Tumor cell killing by macrophages activated in vitro with lymphocyte mediators. III. Inhibition by cytochalasins, colchicine, and vinblastine.

The effect of cytochalasin A and B, colchicine and vinblastine on tumor cell killing by macrophages activated in vitro with lymphocyte mediators was examined. Both cytochalasins reversibly inhibited the killing of tumor cells by activated macrophages. Kinetic studies with cytochalasin B suggested that this drug exerts its effect on an early step of the cytotoxic process. Additional studies revealed that the drug inhibited the binding of tumor cells by activated macrophages.Colchicine inhibited both the binding and the killing of tumor cells by activated macrophages, whereas its structural analogue, lumicolchicine, had no effect on either macrophage function.Vinblastine also inhibited the binding and killing of tumor cells. However, this drug no longer inhibited tumor cell binding at low concentrations (<10^?6M) that still inhibited tumor cell killing. Further, vinblastine inhibited tumor cell killing when added late to an ongoing cytolytic reaction.These results suggest that the cytochalasins, colchicine and vinblastine inhibit macrophage mediated cytotoxicity by preventing intimate contact between the effector macrophages and their targets. In addition, vinblastine also appears to inhibit a later step of the cytolytic process, possibly the secretion of a cytotoxic macrophage product.     

Influence of colchicine and vinblastine on the intracellular migration of secretory and membrane glycoproteins: III. Inhibition of intracellular migration of membrane glycoproteins in rat intestinal columnar cells and hepatocytes as visualized by light and electron-microscope radioautography after 3H-fucose injection

In the first paper of this series (Bennett et al., 1984), light-microscope radioautographic studies showed that colchicine or vinblastine inhibited intracellular migration of glycoproteins out of the Golgi region in a variety of cell types. In the present work, the effects of these drugs on migration of membrane glycoproteins have been examined at the ultrastructural level in duodenal villous columnar cells and hepatocytes. Young (40 gm) rats were given a single intravenous injection of colchicine (4.0 mg) or vinblastine (2.0 mg). At 10 min after colchicine and 30 min after vinblastine administration, the rats were injected with 3H-fucose. Control rats received 3H-fucose only. All rats were sacrificed 90 min after 3H-fucose injection and their tissues processed for radioautography. In duodenal villous columnar cells, 3H-fucose labeling of the apical plasma membrane was reduced by 51% after colchicine and by 67% after vinblastine treatment; but there was little change in labeling of the lateral plasma membrane. Labeling of the Golgi apparatus increased. This suggests that labeled glycoproteins destined for the apical plasma membrane were inhibited from leaving the Golgi region, while migration to the lateral plasma membrane was not impaired. In hepatocytes, labeling of the sinusoidal plasma membrane was reduced by 83% after colchicine and by 85% after vinblastine treatment. Labeling of the lateral plasma membrane also decreased, although not so dramatically. Labeling of the Golgi apparatus and neighboring secretory vesicles increased. This indicates that the drugs inhibited migration of membrane glycoproteins from the Golgi region to the various portions of the plasma membrane. Accumulation of secretory vesicles at the sinusoidal front suggests that exocytosis may also have been partially inhibited. In both cell types, microtubules almost completely disappeared after drug treatment. Microtubules may, therefore, be necessary for intracellular transport of membrane glycoproteins, although the possibility of a direct action of these drugs on Golgi or plasma membranes must also be considered.     

The Mechanism of Somatic Association in Common Wheat, TRITICUM AESTIVUM L. IV. Further Evidence for Modification of Spindle Tubulin through the Somatic-Association Genes as Measured by Vinblastine Binding

Treatment with the antitubulin vinblastine was found to disrupt the spindle system in dividing root-tip cells of common wheat, Triticum aestivum L. Genotypes lacking the somatic association suppressor gene on 5B^L, or containing the somatic-association promoter on 5B^S, were found to be more sensitive to the treatment. In genetic lines carrying the somatic association suppressor, sensitivity to vinblastine was lower and there was a direct correlation between dosage of the suppressor gene (0, 2, and 4) and the decrease in spindle disruption on exposure to various concentrations of vinblastine. It is concluded that the somatic association genes affect binding ability of spindle tubulin to vinblastine. Since the same genes affect binding of colchicine to tubulin and since the two alkaloids attach to different sites it is assumed that the somatic association suppressor gene has a broad effect on the tubulin molecules which is not confined to a single site. The relevance of genetic control of antitubulin binding to somatic association is discussed.     

