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PubMed1976

The effect of Mycobacterium bovis (Bacillus Calmette-Guérin) on macrophage random migration, chemotaxis, and pinocytosis.

Macrophage random migration, chemotactic responsiveness, and pinocytosis were studied in the guinea pig after Bacillus Calmette-Guérin (BCG) administration. Oil-induced peritoneal exudate cells were evaluated for random migration and chemotaxis in modified Boyden chambers. Pinocytosis was measured by the in vitro uptake of colloidal 198Au. There was a parallel increase in random migration and chemotaxis after an i.p. dose of 1.0 X 10(7) COLONY-FORMING UNIts of BCG. This was seen as early as Day 4, sustained through Day 21, and absent by Day 44. A number of different strains and preparations of BCG vaccine exerted a similar augmentation of macrophage mobility. In parallel assays, aliquots of macrophages from BCG-treated animals showed increased pinocytosis, first noted at Day 14, peaking at Day 28, and no longer measurable at Day 44. These alterations in macrophage function following BCG administration may in part contribute to the antitumor effect of BCG adjuvant therapy.

Animals↗
PubMed1994

Human high-affinity Fc IgG receptor (Fc gamma RI)-mediated phagocytosis and pinocytosis in COS cells.

The high-affinity receptor (Fc gamma RI) for the constant (Fc) portion of immunoglobulin G (IgG) is one of three Fc IgG receptor classes (Fc gamma Rs) found on mononuclear phagocytes. The functional specialization of each of the Fc gamma R classes is not well understood. Previous studies utilizing anti-Fc gamma R monoclonal antibodies (mAbs) as opsonins suggest that Fc gamma RI, like the other Fc gamma Rs expressed by macrophages, is able to mediate phagocytosis. The ability of Fc gamma RI to mediate pinocytosis, however, had not been certain, since it binds, but does not mediate, internalization of monomeric IgG in the monocytoid U937 cells. We studied Fc gamma RI-mediated internalization by introducing it into the Fc gamma R-negative fibroblastic COS cells. We found, using electron microscopy and fluorescence microscopy, that COS cells expressing Fc gamma RI are able to phagocytose IgG-coated zymosan particles and sheep red blood cells (SRBC), as well as pinocytose cross-linked IgG. There was no intracellular accumulation of monomeric IgG. Chimeric receptors which retain the extracellular domains of Fc gamma RI but lack the entire wild-type transmembrane and intracellular regions of the receptor mediated both phagocytosis and pinocytosis with equal or increased efficiency when compared to the wild-type receptor. Control COS cells transfected with CD2 rosetted, but did not phagocytose, SRBC. Attachment of phagocytic targets to COS cells is therefore not sufficient for phagocytosis. Taken together, this suggests that the extracellular domain of Fc gamma RI is sufficient for it to mediate phagocytosis and pinocytosis in this system.

Amino Acid Sequence↗
PubMed1965

THE IN VITRO DIFFERENTIATION OF MONONUCLEAR PHAGOCYTES. II. THE INFLUENCE OF SERUM ON GRANULE FORMATION, HYDROLASE PRODUCTION, AND PINOCYTOSIS.

The concentration of newborn calf serum in the medium has marked effects on the morphological and biochemical properties of mouse mononuclear phagocytes. At a low serum concentration, the cells developed small numbers of tiny cytoplasmic granules and little or no increase in acid phosphatase, cathepsin, and beta-glucuronidase. As the serum concentration was raised, granules were formed at a more rapid rate and were larger in size. The rate of production and total amount of three hydrolytic enzymes was increased at higher levels of serum. Observations on living cells indicated that the phase-dense granules which accumulated in the perinuclear region were derived from pinocytic vesicles. These clear vesicles fused and migrated to the centrosphere where they underwent a gradual increase in phase density and reacted positively for acid phosphatase. A microscopic technique was described for the evaluation of the pinocytic process. When this method was employed, the rate of pinocytosis increased curvilinearly with elevations in the calf serum concentration of the medium. The comparative influence of bovine, horse, and rabbit serum on mouse cells was evaluated. It is suggested that pinocytosis is a major regulator of granule formation and hydrolytic enzyme production by the mouse macrophage.

Acid Phosphatase↗
PubMed1979

Insights into mechanisms of intracellular protein turnover from studies on pinocytosis.

