Reversible suppression of lactation by colchicine

Intracellular origin and secretion of milk fat globules

The cream or fat fraction of milk consists of fat droplets composed primarily of triacylglycerols that are surrounded by cellular membranes. In this review we discuss what is known about how these droplets are formed in and secreted by mammary epithelial cells during lactation. This secretion mechanism, which appears to be unique, is unlike the exocytotic mechanism used by other cell types to secrete lipids. Milk fat globules originate as small, triacylglycerol-rich, droplets that are formed on or in endoplasmic reticulum membranes. These droplets are released from endoplasmic reticulum into the cytosol as microlipid droplets coated by proteins and polar lipids. Microlipid droplets can fuse with each other to form larger cytoplasmic lipid droplets. Droplets of all sizes appear to be unidirectionally transported to apical cell regions by as yet unknown mechanisms that may involve cytoskeletal elements. These lipid droplets appear to be secreted from the cell in which they were formed by being progressively enveloped in differentiated regions of apical plasma membrane. While plasma membrane envelopment appears to be the primary mechanism by which lipid droplets are released from the cell, a mechanism involving exocytosis of lipid droplets from cytoplasmic vacuoles also has been described. As discussed herein, while we have a general overview of the steps leading to the fat globules of milk, virtually nothing is known about the molecular mechanisms involved in milk fat globule formation, intracellular transit, and secretion.

Secretion of milk proteins

Mammary epithelial cells secrete milk proteins in a polarized manner from their apical surface during lactation. These secreted proteins are either synthesized by the mammary cells or are transported by transcytosis from blood plasma. The intracellular trafficking pathways by which milk proteins are secreted are known in general outline. In this review the basic cell biology of the mammary epithelial cell secretory pathway is considered in relation to what is known in more detail for other cell types. In addition, potential points of control of protein secretion are examined. The secretory biology of mammary epithelial cells has not been characterized extensively in recent years and, while some aspects are well understood, other key issues, which still remain to be resolved, have been highlighted.

Origin and secretion of milk lipids

The cream fraction of milk comprises droplets of triacylglycerol coated with cellular membranes. In this review, we discuss how these droplets are formed and secreted from mammary epithelial cells during lactation. This secretory system is especially interesting because the assembled lipid droplets are secreted from the cytoplasm enveloped by cellular membranes. In other cells, such as hepatocytes and enterocytes, lipid is secreted by exocytosis from membrane-bounded compartments of the secretory pathway. Milk lipids originate as small droplets of triacylglycerol, synthesized in or on the surfaces of rough endoplasmic reticulum (ER)4 membranes. These droplets are released into the cytoplasm as microlipid droplets (MLDs) with a surface coat of protein and polar lipid. MLDs may fuse with each other to form larger cytoplasmic lipid droplets (CLDs). Droplets of varying size, are transported to the apical cytoplasm by unknown mechanisms and are secreted from the cell coated with an outer bilayer membrane. CLDs may increase in size in all regions of the cell, especially at the plasma membrane during secretion. Two possible mechanisms for lipid secretion have been proposed: an apical mechanism, in which lipid droplets are enveloped with apical plasma membrane, and a secretory-vesicle mechanism, in which fat droplets are surrounded by secretory vesicles in the cytoplasm and are released from the surface by exocytosis from intracytoplasmic vacuoles. A combination of both mechanisms may be possible. Following secretion, a fraction of the membrane surrounding the globules may be shed from the droplets and give rise to membrane fragments in the skim milk phase.

Approaches to the manipulation of mammary involution

Mammary involution is a gradual process that occurs following cessation of milking. Regression of mammary secretory tissue accompanies dramatic changes in secretion composition during the transition from lactation to involution. Conversely, rapid differentiation of secretory tissue and copious accumulation of colostrum occur as parturition approaches. The duration of the nonlactating period, mammary gland health, and secretory cell response to hormones influence subsequent lactational performance in most species. Manipulation of the bovine mammary gland in an attempt to hasten involution has been studied. The primary objective of these studies was to determine if hastened involution would decrease new intramammary infections during the early nonlactating period. Results of these studies have also led to a more fundamental understanding of events that occur during physiological transition of the mammary gland. Adequate regression, proliferation, and differentiation of mammary secretory epithelium during the nonlactating period of ruminants appear to be essential for maximal milk production during lactation. Factors that interfere with these mechanisms can adversely affect mammary function during the impending lactation. A greater understanding of these processes may provide new approaches for increasing milk production in dairy cattle.