Effect of Colchicine,Vinblastine, and Cytochalasin B on Cell Surface Anionic Sites of Tritrichomonas foetus1

ABSTRACT. Drugs that interact with microtubules (colchicine and vinblastine) and microfilaments (cytochalasin B) partially inhibited cell growth and motility of Tritrichomonas foetus. Parasites incubated with these substances became rounded and cell division was blocked. Neither colchicine nor vinblastine disrupted the microtubules that form the peltar-axostylar system. Any one of these drugs interfered with the net negative surface charge of T. foetus as evaluated by determination of the cellular electrophoretic mobility (EPM). The decrease in the EPM of cytochalasin B-treated cells was caused by dimethylsulfoxide, which was used as solvent. Untreated cells as well as cytochalasin B-treated cells showed a uniform distribution of anionic sites on the plasma membrane as seen with cationized ferritin particles. In cells treated with colchicine or vinblastine the anionic sites were distributed in patches. These results are discussed in terms of participation of labile cytoplasmic microtubules and microfilaments in the control of the distribution of anionic sitecontaining macromolecules located on the cell surface of T. foetus.     

Vinca alkaloids inhibit conversion of arachidonic acid to thromboxane by human platelet microsomes: comparison with other microtubule-active drugs

The Vinca alkaloid vinblastine causes dose-dependent inhibition of malondialdehyde formation and aggregation in activated human platelets as a result of inhibition of arachidonic acid metabolism via the thromboxane pathway (Brammer, J.P., Kerecsen, L. and Maguire, M.H. (1982) Eur. J. Pharmacol. 81, 577). The nature of the inhibition by vinblastine has been investigated with human platelet microsomes, measuring conversion of arachidonic acid to malondialdehyde and thromboxane B2 via spectrophotometric assay and RIA, respectively, determining arachidonate oxygenation by monitoring oxygen consumption, and identifying metabolites formed from [1-14C]arachidonic acid. Vinblastine was compared with other Vinca alkaloids and with structurally unrelated microtubule-active drugs. Vinca alkaloids were unique in causing dose-dependent inhibition of both malondialdehyde and thromboxane B2. Order of potency was vinblastine = vincristine = vindesine greater than leurosine greater than vinepidine. Inhibition of malondialdehyde and thromboxane B2 by 50 microM vinblastine was at least 60%. Microsomal cyclooxygenase was not inhibited by 200 microM vinblastine. Inhibition by vinblastine of [1-14C]arachidonic acid conversion to thromboxane B2 was associated with a 4-fold increase in prostaglandin E2 formation. Thromboxane B2, but not malondialdehyde, formation was inhibited by colchicine less than nocodazole much less than vinblastine. Results indicate that microsomal thromboxane synthetase is inhibited by Vinca alkaloids and other tubulin-binding drugs, and suggest that the action of vinblastine in inhibiting thromboxane synthesis, aggregation and release in intact platelets is not dependent upon its antimicrotubular actions.     

Effect of colchicine, vinblastine, and cytochalasin B on cell surface anionic sites of Tritrichomonas foetus

Drugs that interact with microtubules (colchicine and vinblastine) and microfilaments (cytochalasin B) partially inhibited cell growth and motility of Tritrichomonas foetus. Parasites incubated with these substances became rounded and cell division was blocked. Neither colchicine nor vinblastine disrupted the microtubules that form the peltar-axostylar system. Any one of these drugs interfered with the net negative surface charge of T. foetus as evaluated by determination of the cellular electrophoretic mobility (EPM). The decrease in the EPM of cytochalasin B-treated cells was caused by dimethylsulfoxide, which was used as solvent. Untreated cells as well as cytochalasin B-treated cells showed a uniform distribution of anionic sites on the plasma membrane as seen with cationized ferritin particles. In cells treated with colchicine or vinblastine the anionic sites were distributed in patches. These results are discussed in terms of participation of labile cytoplasmic microtubules and microfilaments in the control of the distribution of anionic site-containing macromolecules located on the cell surface of T. foetus.     

