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R Ekholm

Publications and source records attributed to R Ekholm.

At least 37 records · Page 2Linked to original sources
PubMed1981

Site of iodination in hyperplastic thyroid glands deduced from autoradiographs.

We have tried to ascertain the site of iodination in the chronically stimulated, hyperplastic thyroid gland of rats. Rats were fed propylthiouracil in a commercial rat diet for 10 days. Then the diet was changed to a low iodine diet for 5 days. To label the gland, 10 mCi of 125I-iodide was injected into the left heart ventricle. Ten seconds later the animal was perfused through the left ventricle with a fixative solution containing a goitrogen to block further iodination, and stable iodide to help extract uncombined radioiodide. Electron microscopic autoradiographs prepared from the fixed thyroids show strong labeling over the lumen of the follicle and no consistent labeling of any other site or organelle. We conclude that the site of iodination in the chronically stimulated, hyperplastic thyroid is the follicular lumen, i.e. the same as that in the normal gland.

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PubMed1980

The structure of newly synthesized intracellular thyroglobulin molecules.

Thyroglobulin molecules from normal rats and rats treated with propylthiouracil (PTU) were studied in the electron microscope by the negative staining technique. Intracellular molecules were prepared from exocytotic vesicles, and extracellular molecules from the high-speed centrifugation supernatant. In accord with our previous findings, extracellular thyroglobulin molecules from normal glands had an ovoid conformation, whereas extracellular molecules from PTU-treated glands had a different, cylindrical shape. Thyroglobulin molecules from exocytotic vesicles were of the ovoid variety in normal as well as in PTU-exposed thyroids. Two interpretations are discussed. One would be that PTU exerts a non-physiological effect on ovoid molecules in connection with exocytosis of thyroglobulin. The other interpretation is that thyroglobulin molecules normally undergo 2 transformations (ovoid-cylinder-ovoid) during the process of exocytosis; PTU would inhibit the second transformation and cause accumulation of cylinders in the colloid.

Animals↗
PubMed1980

Conformational change of the thyroglobulin molecule induced by oxidation in vitro.

Previous electron-microscopical studies from this laboratory have shown that the thyroglobulin molecule can occur in two different conformations, one ovoid and the other cylindrical. Ovoid molecules are characteristic of well-iodinated thyroglobulin whereas cylindrical molecules are found after low-iodine diet or blocking of iodination. The present study was performed in order to elucidate the possible relation between the molecule conformation and the peroxidase-catalyzed reactions that occur in the thyroid in connection with hormone synthesis. Cylindrical thyroglobulin molecules (from PTU-exposed thyroids) were incubated in different media and the proportion of cylindrical and ovoid molecules after incubation was estimated in electron micrographs. It was found that incubation with glucose-glucose oxidase caused an extensive conversion of cylindrical molecules into ovoid molecules. Peroxidase and/or iodide were not necessary for this change of conformation. It is suggested that this in vitro molecule transformation was the result of an oxidation reaction.

Animals↗
PubMed1978

Effects of thyrotropin on thyroglobulin exocytosis and iodination in the rat thyroid gland.

Rats, pretreated with thyroxine for 2 days, were given one or two iv injections of 500 mU of TSH; in some groups the second TSH dose was replaced by 0.75 micronmol isoproternol. The effects of the thyroid stimulators on the following parameters were studied: the number of exocytotic vesicles in the follicle cells; the incorporation of 125I into thyroid proteins, measured over periods of 5 min; and the thyroidal cAMP contents. At 2 h after TSH administration, a second dose of TSH failed to stimulate iodination while at 8 h the iodination response was "normal". Two hours after TSH the follicle cells contained practically no exocytotic vesicles but at 8 h they had a full supply of vesicles, and this was emptied by the second TSH injection. THE CAMP content was less increased by the second TSH injection than by the first one, but the stimulatory effect of the second TSH dose on cAMP was the same at 2 h and at 8 h; this indicates that the lack of iodination response at 2 h was not simply due to blocking of TSH receptors. Isoproternol, which acts on other receptors than does TSH, cause a similar cAMP increase incontrols and at 2 h and 8 h after TSH, but stimulated iodination only in controls and at 8 h after TSH; this supported the conclusion that the lack of iodination response to a second TSH dose at 2 h was not due to impairment of the adenylate cyclase-cAMP system. These observations taken together strongly indicate that a rapid iodination response to TSH depends on stimulated exocytosis which, in turn, requires a pool of exocytotic vesicles in the follicle cells. Such a coupling between exocytosis and iodination seems appropriate since by exocytosis uniodinated thyroglobulin and membrane, showing peroxidase activity histochemically, are delivered to the site of iodination, the apical cell surface.

Animals↗
PubMed1977

Ultrastructural and functional aspects of frog heart lesions after isoproterenol.

The purpose of the present study was to explore the possible correlation between the ultrastructural changes and the functional changes induced by isoproterenol (IPR) in the myocardium of the frog. The study was performed on winter frogs (R. pipiens), The experimental animals wend then given 2 daily doses of IPR (200 mg/kg b,w.). Twenty-four hours after the second IPR dose, specimens were taken from the heart ventricle and subjected to electron microscopical examination and to analysis of the isometric force decay in response to anoxia produced by sodium cyanide. Specimens were taken from macroscopically non-damaged portions of the ventricle as well as from aneurysms. Control animals were kept at + 8 degrees C and + 25 degrees C and injected with saline. In all IPR-injected animals the specimens from macroscopically non-damaged parts of the myocardium showed ultrastructural changes characterized by a relative increase in mitochondrial mass and a pronounced reduction of glycogen. Specimens from aneurysms showed similar changes but in some fibres signs of more severe injury were observed such as fragmentation of the cristae of the mitochondria and disintegration of myofibrils. The functional analyses showed that the isometric force decay of the heart muscle strips induced by the respiratory block by cyanide was much greater in specimens from IPR-treated animals than in those from control frogs. The augmented force decay was found in both strips from macroscopically normal parts of the myocardium and in those from aneurysms although it was somewhat greater and more rapid in the latter. The observed, minor functional difference between macroscopically non-damaged portions of the myocardium and aneurysms is remarkable with respect to the different preservation of myofilaments in these regions. This similarity in functional decline could possibly be due to the observed homogenous reduction of glycogen in the IPR-exposed myocardium since glycogen was the only source of ATP under the experimental conditions of current interest.