Mammary gland function during involution

The process of mammary gland involution occurs during the transition from a lactating to a nonlactating state. This transition phase begins after cessation of milk removal and results in changes in mammary secretion composition. Secretion volume declines during involution, as does the concentration of most milk-specific components. Lactoferrin, hydrolytic enzymes, immunoglobulins, and serum-derived components increase in concentration in the secretions during involution. Changes in mammary secretion composition may reflect changes in function of alveolar epithelial cells and have implications for the disease resistance of the gland. Histological and ultrastructural changes occurring in the gland are consistent with a decline in secretion of milk components from epithelial cells. Autophagocytic mechanisms may be involved in this decline in the lactation function. Ultrastructurally, there is little evidence for an extensive loss of epithelia in the bovine mammary gland during involution. Completion of the functional changes occurring in the gland during the process of involution may be required for the gland to redevelop fully for maximal milk yield in the subsequent lactation. Cellular mechanisms involved in mammary involution and relationships between the processes of involution and redevelopment should be areas of particular interest in the mammary function of dairy cattle.

The effects of concanavalin A on milk secretion and mammary metabolism in the goat

The effects of concanavalin A on the rate of milk secretion and the concentration of metabolites in milk were studied following intramammary injection of the lectin via the teat canal into one mammary gland of lactating goats. Concanavalin A decreased milk secretion from the treated gland, reduced the concentrations of phosphoenolpyruvate, nucleoside diphosphate and 2-oxoglutarate in milk and increased the concentrations of glucose, galactose, glycerol, L-lactate, pyruvate, isocitrate and citrate. The changes in the concentrations of the metabolites in milk are discussed in relation to biochemical changes occurring in the mammary gland during the suppression of milk secretion. It is suggested that, when lactose synthesis and secretion is decreased, substantial metabolism of glucose via glycolysis occurs.

Secretion-coupled protein degradation: studies on mammary casein

Mammary explants from midpregnant rabbits were cultured for 18 h at 37 degrees C with insulin, prolactin and cortisol. Subsequently, explants were labelled for 2 h with inorganic [32P]phosphate, L-[5-3H]proline or L-[4,5-3H]leucine, washed and chased for up to 3 h. The radiolabelling profile of [32P]casein or [3H]casein during the chase period, obtained by isoelectric focussing or immunoprecipitation indicates extensive destruction of neosynthesized casein. The extent of casein destruction in mammary explants in culture (measured after radiolabelling with L-[5-3H]proline), is inversely related to casein secretion. Least casein degradation is observed in explants after 48 h in culture when casein secretion is maximal (observed histochemically). Subsequently, when the extracellular alveolar lumen is filled with secretion products (72 h), rapid intracellular casein destruction is again observed. When the chase was carried out in the presence of drugs which inhibit degradation and/or secretion, the results indicate that secretion-coupled casein degradation is dependent on an intact functional microfilamentous-microtubular network, casein is not degraded by an autophagosome requiring process, degradation is inhibited by leupeptin, amino-acid analogue containing casein does not undergo secretion-coupled degradation and inhibition of N-glycosylation of intracellular vesicular membrane proteins prevents secretion-coupled degradation. Secretion-coupled protein destruction is discussed in relation to the post-translational regulation of the net production of secretory proteins in eukaryotic cells.

Changes in bovine milk secretion following intramammary infusions of concanavalin A, oyster glycogen, or water

Effects of sterile intramammary infusion of Concanavalin A on milk secretion were contrasted with infusion of oyster glycogen or water. Twenty-four cows were infused intramammary with 100 mg Concanavalin A, oyster glycogen in 20 ml water, or with 20 ml water alone. Concentrations of lactose, somatic cells, immunoglobulins G and A, serum albumin, and activity of N-acetyl-B-D-glucosaminidase were determined in milk. Blood N-acetyl-B-D-glucosaminidase activity and concentrations of blood immunoglobulins G and A and serum albumin were determined. Oyster glycogen and concanavalin A caused inflammation in treated quarters; peak elevations of milk somatic cell counts, serum albumin, immunoglobulin G concentrations, and N-acetyl-B-D-glucosaminidase activity were at 12 to 36 h following treatment. Milk production and lactose concentration were reduced by oyster glycogen and Concanavalin A. Selective indices of relative accumulation of milk immunoglobulins decreased following Concanavalin A and oyster glycogen, whereas the N-acetyl-B-D-glucosaminidase activity selective index generally remained unchanged. Inflammation reduced the selective accumulation of immunoglobulins, and absence of change in the N-acetyl-B-D-glucosaminidase selective index indicated that blood is not a major source of milk N-acetyl-B-D-glucosaminidase.

