was stereospecific, but not specific for rT3, as unlabelled L stereoisomers of thyroid hormone analogues were more effective as inhibitors of 125I-rT3 uptake than rT3. Unlabelled T3 and thyroxine (T4) (10 microM) reduced cellular uptake of 125I-rT3 by around 82 and 74 per cent, respectively. The calculated inhibition constants Ki were 1.23+/-0.29 microM (n=4) and 0.66+/-0.19 microM (n=4) for T3 and T4, respectively. Similarly, rT3 reduced cellular uptake of 125I-T3 and 125I-T4 by 34 and 23 per cent, respectively. The calculated inhibition constants Ki were 1.75+/-0.55 microM (n=8) and 1.08+/-0.36 microM (n=8) for the inhibition of 125I-T3 and 125I-T4 uptake, respectively. Reverse T3 inhibited efflux of 125I-T3 from the cells by around 20 per cent, but did not inhibit efflux of 125I-T4. These results suggest that uptake of rT3 in JAr cells may occur via a single, saturable membrane carrier, which also interacts with T3 and T4, while efflux of rT3 may occur by passive diffusion.

Thyroid hormone efflux from placental tissue is not stimulated during cell volume regulation

The effects of cell swelling induced by hyposmotic shock on efflux of hybrid hormones and selected amino acids from human placental tissue were examined. Decreasing the osmolarity of external medium from 290 to 140 mOsm/kg stimulated release of taurine, tryptophan and glutamine from placental tissue fragments. The efflux rate constant for taurine increased from 0.0069 +/- 0.0012/min to 0.0646 +/- 0.0217/min (n = 6) (P < 0.001), for tryptophan from 0.016 +/- 0.0010/min to 0.0295 +/- 0.0016/min (n = 6) (P < 0.001), and for glutamine from 0.0267 +/- 0.0027/min to 0.0659 +/- 0.0043/min (n = 4) (P < 0.001). In contrast, hyposmotic challenge did not affect release of triiodothyronine, thyroxine and leucine. These results indicate that transport processes involved in the regulation of cellular volume are unlikely to facilitate efflux of thyroid hormones from placental tissue, and therefore are unlikely to mediate transfer of thyroid hormones across the placenta. In addition, it is unlikely that the transport system facilitating the release of amino acids from placental tissue during regulatory volume decrease is one of the known amino acid carriers.

Protein tyrosine phosphorylation influences adhesive junction assembly and follicular organization of cultured thyroid epithelial cells

The follicular histoarchitecture of the thyroid forms the anatomical basis for thyroid physiology and is commonly disturbed in diseases of the thyroid. We have used cultured porcine thyroid cells to study thyroid epithelial morphogenesis and its regulation. When cultured in the presence of TSH, freshly isolated thyroid cells reorganize to form follicles within three-dimensional cell aggregates. However, when established follicles are washed into TSH-free medium, thyroid cells spread and migrate to convert follicles into confluent epithelioid monolayers, activating morphogenetic mechanisms, such as cell locomotility, that may be relevant to thyroid inflammation and tumor invasiveness. The phenomenon of follicle to monolayer conversion, therefore, provides an opportunity to identify morphogenetic mechanisms that 1) must be tonically inhibited to maintain follicular organization and 2) may contribute to pathogenetic disturbances of follicular architecture when functioning aberrantly. In this study we found that follicle to monolayer conversion is associated with an increase in cellular phosphotyrosine. This was particularly evident at nascent focal adhesions (cell-substrate adhesive junctions) and later at cell-cell junctions. Focal adhesion assembly was accompanied by reorganization of the actin cytoskeleton, with the appearance of prominent stress fibers. Genistein, a potent inhibitor of protein tyrosine kinases, inhibited the accumulation of phosphotyrosine, focal adhesion assembly, and follicle to monolayer conversion. We conclude that tyrosine phosphorylation exerts an important influence on thyroid epithelial organization in culture, at least partly mediated through regulation of focal adhesion assembly.

Sodium channel heterogeneity in the apical membrane of porcine thyroid epithelial cells

Porcine thyroid epithelial cells cultured as a monolayer with their apical membranes facing the medium are known to absorb Na+ and secrete Cl-. Two types of Na+ channels were found in cell-attached patches of apical membrane. A low conductance Na+ channel (conductance g = 4 picosiemens (pS)) remained open for seconds and showed a high selectivity for Na+ compared with K+. In contrast, a high conductance Na+ channel (g = 10 pS) flickered rapidly and had reduced selectivity. Both types of Na+ channel became more prevalent when the cells were exposed to Na(+)-free medium, though only the high conductance channel increased in prevalence on addition of prostaglandin E2, a stimulator of adenylate cyclase which increases Na+ absorption in this cultured epithelium. Two minority types of channel were also found: a non-selective small conductance cation channel which had been reported previously, and an intermediate conductance channel found only in Na(+)-free medium. It was concluded that passage of Na+ across the apical membrane of thyroid cells is mediated by typical epithelial Na+ channels, but that the two types of channel are differentially regulated.

Chloride channels in the apical membrane of thyroid epithelial cells are regulated by cyclic AMP

Porcine thyroid epithelial cells cultured as a monolayer with their apical membranes facing the medium are known to absorb Na+ and to secrete the anions Cl- and HCO3-. Chloride channels were found in the apical membrane, and displayed a reversal potential close to the resting membrane potential, linear current-voltage relationships, a conductance at physiological temperature of 6.5 pS, and a small but significant permeability to HCO3-. Stimulation of ion transport with prostaglandin E2 or 8-(4-chlorophenylthio) adenosine 3':5'-cyclic monophosphate promoted activation of Cl- channels in cell-attached patches, and excised patches were reactivated by exposure of their cytoplasmic surface to protein kinase A and ATP. Physiological temperatures were necessary for activation of Cl- channels in cell-attached patches. The channels exhibited sub-states with a conductance exactly half that of the full unit conductance, suggesting a dual-barrelled channel structure. It is concluded that the apical membrane of thyroid epithelial cells contains cyclic AMP-activated Cl- channels controlling anion transport.