Animals↗
PubMed1977

Protein concentration of the rat thyroid colloid during thyroxine treatment.

Sprague-Dawley rats were given daily doses of 20 mug thyroxine for varying periods between 1 and 21 days. Samples of colloid were obtained by micropuncture in vivo of follicles in the thyroid isthmus. The protein content of the samples was determined by gel electrophoresis and the volume by a technique based on radioactivity measurement. The relative volume of the follicle lumina was determined by light microscopy. Treatment with thyroxine resulted in an increase in the protein concentration of the colloid. This increase was statistically significant after 2 days and the concentration remained elevated throughout the experimental period. The size of the follicle lumina also increased, most rapidly during the first week. It is suggested that the increase in protein concentration of the colloid during thyroxine treatment is due to inhibition of endocytosis of colloid in combination with the discharge into the follicle lumen of highly concentrated protein.

Animals↗
PubMed1975

Electron microscopy of low iodinated thyroglobulin molecules.

Thyroglobulin molecules were studied in the electron microscope with negative staining technique. In a first series of experiments samples of thyroglobulin varying in iodine content from 0.5 to 0.03% were prepared from the thyroids of mice and rats kept on iodine-poor diets. All samples contained thyroglobulin molecules of the normal ovoid shape, not deviating in size or shape from molecules obtained from normal thyroids. However, in addition, another type of molecule having a cylindrical shape was observed in all samples. The proportion of these cylindrical molecules increased from a few per cent in the moderately iodine-poor thyroglobulin samples to more than 80% in the highly iodine-deficient thyroglobulin (0.03%). In a second series of experiments extremely iodine-poor thyroglobulin (smaller than 0.005%) was obtained from propylthiouracil-treated rats. In these preparations practically all molecules had a cylindrical shape. These samples also contained smaller particles interpreted to be dissociation products. The cylindrical molecules were of two types, one appearing compact and measuring 250 times 135 A (length times diameter) and the other appearing porous and having a length of 145 and a diameter of 205 A. It is concluded that the cylindrical molecules represent non- or low-iodinated thyroglobulin and it is suggested that the porous cylindrical molecule is an unfolded form of the compact cylinder.

Animals↗
PubMed1975

Exocytosis of protein into the thyroid follicle lumen: an early effect of TSH.

As shown in a preceding paper, only exocytotic vesicles conveying newly synthesized protein to the follicle lumen remain in the apical part of rat thyroid follicle cells following elimination of TSH secretion. In the present paper the effect of TSH on these exocytotic vesicles was investigated. TSH secretion was suppressed by administration of thyroxine for 2 days. In the electron microscope administration of TSH was seen to induce well-known signs of endocytosis, such as formation of pseudopods and colloid droplets. In addition, a previously unrecognized change was noted, namely a progressive decrease in the number of exocytotic vesicles. At 5 min after TSH the number was obviously reduced and at 20 min less than 10% of the original number of vesicles remained. Quantitative electron microscopic autoradiography after administration of [3H]leucine showed that TSH caused, concomitant with the disappearance of vesicles, a transfer of radioactivity from the apical region of the follicle cell to the periphery of the follicle lumen. The distribution of labeled protein in thyroid subcellular fractions was studied 1.5 h after administration of [14C]leucine. At 5 min after administration of TSH there was an increase of protein-bound label in the supernatant fraction, containing the luminal colloid, and a corresponding decrease of label in the particle fraction which contained most of the cell organelles, including the exocytotic vesicles. This TSH-induced redistribution of labeled proteins was more pronounced at 10 min and still more at 20 min and appeared to be dose-dependent. These observations taken together are considered to justify the conclusion that TSH induces transfer of newly synthesized protein from the follicle cells to the follicle lumen by exocytosis (i.e., emptying of specific apical vesicles). It is suggested that a causal and functional interrelation may exist between the exocytotic and endocytotic processes.

Animals↗
PubMed1975

Site of iodination in the rat thyroid gland deduced from electron microscopic autoradiographs.

The site of iodination of protein in the thyroid gland (whether intracellular or intraluminal) was ascertained by autoradiographic studies using iodide-125I. In tissue fixed within about 40 sec after intravenous injection of radioiodide the silver grains of autoradiographs were concentrated over the follicular lumen generally as a ring of grains close to the apical border of the follicular cells. The zone of grains was sharply limited toward the cells. No concentration of silver grains was detected associated with any intracellular organelle. The autoradiographic ring which had a minimum width of about 2 mum was continuous along the apical plasma membrane of the follicle cells but there was a drastic reduction in grain density along the plasma membrane of the distal portion of pseudopods. Tissue was fixed so soon after radioiodide injection that it appeared likely that a negligible fraction of radioiodoprotein, if formed in the cell, could have been transferred to the lumen. The observations strongly indicate that the iodination of thyroglobulin occurs in the follicle lumen, probably at the apical surface of the follicle cells. Since in the TSH-treated animals the distribution of the labeling along the apical plasma membrane agrees well with the reported histochemical distribution of thyroperoxidase in this membrane, it is further concluded that iodination may well be catalyzed by peroxidase in the apical plasma membrane.

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