Microtubules and the organization of the Golgi complex

Electron microscopic and cytochemical studies indicate that microtubules play an important role in the organization of the Golgi complex in mammalian cells. During interphase microtubules form a radiating pattern in the cytoplasm, originating from the pericentriolar region (microtubule-organizing centre). The stacks of Golgi cisternae and the associated secretory vesicles and lysosomes are arranged in a circumscribed juxtanuclear area, usually centered around the centrioles, and show a defined orientation in relation to the rough endoplasmic reticulum. Exposure of cells to drugs such as colchicine, vinblastine and nocodazole leads to disassembly of microtubules and disorganization of the Golgi complex, most typically a dispersion of its stacks of cisternae throughout the cytoplasm. These alterations are accompanied by disturbances in the intracellular transport, processing and release of secretory products as well as inhibition of endocytosis. The observations suggest that microtubules are partly responsible for the maintenance and functioning of the Golgi complex, possibly by arranging its stacks of cisternae three-dimensionally within the cell and in relation to other organelles and ensuring a normal flow of material into and away from them. During mitosis, microtubules disassemble (prophase) and a mitotic spindle is built up (metaphase) to take care of the subsequent separation of the chromosomes (anaphase). The breaking up of the microtubular cytoskeleton is followed by vesiculation of the rough endoplasmic reticulum and partial atrophy, as well as dispersion of the stacks of Golgi cisternae. After completion of the nuclear division (telophase), the radiating microtubule pattern is re-established and the rough endoplasmic reticulum and the Golgi complex resume their normal interphase structure. This sequence of events is believed to fulfil the double function to provide tubulin units and space for construction of the mitotic spindle and to guarantee an approximately equal distribution of the rough endoplasmic reticulum and the Golgi complex on the two daughter cells.

Isolation and characterization of new proteins produced by the infusion of colchicine in goat mammary gland

Three new proteins have now been isolated from goat milk obtained after colchicine is infused into the mammary gland. Two of the proteins are proline-rich, and a third is a very acidic phosphoprotein. One of the proline-rich proteins is related compositionally to a sheep colostrum proline-rich protein, which has been shown to have a regulatory effect on the immune response (Janusz, M., Stavoscik, K., Zimecki, M., Wieczorek, Z., and Lisowski, J. (1981) Biochem. J. 199, 9-15). Other aspects of colchicine-treated milks are described.

Lactogenic hormones: binding sites, mammary growth, secretory cell differentiation, and milk biosynthesis in ruminants

Roles of the lactogenic hormones prolactin and placental lactogen in mammary development in ruminants were reviewed. In contrast with other ruminants, failure to detect lactogenic activity in the serum of pregnant cows (in excess of that attributed to prolactin) suggests that placental lactogen may have little direct effect on mammary growth or lactogenesis. However, replacement and ablation experiments using ergocryptine provide definitive evidence that increased periparturient secretion of prolactin is necessary for maximal milk production in cattle. Quantitative microscopy indicates a relative failure of mammary cells in cows with inhibited secretion of prolactin to differentiate structurally. Prolactin induces synthesis and secretion of alpha-lactalbumin in prepartum bovine mammary tissue. Temporary disruption of mammary microtubules immediately prepartum in pregnant heifers reduced subsequent milk production, biosynthetic capacity, and cellular differentiation. For maximal milk production, mammary secretory cells apparently must respond to lactogenic hormone stimulation during the immediate periparturient period. Colchicine may desensitize the mammary epithelium to prolactin action. Membrane binding of radiolabeled human growth hormone to ruminant mammary gland provides a measure of lactogenic hormone binding sites. Specific binding to 600 micrograms of mammary membrane protein was 296% greater in lactating, compared with nonlactating, pregnant (65 days of gestation) ewes. Binding capacity (fmol/mg membrane protein) averaged 275 +/- 57 in mammary membranes from nonlactating, pregnant ewes (100 days gestation, n = 2) and 2,325 +/- 521 in mammary membranes from lactating ewes (n = 6, 14 to 21 days postpartum). Greater understanding of hormonal regulation of the ruminant mammary gland likely will result in development of techniques to produce milk more efficiently and perhaps capability to evaluate production potential of young animals.