Cadherin-mediated adhesion and apical membrane assembly define distinct steps during thyroid epithelial polarization and lumen formation

The biogenesis of follicles from aggregates of precursor cells is an important morphogenetic process in thyroid embryology. It necessitates the creation of a polarized cell phenotype, assembly of specialized cell-cell junctions, and generation of follicular lumena. In this study we sought to investigate the relationship between cell polarization and lumen formation by studying the cell surface events that occurred when freshly isolated adult porcine thyroid cells reorganized to form follicles in primary culture. Follicular reorganization entailed the initial formation of solid three-dimensional cell aggregates and the subsequent appearance of lumena within aggregates. During the initial stage of cell aggregation, the adhesion molecule, E-cadherin, became expressed at all surfaces involved in cell-cell contact. Aggregation was inhibited by monoclonal antibodies that block cadherin function, indicating directly that E-cadherin is a dominant initial cell-cell adhesion molecule. Cell aggregation was also associated with the recruitment to the cell surface of ZO-1, a tight junction-associated protein, and Na+/K(+)-adenosine triphosphatase. These proteins were initially found throughout regions of cell-cell contact and only subsequently redistributed to their mature locations in tight junctions and the basolateral cell surface, respectively. In contrast, components associated with the apical membrane were first detected within large intracellular vacuoles, which subsequently fused with the cell surface between maturing tight junctions to yield the apical membrane domain and nascent follicular lumena. Follicle formation occurred independently of basal lamina assembly and TSH, although maintenance of follicular architecture required the presence of this hormone. These findings indicate that cultured follicles form in two distinct stages: 1) initial aggregation mediated by E-cadherin and associated with recruitment of components of both tight junctions and the basolateral membrane domain, and 2) subsequent formation of a specialized apical membrane domain by coordinated fusion of intracellular vacuoles at sites of the cell surface where tight junctions are maturing. We propose that follicular morphogenesis may arise as a consequence of epithelial cell polarization within coherent three-dimensional cell aggregates.

Uptake of thyroxine in the human choriocarcinoma cell line JAR

We have studied the uptake of 125I-thyroxine (125I-T4) in the human choriocarcinoma cell line JAR. Uptake of 125I-T4 was time-dependent, stereospecific and reversible, with a saturable component of 33% after 120 min of incubation. Kinetic analysis of the initial specific uptake rates indicated the presence of a single uptake process with a Michaelis constant of 59.4 +/- 13.9 nM (n = 12) and maximum velocity of 0.29 +/- 0.06 pmol/min per mg protein. Uptake was dependent on intracellular energy as, in the presence of 2 nM potassium cyanide, saturable uptake was reduced to 60.6 +/- 8.5% (n = 4) of control uptake. Uptake was also temperature-dependent. Saturable 125I-T4 uptake after 60 min of incubation was 26.1 +/- 3.0% at 25 degrees C (n = 6) and 27.3 +/- 5.7% at 4 degrees C of control uptake at 37 degrees C. Ouabain did not inhibit 125I-T4 uptake indicating that the uptake was independent of the Na+ gradient across the cell membrane. Although T4 uptake was stereospecific, as D-T4 failed to inhibit 125I-L-T4 uptake, it was not specific for T4, as tri-iodothyronine (T3) and reverse T3 also inhibited 125I-T4 uptake. We conclude that JAR cells have a saturable, stereospecific and reversible membrane transport mechanism for T4 which is dependent on intracellular energy, but independent of the Na+ gradient across the cell membrane.

Microtubule integrity is necessary for the epithelial barrier function of cultured thyroid cell monolayers

Vectorial transport in the thyroid epithelium requires an efficient barrier against passive paracellular flux, a role which is principally performed by the tight junction (zonula occludens). There is increasing evidence that tight junction integrity is determined by integral and peripheral membrane proteins which interact with the cell cytoskeleton. Although the contribution of the actin cytoskeleton to tight junction physiology has been intensively studied, less is known about possible interactions with microtubules. In the present study we used electrophysiological and immunohistochemical approaches to investigate the contribution of microtubules to the paracellular barrier in cultured thyroid cell monolayers which displayed a high transepithelial electrical resistance (6000-9000 ohm.cm2). Colchicine (1 microM) caused a progressive fall in electrical resistance to < 10% of baseline after 6 h and depolarization of the transepithelial electrical potential difference consistent with a significant increase in paracellular permeability. The effect of colchicine on TER was not affected by agents which inhibit the major apical conductances of thyroid cells but was reversed upon removal of the drug. Immunofluorescent staining for tubulin combined with confocal laser scanning microscopy demonstrated that thyroid cells possessed a dense microtubule network extending throughout the cytoplasm which was destroyed by colchicine. Colchicine also produced changes in the localization of the tight junction-associated protein, ZO-1: its normally continuous junctional distribution was disrupted by striking discontinuities and the appearance of many fine strands which extended into the cytoplasm. A similar disruption in E-cadherin staining was also observed, but colchicine did not affect the distribution of vinculin associated with adherens junctions nor the integrity of the perijunctional actin ring. We conclude that microtubules are necessary for the functional and structural integrity of tight junctions in this electrically tight, transporting epithelium.

Vinculin localization and actin stress fibers differ in thyroid cells organized as monolayers or follicles

In epithelial cells interactions between the actin cytoskeleton and cell-cell junctions regulate paracellular permeability and participate in morphogenesis. We have studied the relationship between supracellular morphology and actin-junction interactions using primary cultures of porcine thyroid cells grown either as three-dimensional follicles or as open monolayers. Regardless of morphology, thyroid cells assembled occluding and adhesive junctions containing ZO-1 and E-cadherin, respectively, and showed F-actin staining in apical microvilli and a perijunctional ring. In monolayers, actin stress fibers were also observed in the apical and basal poles of cells, where they terminated in the vinculin-rich zonula adherens and in cell-substrate focal adhesions, respectively. Surprisingly, we were unable to detect vinculin localization in follicular cells, which also did not form stress fibers. Immunoblotting confirmed significantly greater vinculin in triton-insoluble fractions from monolayer cells compared with follicular cells. Incubation of monolayers with 8 chloro(phenylthio)-cyclic AMP decreased the level of immunodetectable vinculin in the zonula adherens, indicating that junctional incorporation of vinculin was regulated by cyclic AMP. In monolayer cultures, cytochalasin D (1 microM) cause actin filaments to aggregate associated with retraction of cells from one another and the disruption of cell junctions. Despite morphologically similar perturbations of actin organization in follicular cultures treated with cytochalasin D, junctional staining of ZO-1 and E-cadherin was preserved and cells remained adherent to one another. We conclude that in cultured thyroid cells structural and functional associations between actin filaments and cellular junctions differ depending upon the supracellular morphology in which cells are grown. One important underlying mechanism appears to be regulation of vinculin incorporation into adhesive junctions by cyclic AMP.