Effects of colchicine, vinblastine and nocodazole on polarity, motility, chemotaxis and cAMP levels of human polymorphonuclear leukocytes

We present evidence for intrinsic polymorphonuclear leukocyte (PMN) polarity manifested in presence of microtubule-disrupting drugs. Polarization in response to colchicine correlated with the known dose-dependent effects of this drug on microtubule disassembly. The response to 10(-5) M colchicine, 10(-5) M vinblastine and 10(-6) M nocodazole was associated with stimulated motility and random locomotion. Responses elicited by microtubule-disrupting drugs differed from f-Met-Leu-Phe (fMLP)-induced polarization by functional and morphological criteria. Polarization, motility and orthokinesis responses were much weaker. Furthermore, ruffling was almost absent in PMNs polarized in response to colchicine, vinblastine or nocodazole. The response was inhibited by cytochalasin B, indicating that it is microfilament-dependent. We suggest that microtubule-disrupting drugs induce motility via structural changes in the cytoskeleton which act as signals for the motor apparatus. The intrinsic polarity manifested in the presence of microtubule-disrupting drugs could be reversed by an extracellular chemotactic gradient. Stimulated locomotion and motility in response to microtubule-disrupting drugs was only observed with initially spherical PMNs but not with initially motile cells. The findings provide an explanation for the numerous conflicting statements on the chemokinetic activities of these drugs. The role of cAMP in stimulated polarization and motility has been studied. Colchicine, vinblastine and nocodazole elicited a transient elevation of cAMP levels within 1 min of stimulation. cAMP elevation and stimulated motility were not quantitatively correlated.     

Macrophage spreading in vitro. III. The effect of metabolic inhibitors, anesthetics and other drugs on spreading induced by subtilisin

The effect of certain drugs on macrophage spreading induced by the proteolytic enzyme sub-tilisin was quantitatively examined and the 50 and 90 % inhibitory concentrations of the drugs were determined. In most instances the viability of the macrophages was preserved, as shown by phagocytic tests and by experiments in which cells pretreated with the drugs and washed were shown to spread when exposed to subtilisin. Inhibitors of electron transport, oxidative phosphorylation or uncouplers at rather small concentrations all effectively blocked macrophage spreading, indicating an ATP requirement. Spreading was also inhibited by neutral or cationic anesthetics and the reciprocal of their 50 % inhibitory concentrations was linearly related to their octanol-water partition coefficients. Inhibition by the anesthetics paralleled their effects on other membrane phenomena, such as nerve conduction, osmotic lysis of erythrocytes, viral induced cell fusion, or Sarcoma I cell to substrate adhesion, also suggesting a membrane target. Spreading was reduced by the anti-inflammatories indomethacin, or phenylbutazone, by high doses of colchicine or vinblastine, by the putative microfilament-acting drug cytochalasin B or by the SH- reagent n-ethyl maleimide. Colchicine and vinblastine effects may involve mechanisms other than their microtubular actions. Several other drugs, including inhibitors of protein synthesis, did not inhibit the spreading of macrophages. Spreading induced by substrate-bound immune complexes was also inhibited by representative metabolic inhibitors or anesthetics.     

Transport properties of P-glycoprotein in plasma membrane vesicles from multidrug-resistant Chinese hamster ovary cells.

Multidrug resistant (MDR) cells overexpress a 170-180 kDa membrane glycoprotein, the P-glycoprotein, which is believed to export drugs in an ATP-dependent manner. Plasma membrane vesicles from the MDR CHRC5 cell line, but not the AuxB1 drug-sensitive parent, showed uptake of [3H]colchicine and [3H]vinblastine that was stimulated by the presence of ATP and an ATP-regenerating system. Steady-state uptake of drugs was achieved by 10 min and was stable for greater than 30 min. Non-hydrolysable ATP analogues were unable to support drug uptake, indicating that ATP hydrolysis is essential for transport. ATP-stimulated drug uptake appeared to result from drug transport into inside-out vesicles, since uptake was osmotically sensitive and could be prevented by detergent permeabilization. Steady-state uptake was half-maximal at 100 microM colchicine and 200 nM vinblastine and was inhibited by a 10-100-fold excess of MDR drugs and chemosensitizers, in the order vinblastine greater than verapamil greater than daunomycin greater than colchicine. In addition to being vanadate-sensitive, drug uptake was inhibited by 10-200 microM concentrations of several sulfhydryl-modifying reagents, suggesting that cysteine residues play an important role in drug transport. Vesicular colchicine was rapidly exchanged by an excess of unlabelled drug, demonstrating that drug association is the net result of opposing colchicine fluxes across the membrane.     