Intramammary infusion technique for genetic engineering of the mammary gland

The mammary gland is an appropriate substrate for genetic engineering because of its capacity to synthesize and secrete molecules of biological importance. An approach to mass production of such molecules involves transfer of genes into the lactating cell by infusion via the teat and duct system. We describe an infusion technique with the rat, a useful animal in which to develop such technology. By dye maker, trypan blue, and the ultrastructural marker, ferritin-concanavalin A, infusions by this route can permeate the entire gland and deliver molecules to apical membranes of lactating cells.

In vivo effects of colchicine on milk fat globule membrane

Milk secretion in lactating goats was suppressed reversibly by infusing colchicine (2.5 to 5 mg) into one half of the udder via the teat canal. Fat globules were isolated from milks before, during and after (96 h post-infusion) this suppression. Protein, phospholipid, cholesterol (free and esterified), 5'-nucleotidase activity and peptide patterns by gel electrophoresis of these globule samples were determined. Association of [14C]colchicine with milk fat globules in vivo and in vitro also was investigated. Amounts of protein, phospholipid and free cholesterol per g of globule and 5'-nucleotidase per mg of globule protein fall following colchicine infusion. The nature of these changes suggests that the supply of membrane for milk secretion is restricted as a result of the drug treatment. Patterns of globule peptides by gel electrophoresis were qualitatively similar during the experimental period. However, a major globule glycoprotein, Mr = 52 000, showed a significant (3-fold) increase relative to the other principal peptide bands during the period of reduced milk flow. Analysis of milks for radioactivity following infusion of [14C]colchicine revealed that a portion of activity returning in milk is associated with fat globules. This activity peaked at 72 h post-infusion. Evaluation of [14C]colchicine binding to milk fat globules in vitro yielded evidence that the drug binds to the cytoplasmic, but not the exterior surface of the globule membrane. Colchicine's inhibition of milk synthesis and secretion is discussed.

Effect of prepartum blockade of microtubule formation on milk production and biochemical differentiation of the mammary epithelium in holstein heifers

1. Prepartum intramammary infusions of colchicine markedly reduced subsequent milk production compared with untreated mammary glands in the same animal. 2. After the first week postpartum, milk composition was similar in control and treated mammary glands. 3. Rates of fatty acid synthesis, CO2 production and protein synthesis at either 5 or 21 days postpartum were lower in mammary tissue slices taken from mammary glands treated prepartum with colchicine. 4. We conclude that an intact microtubule system is necessary for initiation of milk synthesis and secretion at parturition.

Compensatory increases in milk secretion in response to unilateral inhibition by colchicine during lactation in the goat

1. Milk secretion in one mammary gland of goats was temporarily inhibited by intramammary treatment with colchicine at weeks 6, 12, 18, 24 and 30 of lactation. The magnitude of the inhibition was similar at all stages when compared with milk yield before the experiment. 2. The untreated gland partially compensated for the decreased rate of milk secretion by significantly increasing its own rate of secretion. There was no significant effect of stage of lactation on this response. 3. The rapidity of the compensatory response suggests that existing secretory cells increased their rate of secretion, and therefore that the mammary secretory cells are not secreting at their intrinsic maximum at any stage of lactation. 4. In later stages of lactation (weeks 18, 24 and 30) the increases in the untreated gland were more pronounced in afternoon milkings (8 h milking interval) than in morning milkings (16 h interval. At week 30 only, the rate of secretion before treatment was similarly lower in the 16 h period. It is proposed that an additional limitation is imposed on secretion during the relatively long period of milk accumulation as lactation advances and declines. 5. The results are discussed in relation to rate-limitations on milk secretion with respect to substrate supply and possible systemically active mammary factors.