Characterization of cell polarity and epithelial junctions in the choriocarcinoma cell line, JAR

In the placenta the trophoblast cell layer separates maternal and fetal circulations and is involved in the active transport of selected substances across this barrier. We have used the JAR choriocarcinoma cell line to study aspects of trophoblast membrane transport. To determine whether JAR cells could be used in studies of vectorial transepithelial transport it was necessary to determine whether these cells were polarized and assembled tight junctions. In the present study we investigated JAR cells using a range of markers for specific cell surface domains combined with confocal laser scanning microscopy. Freshly isolated cells initially formed a confluent epithelial monolayer with recruitment of a tight junction-associated protein, ZO-1, and a cell adhesion molecule, E-cadherin, to the surface at sites of cell-cell contact. They did not, however, display cell surface polarization, as NaK-ATPase was not segregated in the basolateral domain, and a differentiated apical cell surface was not assembled. The monolayer stage was also unstable, as continued proliferation resulted in the formation of multilayered aggregates where ZO-1 and E-cadherin were lost from the cell surface. These results suggest that the JAR cell line is unlikely to be a suitable model for studies of transepithelial transport in the placenta.

Thyrotropin inhibits the intrinsic locomotility of thyroid cells organized as follicles in primary culture

The regulation of cell locomotion is a fundamental determinant of tissue architecture. Even in solid tissues of adult organisms cells often retain an intrinsic locomotor capacity which is activated during wound healing or tumor metastasis. In this study we have examined the role of cell locomotion in an in vitro model of thyroid epithelial pattern generation. Primary cultures of adult porcine thyroid cells reorganize to form follicles within three-dimensional cell aggregates when stimulated by thyrotropin (thyroid-stimulating hormone, TSH). Removal of TSH from the culture medium caused established follicles to reorganize into a confluent, two-dimensional epithelioid monolayer. The earliest observed change was the appearance of spreading cells at the peripheries of aggregates. These cells displayed broad lamellipodia whose formation was associated with the redistribution of microfilaments and microtubules and the accumulation of myosin. Spreading cells could migrate into, and fill, artificial wounds several millimeters wide without evidence of cell proliferation, indicating that cells became locomotile as they spread from follicles to form monolayer. Both spreading and migration were inhibited by cytochalasin B. In contrast, cells spread in the presence of colchine, but failed to migrate subsequently. Thyroid cell locomotility from follicles was inhibited by TSH, a cAMP analog, and a cell-free membrane fraction. However, migration from established monolayer cultures was not affected by these regulatory agents. This indicated that cell spreading was an important regulatory locus in thyroid cell patterning. We conclude that the tonic inhibition of thyroid cell locomotility contributes to the maintenance of follicular architecture in vitro. TSH and cell-cell contact may inhibit locomotion by preventing follicular cells from spreading, the earliest step in the morphogenetic conversion of follicles to monolayer.

Thyroid epithelial morphogenesis in vitro: a role for bumetanide-sensitive Cl- secretion during follicular lumen development

The structural and functional unit of the thyroid gland is the follicle, consisting of a closed lumen surrounded by a single layer of polarized epithelial cells. In this paper we have attempted to characterize the process of lumenal development when primary cultures of porcine thyroid cells reorganized to form follicles. Cells incubated with the loop diuretic, bumetanide, an inhibitor of NaK2Cl cotransport, aggregated but failed to form normal follicles. Laser scanning confocal microscopy combined with immunohistochemical markers of thyroid cell-surface proteins demonstrated that in the presence of bumetanide cells polarized and assembled ZO-1-containing tight junctions separating their apical and basolateral membrane domains. Cultures formed small lumena but their subsequent growth was inhibited by bumetanide. Electrophysiological studies confirmed that bumetanide-sensitive Cl- transport was the major contributor to the transepithelial electrical potential difference across the follicular wall after 48 h incubation. Other potential mechanisms did not contribute significantly to follicular lumenal growth. In particular, bumetanide did not affect cell proliferation and, in contrast to tissue follicles, thyroglobulin could not be detected within the lumena of cultured follicles. We conclude that thyroid follicular reorganization involves two distinct and separate phases of lumenal development: initial lumen formation which probably reflects the assembly of a specialized apical membrane domain; and subsequent lumenal growth which is mediated by the inward transport of Cl- by polarized epithelial cells.

Interactions between transport of triiodothyronine and tryptophan in JAR cells

We studied the effect of a number of amino acids on uptake of L-triiodothyronine (T3) in the human choriocarcinoma cell line, JAR. Tryptophan inhibited saturable T3 uptake by about 57% without any significant effect on the non-saturable uptake. Michaelis constant (Km) for T3 uptake was 1.06 +/- 0.15 microM (n = 15) with the corresponding maximum velocity (Vmax) of 24.2 +/- 3.1 pmol/min/mg cellular protein. For tryptophan uptake the Km was 1.31 +/- 0.26 microM (n = 7) and Vmax was 166.4 +/- 35.7 pmol/min/mg protein. The kinetic parameters for both uptake processes were similar to those reported in normal placenta. Uptake of T3 was inhibited by tryptophan but not phenylalanine, but tryptophan uptake was inhibited both by T3 and phenylalanine. Inhibition of T3 uptake by tryptophan was dose dependent, with an inhibition constant (Ki) of 2.9 +/- 0.5 mM. Similarly, tryptophan uptake was inhibited by T3 and phenylalanine in a dose dependent way with Ki values of 4.9 +/- 0.5 microM and 15.6 +/- 4.8 microM respectively. Km for T3 uptake was significantly increased to 1.86 +/- 0.42 microM (n = 4) in the presence of 3 mM unlabelled tryptophan and, similarly, Km for tryptophan uptake was significantly increased to 9.91 +/- 2.57 microM (n = 3) in the presence of 5 microM unlabelled T3. Efflux of T3 was progressively inhibited by increasing concentrations of both ligands, i.e. was saturable. We conclude that there is mutual competitive inhibition between uptake systems for T3 and tryptophan in JAR cells, but the kinetic parameters of cross-inhibition of uptake by the substrates suggest that the carriers are distinct. T3 may be transported in JAR cells by at least two transport systems with differing substrate specificities. We also demonstrated the presence of a saturable membrane carrier mediating the efflux of T3 from the cells which was subject to trans-inhibition by T3 and tryptophan.

Thyroid cell spreading and focal adhesion formation depend upon protein tyrosine phosphorylation and actin microfilaments

Adhesion to proteins of the extracellular matrix exerts a profound influence upon cell function and behavior. Similar adhesive interactions mediate the spreading of cultured cells upon artificial substrata. Recently we observed that thyrotropin (TSH) and intercellular contact regulated thyroid cell-substrate adhesion to inhibit cell spreading, but not initial attachment. This is a mechanism which preserves thyroid follicular differentiation in culture. In the present study we have investigated the role of cytoplasmic components in mediating thyroid cell adhesion to collagen. The earliest change associated with cell spreading was the accumulation of vinculin and phosphotyrosine in developing focal adhesions, which was followed by stress fiber and microtubule assembly. Genistein, an inhibitor of tyrosine kinases, and cytochalasin B inhibited cell spreading and focal adhesion formation without affecting initial attachment to substrate. In contrast microtubule disorganization by colchicine did not alter any parameter of thyroid cell-substrate adhesion. These observations indicate that protein tyrosine phosphorylation and dynamic microfilament integrity are essential for attached thyroid cells to spread upon substrate. They are therefore potential intracellular loci at which TSH and intercellular contact may regulate cell adhesion to extracellular matrix and influence thyroid cell behavior.