Pinocytosis is a widespread phenomenon and involves the internalization of large amounts of plasma membrane. Much of this membrane is probably returned to the plasma membrane in vesicular form, and reasons are advanced why this recycling may occur chiefly before pinosome--lysosome fusion and may be related to vacuole fusion events. Some internalized membrane must nevertheless enter the lysosomal compartment and, in order to maintain a steady state, must be removed at the same rates as it enters. This constant lysosomal involution probably occurs by the budding of vesicles either inward (for digestion) or outward (for return to the plasma membrane). The former process allows for the transfer of cytosol components into the lysosome. Quantitative studies on pinocytosis have shown that selective substrate capture is commonly achieved by the adsorption of substances to the plasma membrane being internalized. In contrast the basal rate of uptake of liquid and membrane is not easily modified. If similar considerations apply to uptake by the quasi-pinocytic lysosomal involution, modification of proteins leading to adsorption to the cytoplasmic face of the lysosome could be the rate-determining step in the degradation of endogenous cytoplasmic proteins by lysosomes.

Animals↗
PubMed1991

Characterization of ramified microglia in tissue culture: pinocytosis and motility.

Functional properties of ramified microglia were investigated in primary cultures of rat cerebral cortical cells. These microglia could be readily identified in both fixed and living cultures through previously established features. Based on their destruction by 5 mM L-leucine methyl ester, a high level of intrinsic endocytotic activity was established. When cultures were incubated with fluorescent latex beads to assess phagocytosis, little or no such activity was exhibited by ramified cells. However, when cultures were incubated with dyes or other soluble tracer compounds, these cells always exhibited labeling. This labeling was selective for ramified microglia in the cultures and was demonstrated using a variety of compounds, including trypan blue, lucifer yellow, horseradish peroxidase (HRP), and India ink. Intracellular label could be observed in vesicular structures; this localization corresponded to an active cellular process. Also, cellular labeling was inhibited by the presence of colchicine. These features supported the inference that the labeling was attributable to pinocytosis, and this process appeared to account for the vast majority of endocytotic activity in the ramified microglia. Possible physiological significance of this pinocytotic activity was indicated by the accumulation of various neurotransmitters/modulators: gamma-aminobutyric acid and vasoactive intestinal polypeptide (VIP). Ramified cells in these cultures have been previously noted to exhibit a constant and rapid pattern of motility, which was consistently observed here through time-lapse video recording; pinocytosis and rapid motility were shown to concur in individual cells. Based on their high intrinsic pinocytotic activity and pattern of cellular motility, the ramified microglia specifically are suggested to serve a constitutive function of fluid cleansing within the interstitial spaces of brain tissue.

Animals↗
PubMed1982

Cytophotometric assessment of phagocytosis and pinocytosis on single resident and exudate murine peritoneal macrophages.

Computerised scanning cytophotometry was used to evaluate the phagocytic and pinocytic performance of single resident and exudate macrophages. The results indicate that both receptor mediated phagocytosis and fluid phase pinocytosis are more efficiently performed by activated than resident murine peritoneal macrophages. Exudate macrophages phagocytise at approximately twice the rate of resident macrophages while pinocytosis is five times more prominent in exudate than in resident macrophages. The results are consistent with other published investigations and in addition reflect individual cellular variation in macrophage populations.

Animals↗
PubMed1976

Inhibition of pinocytosis in rat yolk sac by trypan blue.

Day 17.5 yolk sacs from rats injected with partially denatured 125I-labeled bovine serum albumin (I-BSA) were cultured in vitro by a raft technique. The rates of release of [125I]iodotyrosine were similar in control yolk sacs and in yolk sacs from rats preinjected with trypan blue. Day 17.5 rat yolk sacs were also cultured in medium containing I-BSA. Following pinocytic uptake the substrate was degraded intracellularly and [135I]iodotyrosine released into the medium. Trypan blue, when present in the medium in concentrations above 100 mug/ml, inhibited pinocytosis of I-BSA and so decreased the rate of [125I]iodotyrosine production. Trypan blue similarly decreased the rate of pinocytic uptake of 125I-labeled polyvinylpyrrolidone. Pinocytic uptake of macromolecules was not decreased in yolk sacs from rats pretreated with trypan blue. The relevance of these results to the mechanism of teratogenic action of trypan blue is discussed. It is proposed that if trypan blue in teratogenic doses similarly inhibits pinocytosis by the yolk sac during the organogenetic period teratogenesis might result from a transient interruption in the flow of metabolites through the yolk sac to the embryo.

Animals↗
PubMed1979

Quantitative estimate of pinocytosis in experimental acute hypertension.