Detection of natural products that induce aberrations of the mitotic spindle

A number of natural products which inhibit the synthesis and functioning of the mitotic spindle have been shown to be potent antitumour agents. As an aid to the detection and evaluation of these chemicals that produce aberrations of mitotic cell division we have utilized a model screening system based upon the simultaneous visualisation of both the mitotic spindle and the chromosomes.This assay allows the detection of morphological aberrations of both spindle synthesis and function produced by exposure to chemicals active upon the mitotic spindle. This assay has been evaluated for its ability to detect natural spindle damaging agents. Extracts of Catharanthus roseus, which produces the potent spindle toxins vinblastine and vincristine, derived from both leaves and tissue culture samples were assayed for their ability to induce spindle aberrations in human fibroblasts. Samples of extract which lacked natural vinblastine were spiked with pure vinblastine sulphate to estimate the sensitivity of the assay to detect low concentrations of known spindle toxins.This study indicates that spindle toxins may be identified in specific plant extracts at high levels of sensitivity thus providing an effective screening technique for the identification of potentially valuable plant materials for their use as sources of antitumour chemicals.     

Hydrodynamics of cytoplasmic streaming and the cytoskeletal tubulin component in cells of <Emphasis Type="Italic">Elodea densa</Emphasis> internodes

A spin-echo NMR method was applied to study hydrodynamic parameters of cytoplasmic streaming and to examine translational diffusion of cytoplasmic water in stems of elodea (Elodea densa (Planch.) Casp.) treated with antimitotic drugs. Within the first few hours after colchicine treatment, the dynamics of translational displacement followed the curve with a transient maximum. The action of vinblastine was manifested in a monotonic decrease of the translational diffusion. After 24-h treatments with colchicine and vinblastine, the translational diffusion was slowed down. All these changes induced by antimitotic drugs were observed on the background of unaltered rate of cytoplasmic streaming.     

The effect of cytochalasin B,colchicine and vinblastine on the adhesion of Trichomonas vaginalis to glass surfaces

The attachment of Trichomonas vaginalis to glass surfaces was studied in the presence of cytochalasin B, colchicine and vinblastine. Marked inhibition of adhesion was noted with high concentrations of cytochalasin B. Colchicine and vinblastine were without effect. These findings suggest that Trichomonas vaginalis adhesion is at least partially mediated by the extension of cellular probes, due to the action of cytochalasin-sensitive microfilaments.     

Microtubules and pancreatic amylase release by mouse pancreas in vitro

The effects of vinblastine and colchicine on pancreatic acinar cells were studied by use of in vitro mouse pancreatic fragments. Vinblastine inhibited the release of amylase stimulated by bethanechol, caerulein, or ionophore {"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187. Inhibition required preincubation with vinblastine,and maximum inhibition was observed after 90 min. Inhibition was relatively irreversible and could not be overcome by a high concentration of stimulant. Inhibition could also be produced by colchicine although longer preincubation was required and inhibition was only partial. Uptake of [3H]vinblastine and [3H]colchicine by pancreatic fragments was measured and found not to be responsible for the slow onset of inhibition by these drugs. In incubated pancreas, microtubules were present primarily in the apical pole of the cell and in association with the Golgi region. Vinblastine, under time and dose conditions that inhibited the release of stimulated amylase, also reduced the number of microtubules. The only other consistent structural effects of vinblastine were the presence of vinblastine- induced crystals and an increased incidence of autophagy. The remainder of cell structure was not affected nor were overall tissue ATP and electrolyte contents or the stimulant-induced increase in 45Ca++ efflux. It is concluded that the antisecretory effects of vinblastine and colchicine are consistent with a microtubular action, but that acinar cell microtubules are more resistant to the drugs than many other cell types.     

Effect of various antimicrotubular drugs on the osmotic water flow through the wall of frog's urinary bladder]

The action of antimicrotubular drugs (colchicine, vinblastine and copper) on the osmotic water flow through the wall of the urinary bladder of Rana temporaria has been studied. The osmotic gradient was made by five- or tenfold dilution of the internal Ringer solution. The water flow was estimated gravimetrically. The water flow was induced by pituitrin (50 milliunits/ml), cyclic AMP (cAMP, 0.5-10(-3) M) and nystatine (3.5-10(-5) M). Pituitrin and cAMP and all the antimicrotubular drugs were added from the serosal surface of the bladder. Nystatine was introduced with the help of a fixed polyethylene tube. Preincubation with colchicine lasted 4 hours and that with vinblastine and copper (CuSO4), 1 hour. The drug concentrations varied between 10(-5)--10(-4) M. All the drugs studied showed a significant inhibitory effect toward pituitrin. The action of cAMP on the water flow was seen inhibited in the presence of colchicine and copper. The nystatine induced water flow was supressed by copper, colchicine being in this case inactive. A conclusion is drawn that the inhibition of cAMP formation does not cause a decreased pituitrine effect in the presence of antimicrotubular drugs. It has been assumed that the microtubules may be involved in the directed water flow within the cell.     