Contact inhibition of cell spreading: a mechanism for the maintenance of thyroid cell aggregation in vitro

When freshly isolated porcine thyroid cells are stimulated with thyrotropin (TSH) they organize to form functional follicles in conventional substrate-adherent culture. Cell aggregation is essential for follicular reorganization and is likely to be influenced by the balance between cell-cell adhesion (promoting aggregation) and cell-substrate adhesion (favoring spreading and monolayer formation). Recently we observed that TSH potentiated cell-cell adhesion and in the present study we have sought evidence that TSH might also regulate cell-substrate adhesion. Two parameters of cell-substrate adhesion, namely, cell attachment to collagen and cell spreading upon collagen, were measured using preparations of isolated single cells and of multicellular aggregates. TSH had no effect upon the attachment or spreading of single cells, but inhibited aggregate spreading without affecting aggregate attachment. The possibility that cell-cell contact modulated the response to TSH in aggregates, but not in single cells, was confirmed using a cell-free membrane preparation which inhibited the spreading of single cells but not their rate of attachment. Moreover, TSH potentiated the inhibitory effect of membranes on the spreading of single cells. Heparin also specifically inhibited the spreading of both single cells and cell aggregates, suggesting that a heparin-sensitive adhesive mechanism might be recruited as thyroid cells spread. We conclude that thyroid cell-substrate adhesion is regulated by a synergistic interaction between cell-cell contact and TSH which preferentially inhibited cell spreading but not attachment. Such contact-dependent inhibition of cell spreading is predicted to preserve cell aggregates and hence contribute to the maintenance of thyroid follicular differentiation in vitro.

Activation of sodium transport mediates regulation of thyroid follicle volume in response to hypotonic media

The thyroid epithelium possesses a bidirectional fluid transport system capable of absorbing Na+ and secreting Cl-. In the present studies we have examined its possible role in the regulation of thyroid follicular size. When exposed to hypotonic media (200 mosM) cultured porcine thyroid follicles first swelled and then displayed a regulatory volume decrease (RVD) over 60 min. This was associated with a transient depolarization of the transepithelial potential difference (TEP) and subsequent hyperpolarization with a time course similar to RVD. Phenamil (1 microM), an antagonist of epithelial Na+ channels, did not affect initial swelling but prevented the subsequent follicular RVD. Phenamil abolished hyperpolarization of TEP, but the loop diuretic bumetanide, which inhibits Cl- secretion in thyroid cells, did not prevent it. Exposure to hypotonic medium produced a slow hyperpolarization of the intracellular potential (basolateral membrane potential) consistent with an increase in basolateral membrane K+ conductance. Ba2+ and quinidine, which are known to inhibit K+ channels in epithelia, prevented RVD. Addition of the K+ ionophore valinomycin (1 microM) caused follicle shrinkage that was prevented by phenamil (1 microM). We conclude that cultured follicles respond to hypotonically induced stretch by activating outwardly directed Na+ transport through a mechanism which involves change in the basolateral K+ conductance. This response would be characteristic of a system that controlled follicle volume. However, it is not clear from these studies whether the cells responded primarily to the increase in follicle volume or to the change in cell volume that is expected to accompany hypotonic challenge.

Differential regulation of thyroid cell-cell and cell-substrate adhesion by thyrotropin

Preservation of cell aggregation is necessary for thyroid follicular differentiation in vitro and requires stimulation by thyrotropin (TSH). We have tested the hypothesis that TSH preferentially increases thyroid cell-cell adhesion relative to cell-substrate adhesion. Cell-cell adhesion was measured in short-term suspension cultures by the decrease in the fraction of single cells remaining in culture (free cell ratio, FCR). When incubated in medium alone freshly isolated cells showed a progressive fall in FCR but this was accelerated by TSH and the cyclic AMP analog, 8-(4-chlorophenylthio)cyclic AMP. Aggregation was dependent upon extracellular Ca2+ and also promoted by a cell-free membrane extract. In contrast, attachment of cells to plastic dishes treated for tissue culture was not affected by TSH. We conclude that thyroid cells possess a TSH-sensitive cell adhesion system. The preferential increase in cell-cell adhesion may be one mechanism by which TSH stimulates the formation and preservation of follicles in vitro.

Chloride conductance of apical membrane in cultured porcine thyroid cells activated by cyclic AMP

The thyroid epithelium transports fluid bidirectionally using active transport of Na+ ions from apical to basal poles and active transport of Cl- in the reverse direction. In these studies we sought evidence for cyclic AMP activated Cl- channels on the apical membranes of thyroid cells in monolayer culture. A Cl(-)-dependent basal-positive short-circuit current (ISC) was demonstrated in bicameral chambers after blocking Na+ transport with phenamil, and responded to prostaglandin (PG) E2 with a spike of 5-10 min duration followed by a plateau. The onset of the spike coincided with an increase in the conductance of the epithelium. Application of an external Cl- concentration gradient, by replacing the medium in the apical compartment with Cl(-)-free medium, resulted in an increase in ISC after, but not before, addition of PGE2. Forskolin and thyroid-stimulating hormone (TSH), but not A23187, also stimulated Cl- transport. In conjunction with previous observations that Cl- transport was mediated by a bumetanide-sensitive NaKCl2 symporter on the basal membrane, these observations indicated the presence of a cyclic AMP activated Cl- conductance in the apical membrane of thyroid cells.

Membrane transport of thyroid hormone in the human choriocarcinoma cell line, JAR

We studied uptake of L-triiodothyronine (T3) by the human choriocarcinoma cell line, JAR. Uptake was time dependent with a half-time of 56.2 +/- 7.2 min (mean +/- SEM, n = 4). A non-saturable component accounted for about 24% of total uptake. We found a single saturable uptake mechanism with a calculated Michaelis constant (Km) of 586 +/- 206 nM (n = 9) and a corresponding maximum velocity of 17.0 +/- 5.7 pmol/min per mg protein (n = 9), values similar to those we have described recently in cultured normal human trophoblast cells. Uptake was dependent on temperature and intracellular energy, being reduced at lower temperatures and in the presence of potassium cyanide. It was independent of the Na+ gradient across the cell membrane and the presence of Na+ in the external medium, but was affected by the cell membrane potential.