Cerebral cortical arterioles in focal neocortical areas develop increased permeability to plasma proteins and protein tracers in experimental hypertensive encephalopathy. The mechanism underlying this increased permeability has been the subject of several studies. In our previous studies of angiotensin-induced acute hypertension, pinocytosis appeared to be the principal mechanism for the increased blood-brain barrier (BBB) permeability observed. In the present study pinocytotic activity was assessed quantitatively to determine whether enhanced pinocytosis was confined to the permeable arteriolar segments of hypertensive animals. In addition, the effect of horseradish peroxidase (HRP) itself on the pinocytotic activity of normal cerebral cortical arteriolar endothelium was determined. In 12 rats following administration of HRP, hypertension was induced by an infusion of angiotensin. The animals were perfusion-fixed 90 s after the onset of the infusion. Control animals received saline only or HRP only. The area of arteriolar endothelium in cross section was determined by a planimeter from overlapping electron micrographs taken at a constant magnification around the circumference of the vessel wall. Results indicate a significant (P less than 0.001) increase in the number of pinocytotic vesicles in the permeable arteriolar segments of hypertensive animals as compared with nonpermeable arteriolar segments of the same animals and comparable segments of normotensive rats. In addition, eight times as many vesicles appear to be transporting tracer in the permeable arteriolar segments of hypertensive animals as compared to the nonpermeable segments of the same animals and normotensive animals. HRP alone did not affect the pinocytotic index, there being no difference (P greater than 0.05) in the number of vesicles in normotensive animals receiving saline only and those receiving HRP only. Our previous observation that disruption of endothelial cells or their tight junctions did not occur was confirmed.

Animals↗
PubMed1994

Chloroquine accumulation and alterations of proteolysis and pinocytosis in the rat conceptus in vitro.

The teratogenicity of chloroquine (CQ) has been hypothesized to result from its effects on lysosomal function, specifically the ability of the visceral yolk sac (VYS) to capture and degrade external macromolecules. Using the rat whole embryo culture system, we evaluated the ability of CQ to accumulate in conceptual tissues and its effects on aspects of VYS function known to be important in the uptake and processing of nutrients. When CQ was added directly to the culture medium, it was found to accumulate rapidly in conceptual tissues, particularly the VYS. Tissue concentrations of CQ in the embryo proper reached approximately 10-fold those in the medium, whereas concentrations in the VYS exceeded by 100-fold the medium concentration within a 4-hr exposure on gestational day (GD) 10. Embryotoxic concentrations of CQ (10-30 microM) enhanced the activity of lysosomal cysteine proteinases measured in vitro under optimum pH conditions in both embryonic and VYS homogenates after a 26-hr treatment from GD 10-11. A different pattern of response in enzyme activity was observed between embryos and VYSs that could be attributed to the preferential accumulation of CQ in the VYS. Nonembryotoxic concentrations of CQ (1-7.5 microM) induced a concentration-dependent increase in VYS enzyme activity that peaked in conceptuses exposed to 20 microM CQ (an intermediate embryotoxic concentration). The enhanced cysteine proteinase activity was time dependent and appeared to increase gradually in conceptuses exposed to 10-20 microM CQ during the 26-hr culture period. This was in contrast to the rapid accumulation of CQ in conceptual tissues seen on gestational day 10. Protein content in the VYS was increased significantly after a 9-hr exposure of whole conceptuses to CQ (20 microM), indicating an inhibition of VYS proteolytic activity in situ. After 24 hr of exposure to 20 microM CQ, VYS protein content was not significantly different from control, but embryonic protein was reduced significantly by 20%. These observations are consistent with a model of reversible inhibition of VYS proteolysis by CQ followed by a compensatory increase in lysosomal proteinase activity. VYS fluid-phase pinocytosis was also assessed after CQ exposure and found to be inhibited only in the highest CQ concentration tested (30 microM). Lower concentrations of CQ that were still embryotoxic (10-20 microM) did not affect VYS fluid-phase pinocytosis, suggesting that inhibition of this activity is not primarily responsible for CQ embryotoxicity.

Animals↗
PubMed1983

Internalization and recycling of plasma membrane glycoconjugates during pinocytosis in the macrophage cell line, P388D1. Kinetic evidence for compartmentation of internalized membranes.