Effect of colchicine, vinblastine and cytochalasin B on human lymphocyte transformation by phytohemagglutinin

The effect of colchicine, vinblastine, and cytochalasin B has been investigated on the phytohemagglutinin (PHA) induced transformation and DNA synthesis of human lymphocytes. The three drugs produced, at an appropriate concentration, inhibition of DNA synthesis and lymphocyte transformation, if added prior to PHA. Inhibitory concentrations of cytochalasin B were no longer effective in preventing DNA synthesis if added 2 h after exposure to PHA; on the other hand, colchicine and vinblastine were effective even if they were added 16 h after PHA. Studies of lymphocyte aggregation to beads of Sepharose with chemically bound PHA suggest that the effects of these drugs do not seem to lie primarily on blocking PHA binding to the lymphocyte membrane, but rather on a subsequent step(s).     

Microtubule inhibitors alter the secretion of beta-glucuronidase by human blood platelets: involvement of microtubules in release reaction II

The effects of the antimicrotubular drugs colchicine and vinblastine on the blood platelet release reaction were studied by measuring release of ¹⁴C-5-hydroxytryptamine (¹⁴C-5-HT, release I) and β-glucuronidase (release II) from gel-filtered human platelets. β-glucuronidase release induced by thrombin was significantly inhibited by colchicine (0.01-1 mM) or vinblastine (0.05–0.1 mM). Release of ¹⁴C-5-HT, however, was unaffected at low concentrations of colchicine and only slightly inhibited at higher concentrations. Inhibition of β-glucuronidase release depended on colchicine or vinblastine concentrations and decreased with longer time intervals (1′, 5′, 20′) after thrombin stimulation. Levels of the cytoplasmic enzyme, lactic acid dehydrogenase, in supernatants of colchicine treated platelets were not significantly different from controls. Colchicine also inhibited β-glucuronidse release, but not ¹⁴C-5-HT release, induced by trypsin and sodium arachidonate. Binding of ¹⁴C-colchicine by platelets was measured and it was found that platelet aggregation and release of 5-HT induced by adenosine diphosphate, epinephrine and collagen proceeded without any alteration in colchicine binding. However, significant increases in the rate and degree of colchicine binding were observed when platelets were stimulated by thrombin, trypsin and arachidonic acid which induced aggregation, release of both 5-HT and β-glucuronidase. The results suggest that an alteration in platelet microtubules is correlated with the physiologic response resulting in release II and that the cellular mechanisms effecting release I and II by platelets differ qualitatively in that the microtubules may facilitate release II.     

Contrasting effects of maytansine and vinblastine on the alkylation of tubulin sulfhydryls

The ansa macrolide maytansine is a competitive inhibitor of vinblastine for binding to tubulin. Both drugs are potent inhibitors of microtubule assembly in vitro but maytansine, unlike vinblastine, is unable to induce tubulin aggregation or to stabilize colchicine binding. In this study, the effects of maytansine and vinblastine on the accessibility of tubulin's sulfhydryl groups were compared. It was found that 10 μm vinblastine inhibited the reaction of bovine brain tubulin with [¹⁴C]iodoacetamide by 45%. In contrast, maytansine, even up to 100 μm, had no effect on the reaction. However, when the two drugs were tested in combination, maytansine was a potent inhibitor of vinblastine's effect, consistent with the two drugs competing for the same or overlapping sites, but suggesting that the nature of the binding was different. In contrast, maytansine did not affect the suppression of alkylation induced by colchicine and podophylotoxin, consistent with these drugs binding to different sites. Maytansine and vinblastine were each able to increase the formation of $\text{β}{}^1$ by the bifunctional reagent, N,N′-ethylenebis-(iodoacetamide); $\text{β}{}^1$ is the designation for an electrophoretically faster migrating form of β-tubulin which apparently contains an intrachain crosslink. Thus, in at least the portion of the tubulin molecule involved in $\text{β}{}^1$ formation, the two drugs have similar effects. Since maytansine does not appear to suppress any competing alkylation reactions, it is possible that the enhancement of $\text{β}{}^1$ formation represents a genuine conformational effect. Since the sulfhydryl groups of tubulin may be involved in regulating microtubule assembly, it is likely that maytansine and vinblastine differ in the manner in which they inhibit microtubule assembly.     