Uptake of L-tri-iodothyronine by human cultured trophoblast cells

We investigated the uptake of L-tri-iodothyronine (T3) by cultured human trophoblast cells. Uptake was time-dependent, initially linear and approaching equilibrium after 60 min with an approximate half-time of 13 +/- 4.5 min (mean +/- S.E.M., n = 4). It had a non-saturable component accounting for about 50% of total uptake. We demonstrated a single saturable T3 uptake mechanism with a calculated Michaelis constant (Km) of 755 +/- 145 nmol/l (n = 11-13) and a corresponding maximum velocity of 28.8 +/- 5.3 pmol/min per mg protein (n = 11-13). The Km value was similar to those reported in other tissues.

Bidirectional ion transport in thyroid: secretion of anions by monolayer cultures that absorb sodium

Cultured porcine thyroid cell monolayers transport Na+ in an apical-to-basal direction, resulting in the development of a basal-positive transepithelial potential difference (TEP) and the formation of domes (fluid-filled elevations of the cell layer above the culture dish substrate). Stimulation by prostaglandin E2 (PGE2) increases the magnitude of the TEP, the short-circuit current (Isc) measured in Transwell Ussing chambers, and the height of domes in cultures grown on impermeable substrates. A phenamil-resistant, PGE2-stimulated component of the Isc in Transwells and of the TEP in monolayers in conventional culture dishes was inhibitable by bumetanide, a diuretic drug that blocks NaKCl2 symporters, mediating active transport of Cl-. The rate of decrease in height of domes in cultures after addition of phenamil, presumably indicative of transport of fluid in a basal-to-apical direction, was also reduced by bumetanide. Studies with Transwells in Cl(-)-free, HCO(3-)-free or Cl(-)- and HCO(3-)-free media indicated that thyroid cells transported HCO3- as well as Cl- in a basal-to-apical direction. It was concluded that the thyroid epithelium is both sodium absorbing and anion secreting.

Regulation of thyroid follicular volume by bidirectional transepithelial ion transport

Previous studies have shown that thyroid cells in monolayer culture exhibit bidirectional ion transport comprising apical-to-basal amiloride-sensitive Na+ transport and oppositely directed bumetanide-sensitive Cl- transport. We have now investigated the role of ion transport in the regulation of thyroid follicular size using follicular primary porcine thyroid cell cultures. Bumetanide (10 microM) added at the beginning of culture inhibited the formation of follicular lumina and caused a fall in follicle height when added to 3-day-old cultures. In contrast, phenamil (1 microM; an amiloride analog) increased follicle size both in freshly isolated and 3-day-old cultures. The effect of bumetanide was prevented by the prior addition of phenamil. Micropuncture studies showed that follicles had a lumen-negative, basal-positive transepithelial potential difference which was progressively reduced in magnitude by the serial addition of bumetanide (10 microM) and phenamil (1 microM). We conclude that thyroid follicles possess a bidirectional ion transport system which transports Na+ in an apical-to-basal direction and Cl- in the opposite direction. The balance between these two processes determines net solute flux and hence follicular size. A physiological role of ion transport in the thyroid may be to regulate follicular volume suggesting that abnormalities of ion transport may be responsible for disorders of follicular size.

Amiloride analogues induce responses in isolated rat cardiovascular tissues by inhibition of Na+/Ca2+ exchange

The role of inhibition of Na+/Ca2+ exchange in the positive inotropic, negative chronotropic and vasorelaxant responses to amiloride and some of its analogues was investigated in isolated cardiovascular tissues from female Wistar rats. The compounds tested were amiloride, 5-(N-ethyl-N-isopropyl)amiloride (EIPA, a potent inhibitor of Na+/H+ exchange), phenamil and 2',4'-dimethylbenzamil (DMB), both potent Na+ channel inhibitors with activity against Na+/Ca2+ exchange, and 5-(N-4-chlorobenzyl)-2',4'-dimethylbenzamil (CBDMB), a potent inhibitor of Na+/Ca2+ exchange with reduced activity against Na+ channels compared with its parent compound DMB. Phenamil, DMB and CBDMB increased the force of contraction of right ventricular papillary muscles with similar potencies (-log EC50 values: 4.77 +/- 0.06, 5.09 +/- 0.09, 4.97 +/- 0.17 respectively), while amiloride and EIPA gave small negative inotropic responses. All compounds gave negative chronotropic responses at similar concentrations to those which exerted inotropic effects. Inhibition of KCl contraction of endothelium-free aortic rings was observed with all compounds tested. Phenamil, DMB and CBDMB but not amiloride or EIPA showed a shift to the left of the concentration-response curves in the presence of intact endothelium. These results provide further evidence for positive inotropic and endothelium-dependent vasorelaxant effects of amiloride analogues mediated by inhibition of Na+/Ca2+ exchange.

Epidermal growth factor (EGF) inhibits the secretomotor response of the thyroid: effects of EGF on radioiodine turnover and fluid transport in cultured porcine thyroid cells

Thyrotrophin (4-256 microU/ml) promoted an increase in the rate of release of radioiodine from the organic iodine pool of cultured porcine thyroid cells in follicular formations. This action of TSH was antagonized by low concentrations of epidermal growth factor (EGF; 0.1-5 nmol/l). The maximal effect of EGF was reached by 0.5 nmol/l. EGF (0.5-5 nmol/l) also inhibited the stimulatory effect of 8-chloro cyclic AMP (0.06-1.0 nmol/l) on radioiodine turnover. Exposure of thyroid cultures to media with a calcium concentration of 17.7 mumol/l (1% of normal) resulted in a very marked increase in the rate of release of radioiodine. The effect of TSH in low-calcium media was to inhibit the increased release of radioiodine, and EGF (0.5 nmol/l) antagonized this inhibitory effect of TSH. The calcium ionophore, A23187, stimulated radioiodine release in a dose-dependent fashion, and EGF (1.7 nmol/l) inhibited this response. Fluid transport in thyroid monolayers was stimulated by prostaglandin E2 (PGE2; 1 mumol/l). EGF (5 nmol/l) also stimulated fluid transport, but antagonized the effect of PGE2 added subsequently. It was concluded that EGF exerted acute antagonistic effects on thyroid cell responses in vitro to cyclic AMP and agents promoting accumulation of cyclic AMP in time-frames too short for these inhibitory effects to be attributable to the dedifferentiative effect of the growth factor.