An analysis was made of the pinocytosis-derived internalization and recycling of membrane in the macrophage cell line, P388D1. Plasma membrane glycoconjugates, reversibly labelled with [3H]galactose, were used as a membrane marker. Label internalized with the plasma membrane was no longer accessible to release by externally added beta-galactosidase and could therefore be distinguished quantitatively from label remaining on the cell surface. Direct experimental evidence for membrane recycling was obtained by demonstrating that previously internalized label reappeared at the cell surface. The composition of labelled membrane glycoconjugates, as analysed by SDS-polyacrylamide gel electrophoresis, remained unaltered before and after internalization. The label remained membrane-bound in an unmodified way during the entire period of 8 h investigated, corresponding to about twenty-four cycles of membrane flow. Membrane flow led to a steady-state distribution of label between the plasma membrane and intracellular membranes. The redistribution of label occurred with biphasic kinetics, which could be described as the sum of two exponential functions. This behavior is explained by presenting a model of membrane flow between the plasma membrane and two consecutive intracellular membrane compartments, which we assume to consist of pinosomal membranes and of pinosome-derived membrane of secondary lysosomes. The relative membrane surface areas turn out to be in the ratio of 100:12.5:7.3, respectively. At the observed rate of pinocytosis, the equivalent of the plasma membrane is internalized once every 21 min, in the form of primary pinosomes of the size 0.24 micrometer. The residence time of membranes in the pinosome compartment is about 3 min. The rate at which membranes enter the lysosomal compartment is 31 times lower than the rate of membrane internalization. We conclude that only 3% of the amount of membrane internalized at any one time subsequently enters the secondary lysosome compartment. After a residence time of 49 min this membrane fraction is finally recycled to the cell surface. The results are discussed in terms of mixing and sorting-out of pinosomal and lysosomal membranes.

Animals↗
PubMed1985

Degradation of insulin by human fibroblasts: effects of inhibitors of pinocytosis and lysosomal activity.

The role of the pinosome-lysosome pathway in the degradation of 125I-labelled bovine insulin by cultured human fibroblasts was examined by comparing the effects of various known inhibitors of pinocytosis and lysosomal degradation on the uptake and degradation of 125I-labelled polyvinylpyrrolidone, formaldehyde-denatured bovine serum albumin and bovine insulin by these cells. Fibroblasts incubated with polyvinylpyrrolidone steadily accumulate this substrate, whereas incubations with insulin or denatured albumin led to the progressive appearance in the culture medium of [125I]iodotyrosine. Inhibitors of pinocytosis (bacitracin, colchicine and monensin), metabolic inhibitors (2,4-dinitrophenol and NaF), lysosomotropic agents (chloroquine and NH4Cl) and an inhibitor of cysteine-proteinases (leupeptin) decreased the rate of uptake of polyvinylpyrrolidone and denatured albumin very similarly, but only bacitracin had an effect on the processing of insulin. Chloroquine, NH4Cl and leupeptin strongly inhibited the digestion of denatured albumin, but not of insulin. The different responses to the modifiers, with polyvinylpyrrolidone and denatured albumin on the one hand and insulin on the other, suggest that insulin degradation can occur by a non-lysosomal pathway. The very strong inhibitory effect of bacitracin on insulin processing by fibroblasts may point to an important role of plasma membrane proteinases in insulin degradation.

2,4-Dinitrophenol↗
PubMed1981

Analysis of the effect of amines on inhibition of receptor-mediated and fluid-phase pinocytosis in rabbit alveolar macrophages.

Incubation of rabbit alveolar macrophages in vitro with methyl amine led to a time- and concentration-dependent inhibition of uptake of alpha macroglobulin-125I-trypsin complexes (alpha M-125I-T). Upon addition of methyl amine (50 mM) to cells prelabeled with alpha M-125I-T there was a rapid inhibition of lysosomal catabolism of internalized ligand. In the absence of ligand, incubation of cells with 50 mM methyl amine led to a 40-70% decrease in surface-receptor number. The methyl amine-induced decrease in surface-receptor number only occurred in metabolically active cells since cells incubated at 0 degrees C, or treated with N-ethyl maleimide and incubated at 37 degrees C, did not show the effect. Incubation of cells at 37 degrees C with methyl amine also effected a 40-70% decrease in fluid-phase pinocytosis. Although there was a decline in surface-receptor number, the remaining population of receptors were capable of mediating (at least) one round of ligand internalization. However, further ligand uptake was prevented. Data demonstrate that although receptors were present on cell surfaces, they were incapable of mediating ligand internalization. Incubation of macrophages with chloroquine at 37 degrees C for 60 min also led to a disappearance of receptors, and a concomitant reduction in fluid-phase pinocytosis.