Effect of drugs affecting microtubular assembly on microtubules, phospholipid synthesis and physiological indices (signalling, growth, motility and phagocytosis) in Tetrahymena pyriformis

Structural changes of microtubules, incorporation of radioactively labelled components into phospholipids, cell motility, growth and phagocytosis were studied under the effect of four drugs affecting microtubular assembly: colchicine, nocodazole, vinblastine and taxol. Although the first three agents influence microtubules in the direction of depolymerization and the fourth stabilizes them, their effects on the structure of microtubules cannot be explained by this. Using confocal microscopy after an acetylated anti-tubulin label, in nocodazole- and colchicine-treated cells, the basal body cages disappear and longitudinal microtubules (LM) became thinner without changing transversal microtubules (TM). After taxol treatment LM also became thinner, however TM disappeared. Under the effect of vinblastine TM became thinner, without influencing LM. These drugs influence the incorporation of components ([(3)H]-serine, [(3)H]-palmitic acid and (32)P) into phospholipids, however their effect is equivocal and cannot be consequently coupled with the effect on the microtubules. Nocodazole, vinblastine and taxol significantly reduced the cell's motility, however colchicine did so to a lesser degree. Vinblastine and nocodazole totally inhibited, and taxol significantly decreased cell growth, while colchicine in a lower concentration increased the multiplication of cells. Phagocytosis was not significantly influenced after 1 min, but after 5 min all the agents studied (except colchicine) significantly inhibited phagocytosis. After 15 and 30 min each molecule caused highly significant inhibition. The experiments demonstrate that drugs affecting microtubular assembly dynamics influence differently the diverse (longitudinal, transversal etc.) microtubular systems of Tetrahymena and also differently influence microtubule-dependent physiological processes. The latter are more dependent on microtubular dynamics than are changes in phospholipid signalling.     

Colchicine binding in the rat tapeworm,Hymenolepis diminuta

A fraction with enriched colchicine-binding properties prepared from Hymenolepis diminuta was found to possess many of the properties of tubulin isolated from other sources. Colchicine was bound by a simple saturable process with a dissociation constant of 13.2 μM. The binding capability decayed with a half-life of about 5 h. Binding was unaffected by lumicolchicine, was competitively inhibited by podophyllotoxin (inhibition constant of 4.8 μM) and showed an apparent stimulation by vinblastine sulphate. Sodium chloride also appeared to stimulate the binding process. The ligand/receptor complex had a molecular weight of approx. 112 000 as determined by gel filtration.On the basis of this biochemical and pharmacological evidence it was concluded that the colchicine receptor in the supernatant fraction of the H. diminuta homogenate was almost certainly tubulin. Refinement of the preparation should facilitate further studies on the mode of action of certain types of anthelmintic compound.     

Influence of colchicine and vinblastine on the intracellular migration of secretory and membrane glycoproteins: II. Inhibition of secretion of thyroglobulin in rat thyroid follicular cells as visualized by radioautography after 3H-fucose injection

Young (40 gm) rats were given a single intravenous injection of colchicine (4.0 mg) or vinblastine (2.0 mg). At 10 min after colchicine and 30 min after vinblastine administration, the rats were injected with 3H-fucose. Control rats received 3H-fucose only. All rats were sacrificed 90 min after 3H-fucose injection and their tissues processed for radioautography. In thyroid follicular cells of control animals, at this time interval, 57% of the total label was associated with colloid and secretory vesicles in the apical cytoplasm while 27% was localized in the Golgi apparatus and neighboring vesicles. In experimental animals, the proportion of label in colloid and apical vesicles was reduced by more than 69% after colchicine and more than 83% after vinblastine treatment. The proportion of label in the Golgi region, on the other hand, increased by more than 125% after colchicine and more than 179% after vinblastine treatment. Within the Golgi region, the great majority of the label was associated with secretory vesicles which accumulated adjacent to the trans face of the Golgi stacks. It is concluded that the drugs do not interfere with passage of newly synthesized thyroglobulin from the Golgi saccules to nearby secretory vesicles, but do inhibit intracellular migration of these vesicles to the cell apex. In most cells the number of vesicles in the apical cytoplasm diminished, but this was not always the case, suggesting that exocytosis may also be partially inhibited. The loss of microtubules in drug-treated cells suggests that the microtubules may be necessary for intracellular transport of thyroglobulin.