Control of ion transport in the thyroid: prostaglandin E2 activates cation transport on the basal membrane of cultured porcine thyroid cell monolayers

Confluent monolayers of cultured porcine thyroid cells transport fluid from the apical to the basal surface, forming circumscribed zones of detachment (domes) from the culture dish substrate. Stimulation of fluid transport by prostaglandin E2 (PGE2; 1 mumol/l) was associated with an increase in transepithelial potential (TEP). Intracellular potentials (equal to the potential difference across the apical membrane of the cell, Eapical) and the TEP were measured in individual domes so that the potential difference across the basal membrane of the cell (Ebasal) could be calculated from the relationship TEP = Eapical-Ebasal. The PGE2-induced increase in TEP was associated with hyperpolarization of the basal membrane, accompanied by a slight depolarization of the apical membrane. Lines of best fit by least-squares regression showed Eapical = -20.3 mV +0.219 TEP (correlation coefficient r = 0.627; P less than 0.001) and Ebasal = -20.3 mV -0.781 TEP (r = 0.944; P less than 0.001). Phenamil (1 mumol/l), a Na+ channel selective amiloride analogue, reduced the TEP from 13.25 +/- 0.58 (S.E.M.; n = 56) to 2.39 +/- 0.16 mV (n = 51; P less than 0.001) and hyperpolarized the apical membrane potential from -20.7 +/- 0.68 (n = 60) to -32.2 +/- 0.83 mV (n = 105; P less than 0.001). The response of the TEP to phenamil was immediate, and was promptly reversed on washing; in contrast, addition of 5-(N-ethyl-N-isopropyl)amiloride (20 mumol/l; selective for Na+/H+ antiporters) resulted in a slow depolarization over 30 min with a slow recovery after washout.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effects of amiloride and its analogues on prostaglandin E2-stimulated fluid transport by cultured porcine thyroid cells: evidence for apical membrane Na+ channels

Confluent monolayers of cultured porcine thyroid cells transport fluid from the apical to the basal surface, forming circumscribed zones of detachment (domes) from the culture dish substrate. Fluid transport, as measured by increase in dome height, was stimulated by prostaglandin E2 (PGE2; 1 mumol/l) and inhibited by amiloride (0.1-100 mumol/l). Values of the inhibition constant (Ki) with 95% confidence limits for each of a series of amiloride analogues were: 3',4'-dichlorobenzamil (DCB), 0.090 (0.045-0.18) mumol/l; 2',4'-dimethylbenzamil (DMB), 0.14 (0.074-0.27) mumol/l; amiloride, 0.72 (0.33-1.8) mumol/l; 5-(N,N-hexamethylene)amiloride (HMA), 17 (5.9-43) mumol/l; 5-(N-ethyl-N-isopropyl)amiloride (EIPA), 33 (15-71) mumol/l; and 2-guanidinobenzimidazole, 243 (110-570) mumol/l. Triaminopyrimidine was ineffective at concentrations up to 1 mmol/l. Since DCB and DMB are known to have a higher affinity for Na+/H+ channels, while HMA and EIPA show higher affinity for Na+/H+ antiports, it was concluded that PGE2-stimulated fluid transport involved an apical membrane Na+ channel.

Electrophysiological correlates of fluid transport in cultured porcine thyroid cells

Confluent monolayers of cultured porcine thyroid cells transport fluid from the apical to the basal surface, forming circumscribed zones of detachment from the culture dish substrate (domes). The transepithelial potential (TEP), positive on the basal side, was 12.9 +/- 0.4 (S.E.M.) mV (n = 93) under control conditions, increasing to 38.9 +/- 0.3 mV (n = 281) when fluid transport was stimulated by prostaglandin E2 (PGE2; 1 mumol/l). Forskolin (1 mumol/l) and 8-(4-chlorophenylthio) adenosine 3',5'-cyclic monophosphate (0.5 mmol/l) were also effective in increasing TEP. Addition of amiloride in concentrations sufficient to block fluid transport (100 mumol/l) reduced the TEP to 5.8 +/- 0.3 mV (n = 76). Substitution of N-methyl-D-glucamine for sodium in the medium reduced the PGE2-stimulated TEP to 13.4 +/- 0.8 mV (n = 32). Substitution of gluconate for chloride increased the TEP to 40.3 +/- 0.4 mV (n = 160). Removal of bicarbonate or potassium from the medium, or addition of ouabain (200 mumol/l) were also effective in reducing the TEP. In media of low bicarbonate concentration (1 mmol NaHCO3/l), acetazolamide (1 mmol/l) reduced the TEP. Fluid transport by the monolayer as measured by the change in height of domes was increased by PGE2 (1 mumol/l). PGE2-stimulated fluid transport was inhibited by sodium or chloride ion substitution, bicarbonate removal or the addition of ouabain (200 mumol/l) or amiloride (100 mumol/l). It was concluded that fluid transport in thyroid monolayers is mediated by rheogenic sodium transport with chloride transport being passive, electrogenically coupled to sodium transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of calcium in the secretomotor response of the thyroid: effects of calcium ionophore A23187 on radioiodine turnover, membrane potential and fluid transport in cultured porcine thyroid cells

The calcium ionophore A23187 (0.1-1 mumol/l) inhibited membrane electrical polarization, uptake of 125I, fluid transport and TSH-stimulated release of radioiodine from the organic pool in follicular cultures of porcine thyroid cells. At higher concentrations (1-30 mumol/l), A23187 promoted release of radioiodine from the organic pool. Stimulation of release of radioiodine from the organic pool by veratridine (a sodium channel agonist, 0.4-1 mmol/l) and A23187 was dependent on the calcium concentration of the medium, while TSH action was independent. Incubation in medium of very low calcium concentration (0.0177 mmol/l) resulted in enhanced release from the organic pool, which was inhibited by TSH (256 microU/ml), A23187 (25 mumol/l) or veratridine (0.5 mmol/l). These data therefore do not support the hypothesis that calcium acts as a mediator of the secretomotor action of TSH, but suggest the possibility of a TSH-induced increase in intracellular calcium as a regulatory negative-feedback mechanism.

Cyclic AMP-stimulated fluid transport in the thyroid: influence of thyroid stimulators, amiloride and acetazolamide on the dynamics of domes in monolayer cultures of porcine thyroid cells

Confluent monolayer cultures of porcine thyroid cells form dome-shaped elevations by local separation from the plastic culture dish. Formation of domes by epithelial cells in culture is generally considered to be evidence of fluid transport. A computer-controlled data acquisition system was developed to quantitate fluid transport in thyroid cultures by serial measurements of dome elevation. Thyrotrophin (10 mU/ml), prostaglandin E2 (PGE2; 0.01-1 mumol/l), forskolin (1 mumol/l), 8-(4-chlorophenylthio)adenosine 3':5'-cyclic monophosphate (0.5 mmol/l) and 3-isobutyl-1-methyl-xanthine (0.5 mmol/l) promoted increases in dome height over 5-120 min. Dome growth in the presence of PGE2 (1 mumol/l) was inhibited by amiloride (0.1-100 mumol/l), ouabain (200 mumol/l), or by removal of bicarbonate and glucose from the medium. In media of reduced bicarbonate concentration (1 mmol/l compared with the control concentration of 10 mmol/l), dome growth was inhibited by acetazolamide (0.01-1 mmol/l). These data are consistent with cyclic AMP-stimulated transport of fluid from apical to basal pole of the cells, dependent on sodium entry through the apical pole by an Na+/H+ exchanger.