Animals↗
PubMed1986

Kinetics of fluid-phase pinocytosis in Dictyostelium discoideum amoebae.

Kinetics of pinocytosis in Dictyostelium discoideum were investigated over an extended period of time (up to 6 hours) using fluorescein isothiocyanate (FITC)-dextran as a fluid-phase marker. FITC-dextran added to the medium accumulated rapidly inside the cells with a rate of influx equivalent to 9 microns3 of fluid/cell x min. After a period of about 90 min of uptake, the intracellular FITC-dextran level reached a plateau which corresponded to a strict balance between pinocytosis and exocytosis as shown both by efflux measurements and pulse experiments with (3H) dextran. At equilibrium, the amount of internalized marker reached a value equivalent to 790 microns3 of fluid taken up per amoeba, i.e. a volume paradoxically higher than the total aqueous space of the cell (520 microns3 ). FITC-dextran was thus markedly concentrated intracellularly. The endocytic compartment in which the intracellular FITC-dextran was concentrated could be completely washed out when FITC-dextran was removed from the external medium.

Dextrans↗
PubMedDate not supplied

Pinocytosis in the rat visceral yolk sac: potential role in amino acid nutrition during the fetal period.

At 9.5 to 10.5 days post-conception (p.c.) the rat embryo receives amino acids primarily by pinocytosis of maternal proteins by the visceral yolk sac (VYS) endodermal cells and their subsequent digestion in lysosomes. As this mechanism may also contribute to the nutritional support of the rapidly growing fetus later in gestation, this study determines the potential of the VYS to contribute to the amino acid needs of the rat fetus on 13.5, 15.5 and 17.5 days p.c. Wistar rats were killed on 11.5, 13.5, 15.5, 17.5 and 19.5 days p.c. and the protein content of fetuses, VYSs and serum was determined. Additional VYSs were isolated on the same days p.c. and the uptake of [14C]sucrose, a measure of fluid-phase pinocytosis, was determined. The rate of protein accretion by the fetus and the capacity of the VYS to pinocytose and digest proteins were calculated on 13.5, 15.5 and 17.5 days p.c. There was a decrease in pinocytic capacity, per mg protein, over the period of development studied but, owing to the growth of the tissue, the total capacity of the yolk sac to pinocytose and digest proteins increased from 13.5 to 17.5 days p.c. Comparison of the VYSs capacity to pinocytose and digest proteins and the rate of protein accretion by the fetus suggests that the VYS could support a significant proportion of the increase in protein content of the growing fetus. This conclusion is reinforced by the limited capacity of the maternal plasma to supply free amino acids to the fetus.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗
PubMed1987

Sugar uptake by fluid-phase pinocytosis and diffusion in isolated rat hepatocytes.

Measurements of sugar pinocytosis (fluid-phase endocytosis of radiolabelled sucrose, lactose and raffinose) in freshly isolated rat hepatocytes are disturbed by sugar diffusing into the cells through plasma-membrane blebs. Non-pinocytic entry may be even more pronounced at 0 degrees C, and is a major contributor to 'background' radioactivity. By electrodisruption of the plasma membrane, a distinction can be made between pinocytotically sequestered sugar and free sugar that has entered the cytosol by diffusion. Pinocytosis proceeds at a rate of 2%/h (relative to the intracellular fluid volume), whereas the rate of sucrose entry by diffusion is more than twice as high. Three pinocytotic compartments are distinguishable in isolated hepatocytes: (1) a rapidly recycling compartment, which is completely destroyed by electrodisruption, and which may represent pinocytic channels continuous with the plasma membrane; (2) a non-recycling (or very slowly recycling) electrodisruption-resistant compartment, which allows accumulation of the lysosomally hydrolysable sugar lactose, and which therefore must represent non-lysosomal vacuoles (endosomes?); (3) a lysosomal compartment (non-recycling, electrodisruption-resistant), which accumulates raffinose and sucrose, but which hydrolyses lactose. The last two compartments can be partially resolved in metrizamide/sucrose density gradients by the use of different sugar probes.

Animals↗
PubMed1977

Phosphohexosyl components of a lysosomal enzyme are recognized by pinocytosis receptors on human fibroblasts.