Role of cell-cell contact in the preservation of differentiation and response to thyrotrophin in cultured porcine thyroid cells

Cultured porcine thyroid cells did not reassociate into functional follicles in the presence of TSH unless the initial seeding density was adequate. At 0.2 X 10(6) cells/35 mm diameter culture dish the cells rapidly formed a monolayer even in the presence of TSH (128 microunits./ml), and radioiodide uptake was not significantly increased compared with that in control cells. Seeding densities of 1-3 X 10(6) cells/dish resulted in cultures which responded to TSH with follicular development and increased radioiodide uptake. A cell-free membrane fraction of thyroid homogenate restored the ability of cultures seeded at low densities to respond to TSH with development of follicular morphology and increased radioiodide uptake. Delaying the addition of TSH by 48 h markedly reduced the stimulation of follicular development and radioiodide uptake of cultures. Addition of membrane fractions, or an alkali-soluble fraction of membranes, at zero time improved the responses to TSH added after a 48-h delay. It was concluded that maintenance of differentiation and of TSH-responsiveness in cultured thyroid cells was influenced by cell-cell contact.

Ionic mechanisms regulating thyroidal secretion: effects of ouabain and medium sodium concentration on radioiodine release from cultured porcine thyroid cells

Thyrotrophin stimulated release of radioiodine from the organic iodine pool of cultured porcine thyroid cells. The response was well developed within 2-4 h of incubation. Inhibition of the Na+/K+ pump with ouabain or incubation in sodium-free medium inhibited the response to TSH. The magnesium content of cultures was reduced by ouabain, and increasing the magnesium concentration of the medium to 10 mmol/l reversed the inhibition of the TSH response by ouabain. After prolonged incubation (4-6 h), ouabain in magnesium-enriched medium stimulated release of radioiodine. Its effects were not additive with those of TSH. Incubation for 4-6 h in media of reduced sodium concentration (34 mmol/l) also stimulated release. Sodium-free medium alone did not alter basal release rates, but magnesium enrichment of sodium-free medium promoted release after 4 h of incubation. It was concluded that the previously reported inhibition of the TSH response in thyroid tissue by ouabain or sodium-free medium was due to secondary derangements of cellular function rather than to a specific blockade of the secretomotor signal. The data are consistent with the hypothesis that a reduction in the sodium ion electrochemical gradient across the cell membrane mediates the secretomotor effect of TSH.

Dedifferentiation of cultured thyroid cells by epidermal growth factor: some insights into the mechanism

Epidermal growth factor (EGF) has been shown to enhance both the proliferation and dedifferentiation of thyroid cells in culture, leading to a maintained dedifferentiated state, even in the presence of thyrotropin (TSH). Since this maintained loss of differentiated function is not seen with other mitogens, it may relate to a regulatory role for EGF in thyroid function. Therefore, we have examined the loci affected by the dedifferentiative actions of EGF using porcine thyroid cells in culture. EGF (10 ng/ml) induces a loss of thyrotropin (TSH) receptors with a time course identical to the loss in ability to transport iodide. This could account for the difference in extent of iodide uptake and morphological dedifferentiation seen between TSH- and cAMP-supported cells, although the fact that cAMP-supported cells also dedifferentiate implies a lesion distal to the cyclase. Reciprocal plot analysis of iodide uptake in control and EGF-treated cells shows that EGF increases the Km for iodide transport, corresponding to a decreased affinity of iodide pump sites for iodide. These effects on iodide pump affinity and TSH receptor number may result from reversal of thyroid cell polarity in monolayer culture, or they may be the result of more specific actions of EGF at these loci. It has been possible to discriminate between the proliferative and dedifferentiating actions of EGF using amiloride, a non-specific inhibitor of the Na+/H+ antiporter. An optimum concentration of amiloride (0.1 mM) was able to block EGF-stimulated incorporation of [3H]thymidine into DNA without preventing the blockade of iodide uptake, which implies that dedifferentiation is not a consequence of proliferation.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of sodium influx in thyrotrophin action: effects of the sodium channel agonist veratridine and thyrotrophin on radioiodine turnover and membrane potential in cultured porcine thyroid cells

Veratridine, a sodium channel agonist, depolarized cultured thyroid cells and increased the secretion of radioiodine from the organically bound pool. These effects were similar to those of TSH. Depolarization of the cells by increasing the potassium concentration of the medium failed to promote secretion, indicating that the sodium influx, rather than the depolarization itself, mediated the response. Veratridine, like TSH, also acutely reduced the cells' iodide uptake and inhibited the iodide transport pump. Unlike TSH, however, veratridine reduced, rather than increased, the fractional exit rate of iodide anion from the unbound pool. The data are consistent with the hypothesis that a sodium influx mediates some but not all of the actions of TSH on the thyroid gland, including the stimulation of secretion of thyroid hormones.

Effect of epidermal growth factor on the membrane potential of cultured porcine thyroid cells

Cultured porcine thyroid cells maintained in media containing TSH exhibited a membrane potential of -50 mV, and hyperpolarized by about 10 mV within 1 h of the addition of epidermal growth factor (EGF; 10 ng/ml). Follicle cells had depolarized to -45 mV after 4 h of exposure to EGF. Cells maintained in dibutyryl cyclic AMP (dbcAMP) did not alter their membrane potential when exposed to EGF for up to 4 h. Cultures washed to remove the TSH or dbcAMP hyperpolarized to -75 mV within 30 min, and a reversible depolarization to -60 mV was observed on addition of EGF. It was concluded that EGF acts as a physiological antagonist of TSH and also exerts a separate depolarizing influence on cultured thyroid cells.

Sodium dependence of the thyrotrophin-induced depolarization in cultured porcine thyroid cells

Cultured porcine thyroid cells exhibit a resting membrane potential of about -73 mV and depolarize to about -54 mV on exposure to TSH. The depolarizing response to TSH was preserved in a medium consisting only of inorganic salts and buffers, but was abolished in sodium-free medium, demonstrating dependence on an inward sodium current. Increasing the potassium concentration of the medium resulted in a reduction in the resting membrane potential of 60 mV per tenfold change in potassium concentration, and a diminished TSH response. A hyperpolarizing TSH response was observed in a sodium- and bicarbonate-free medium, indicating that a hyperpolarizing ion current (probably carried by potassium) was also enhanced in the presence of TSH. Tetrodotoxin blocked the TSH response. We conclude that the response of the thyroid cell membrane to TSH involves increases in permeability to sodium and potassium, and that the thyroid membrane ion channels bear some similarity to the voltage-dependent sodium channels of excitable tissues, despite the absence of action potentials in the thyroid.

Electrical responses of cultured thyroid cells to serotonin

Cultured porcine thyroid cells, maintained in the differentiated state by dibutyryl cyclic AMP, responded to serotonin (5-HT; 10 nmol/l to 1 mumol/l) with a depolarization of the membrane potential, but did not respond to histamine (100 mumol/l) or dopamine (1 mumol/l). The resting membrane potential of these cells was about -71 mV, maximal concentrations of 5-HT (1 mumol/l) inducing a depolarization to approximately -53 mV. Methysergide or phenoxybenzamine, but not propranolol, abolished the response to 5-HT. Sensitivity to 5-HT was reduced by previous exposure of cultures to TSH, the beta-adrenoceptor agonist salbutamol or 5-HT itself.

Electrical responses of cultured porcine thyroid cells to adrenergic agents

The membrane potential of cultured porcine thyroid follicular cells depolarized by up to 20 mV from the resting value of about -73 mV on exposure to beta-adrenoceptor agonists. A similar response was induced by TSH or dibutyryl cyclic AMP. alpha-Adrenoceptor agonists were without effect. The receptor subtype was shown to be (at least predominantly) beta 2 by the order of potency for beta-agonists (isoprenaline approximately equal to fenoterol much greater than adrenaline greater than noradrenaline) and by the relative potency of selective beta-antagonists (ICI 118,551 much greater than atenolol). The alpha-agonist phenylephrine had no effect on the TSH response but weakly inhibited the beta-agonist response. Rather than a physiological antagonism between alpha- and beta-adrenoceptor-mediated responses, this effect was shown to be due to the weak beta-antagonist effect of phenylephrine since the alpha-antagonist phentolamine failed to potentiate the depolarizing response to the mixed agonist noradrenaline, and also failed to block the inhibitory action of phenylephrine on the beta-agonist effect. Sensitivity to beta-agonist was enhanced by omission of serum from the culture medium and reduced by exposure to beta-agonists or a high concentration of TSH or dibutyryl cyclic AMP.

The species specificity of TSH receptor binding antibodies as measured by radioreceptor study

The species specificity of TSH binding inhibitory antibodies was compared for patients with untreated Graves' hyperthyroidism, past Graves' hyperthyroidism, active ophthalmopathy with past hyperthyroidism, and subacute thyroiditis, by measuring inhibition of TSH binding to plasma membranes prepared from human, guinea-pig, calf, pig, and dog thyroid glands in a radioreceptor assay. Results were expressed as TSH binding inhibition indices (TBII). Broad species reactivity was demonstrated. This was greatest with pig and least with guinea-pig thyroid membranes. Immunoglobulin (Ig) from patients in whom strongly positive tests with human thyroid preparations were demonstrated were usually strongly positive with all other species tested, whereas Ig from patients which were less strongly positive with human were, generally, also less positive with the other species. There was a tendency for greater species reactivity of TSH binding inhibiting antibodies from patients with treated Graves' hyperthyroidism (with or without eye disease) than of those from untreated patients with Graves' hyperthyroidism or subacute thyroiditis. Combining the data from all groups, correlation between TBII for human membranes and those of other species was best for dog and least for guinea-pig. It is concluded that the TSH binding inhibiting antibody is a polyclonal antibody against a single antigen at or near the TSH receptor, and that the degree of reactivity with its antigen in other species depends, mainly, on the amount of antibody present in the serum.

Receptor heterogeneity in the human thyroid: differences between thyrotrophin binding sites in membrane and nuclear fractions

Binding of 125I-labelled bovine TSH to crude membrane fractions of human thyroid tissue was a saturable, hormonally specific process which yielded non-linear Scatchard plots with limiting affinities of approximately 10(9) and 10(7) l/mol. Binding activity in membranes was soluble in Triton X-100, was inhibited specifically by immunoglobulins from patients with Graves's disease, and was increased by the beta-blocking drug, propranolol. In contrast, purified nuclear preparations showed a predominance of lower affinity binding, and their binding activity was insoluble in Triton and insensitive to immunoglobulins from patients with Grave's disease and to propranolol. Tryptic digestion liberated only low affinity binding activity from membranes or nuclei. It was concluded that human thyroid tissue contains independent classes of TSH-binding sites, which differ in their chemical, immunological and hormone-binding properties.

Changes in membrane potential of cultured porcine and human thyroid cells in response to thyrotrophin and other agents

Thyrotrophin (TSH), cyclic AMP, cyclic GMP and 1-methyl-3-isobutyl-xanthine (MIX) promoted the reassociation of isolated porcine and human thyroid cells into follicular structures in culture and stimulated the uptake of radio-iodide. Monolayer cells were present in all cultures, but in decreasing proportions as the concentration of stimulator was increased. The resting membrane potential of porcine thyroid cells cultured for 4 days in the presence of TSH was -54 +/- 3.6 (mean +/- S.D.) mV for follicular cells and -31 +/- 2.6 mV for monolayer cells. In the absence of TSH, only monolayer cells were present and their membrane potential was -24 +/- 2.0 mV. Removal of hormone by washing resulted in hyperpolarization to -70 +/- 2.9 mV (follicular cells) or -59 +/- 3.4 mV (monolayer cells). Subsequent replacement of TSH, or addition of cyclic AMP, MIX, prostaglandin E1 (PGE1) or long-acting thyroid stimulator immunoglobulin resulted in depolarization of previously hyperpolarized cells, to approximately the membrane potential observed before washing. Incubation in MIX resulted in enhanced sensitivity to the depolarizing effect of TSH. Cells cultured in the absence of TSH were unresponsive to TSH or other stimulators. The membrane potential of human thyroid cells behaved similarly in response to TSH, to hormone removal and replacement, and to MIX and PGE1.

Inflammatory peptide in spinal cord: evidence that the mediator of antidromic vasodilatation is not substance P

Extracts from rat and bovine spinal cord were found to have 300-1000 times more cutaneous oedema-inducing activity than could be attributed to their substance P-like activity estimated by assay on guinea-pig ileum. The activity on both assay systems was reduced in cord extracts from rats pretreated as neonates with capsaicin. Results of assays of fractions obtained from gel-filtration chromatography indicated that an agent, in the molecular size range for peptides, was present in spinal cord extracts, which possessed some of the properties of substance P but was not identical to it.