Human beta-glucuronidase (beta-D-glucuronide glucuronosohydrolase, EC 3.2.1.31), like many other glycoprotein lysosomal hydrolases, is specifically taken up from the culture medium by human fibroblasts. Prior work has indicated that the enzyme exhibits charge heterogeneity and that "high-uptake" forms, i.e., those rapidly internalized by human fibroblasts, are more acidic than slowly internalized forms. Here we present two lines of evidence that the acidic group required for the high-uptake property of certain forms of the enzyme is a phosphate on, or in proximity to, a D-mannose-type carbohydrate. The first line of evidence was obtained from analysis of inhibition of enzyme pinocytosis by yeast mannans, phosphorylated sugars, and sugars. Mannans that contained phosphate were more potent inhibitors than those that did not contain phosphate. D-Mannose 6-phosphate was a more potent inhibitor than either D-mannose 1 phosphate or 2-deoxy-D-glucose 6-phosphate. D-Mannose and certain related sugars were weak pinocytosis inhibitors, while 2- and 4-epimers of mannose were noninhibitory. Competitive inhibition was demonstrated and the apparent Kis estimated for the following compounds: Saccharomyces cerevisiae mannan from mutant X2180-mnnl, 3 X 10(-6) M; mannan from wild-type S. cerebisiae, 3 X 10(-5) M; D-mannose 6-phosphate, 6 X 10(-5) M; L-fucose, 4 X 10(-2) M; and D-mannose, 6 X 10(-2) M. The second line of evidence comes from the observation that alkaline phosphatase [orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1] treatment of human platelet beta-glucuronidase abolished its "high-uptake" activity, without diminishing its catalytic activity, and converted some forms of the heterogeneous enzyme to less acidic forms.

Alkaline Phosphatase↗
PubMed1980

Kinetics of membrane internalization and recycling during pinocytosis in Dictyostelium discoideum.

Internalization and recycling of plasma membrane during pinocytosis in Dictyostelium discoideum was analyzed quantitatively. A labeling technique was used by which [3H]galactose could be enzymatically bound to and released from the plasma membrane. Label internalized with the plasma membrane was no longer accessible to enzymatic release and could therefore be distinguished quantitatively from label remaining on the cell surface. Internalization of labeled membrane components was measured as a function of pinocytotic uptake. Direct experimental evidence for membrane recycling was obtained by demonstrating that previously internalized label reappeared at the plasma membrane. The experimental data agree with a kinetic model requiring that a shuttle of membrane between two membrane compartments leads to the same surface concentration of label in both. The two compartments consist of the plasma membrane and of cytoplasmic vacuolar membranes; their relative membrane surface areas are 1 and 0.5, respectively. One surface area equivalent of the plasma membrane is internalized during a pinocytotic uptake amounting to 15% of the cell volume. At the observed rate of pinocytosis, this occurred once every 45 min. The average size of the primary pinosomes, as weighted according to their contribution to pinocytotic uptake, was calculated to be about 0.6 microns.

Cell Membrane↗
PubMed1986

Evidence that pinocytosis in lymphoid cells has a low capacity.

In contrast to adherent cells, human B and T lymphoblasts, marmoset monkey T lymphoblasts, and mouse T lymphoblasts do not form monolayers and have a poor ability to pinocytose. After a 10-min incubation of lymphoblasts at 37 degrees C, the level of internalized medium reached a plateau. During this time, lymphoblasts pinocytosed 3-4 femtoliters (1 fl = 10(-15) l) of medium per cell as calculated by the quantity of the entrapped pinocytic marker 5(6)-carboxyfluorescein. The levels of pinocytosed liquid did not increase during a subsequent 90-min incubation of cells at 37 degrees C. Adherent HeLa cells took up 27 fl of medium per cell per hour. Other types of adherent cells were reported by others to pinocytose 20 to 90 fl of medium per cell per hour. The process of pinocytosis in lymphoblasts appeared to be reversible since cells which were pre-loaded with carboxyfluorescein and then incubated at 37 degrees C in fresh medium lost the marker almost completely within 40 min. Similar results were obtained with horseradish peroxidase as the pinocytic marker. Further evidence that lymphoblasts have a low capacity for pinocytic internalization relative to adherent cells was obtained from the observation that Namalwa lymphoblasts were approximately 100 times more resistant to the cytotoxic action of the protein toxin gelonin than the adherent HeLa cells. Gelonin is a ribosome-inactivating toxin which is not capable of binding to cells, and its only mode for internalization appears to be pinocytosis. Ribosomes in cell lysates of the two lines were equally sensitive to gelonin. It is speculated that the poor pinocytic ability of lymphoid cells may reflect a fundamental difference between adherent and non-adherent cells and that this may impede the targeting of drugs into lymphoid cells.

B-Lymphocytes↗