Metabolic energy and cytoskeletal requirements for synthesis and secretion by acini from rat mammary gland-I. Ultrastructural and biochemical aspects of synthesis and release of milk proteins

1. Incubation of acini (alveoli) from lactating rat mammary gland with metabolic and cytoskeletal inhibitors produced a variety of effects on cell function. Cell viability was maintained during incubation as determined by the measurement of lactate dehydrogenase (LDH) activity in media and by light and electron microscopic examination. Caseins and whey proteins were found to be secreted by acini. 2. Addition of iodoacetate, 2,4-dinitrophenol, cyanide, cycloheximide, vinblastine or cytochalasin B inhibited both synthesis and secretion of milk proteins. Colchicine had no effect on synthesis but specifically inhibited protein secretion. Characteristic ultrastructural changes were produced by each inhibitor. 3. Uptake of 2-amino-isobutyric acid was reduced after incubation with all inhibitors except iodoacetate and dinitrophenol. Uridine incorporation was inhibited by colchicine, vinblastine, cytochalasin B and, at high concentrations, 2,4-dinitrophenol; cyanide and cycloheximide stimulated uridine incorporation. 4. Based on these results, milk protein secretion appeared to depend on continued protein synthesis and both processes were energy coupled. Microtubules and microfilaments also appeared to be involved in milk protein secretion.

Role of microtubules in milk secretion--action of colchicine on microtubules and exocytosis of secretory vesicles in rat mammary epithelial cells

Effect of colchicine on microtubules was studied in mammary epithelial cells treated both in vivo and in vitro with the alkaloid. Three hours after the intramammary infusion of colchicine, secretory activity of mammary epithelia ceased, milk constituents accumulated and were randomly distributed within the cytoplasm, sometimes leaking into the perialveolar connective tissue, and autophagic vacuoles were prevalent. It appeared that an accelerated involutionary process was occurring. No microtubules were observed after this treatment. In vitro treated cells appeared to be less affected by the alkaloid. Although numerous casein-containing secretory vesicles accumulated in the cytoplasm, lipid droplet accumulation was less, and fewer autophagic vacuoles were observed, although lysosomes were commonly observed. Occasionally, obliquely sectioned microtubules were found in cells treated with low concentrations of colchicine but were absent at higher colchicine concentrations; however, paracrystalline inclusions (tubulin aggregates) were observed in some cells at all concentrations of the drug. These observations provide evidence that drugs which interfere with microtubule integrity reduce the secretory activity in mammary epithelia. This evidence is consistent with the concept of an association of the microtubular system and the secretory process.

Effects of lysomotropic agents, and of microfilament- and microtubule-disrupting drugs on the activation of casein-gene expression by prolactin in the mammary gland

The organ-culture technique was used to investigate the effects of lysomotropic agents (NH4Cl and chloroquine) and of modifiers of microfilaments (cytochalasin B) and microtubules (colchicine) on the induction of casein synthesis and the accumulation of casein mRNA by prolactin in the rabbit mammary gland. Neither chloroquine nor NH4Cl altered the lactogenic action of prolactin. Cytochalasin B attenuated the response to prolactin in terms of casein synthesis. However, this drug did not hamper the accumulation of casein mRNA. Colchicine exhibited a marked specific inhibitory effect on the induction of casein synthesis. It also prevented the accumulation of casein mRNA. These results suggest that a putative degradation of the internalized prolactin--receptor complex by lysosomes is not strictly involved in prolactin action. In addition, the integrity of the microfilaments seems unnecessary in the process of casein-gene activation by prolactin. By contrast, the integrity of the microtubule network seems absolutely necessary to ensure the transmission of prolactin information to the nucleus.

Effects of colchicine on ultrastructure of the lactating mammary cell: membrane involvement and stress on the Golgi apparatus

The effects of colchicine on ultrastructure of the lactating mammary cell in the rat and goat were studied by electron microscopy. Changes in tissue of the rat were examined over time (1, 2 and 4 h). The goat gland was evaluated by comparing ultrastructure of tissue at the time of maximum milk flow suppression induced by the drug with that of untreated tissue. Colchicine produced notable changes in the tissue of both species: 1) the secretion of lipid droplets and Golgi vesicle contents (exocytosis) was inhibited and the droplets and vesicles became randomly distributed throughout the cell, 2) the Golgi apparatus was significantly reduced in size, 3) casein and lipid continued to be synthesized as evidenced by greater numbers of secretory vesicles and increased sizes of casein micelles and lipid droplets, 4) secretory vesicles showed a propensity to cluster around lipid droplets, 5) isolated microtubules were found occasionally in the control tissue, ordinarily in the vicinity of the Golgi apparatus, but rarely in the colchicine-treated tissue. These observations indicate that colchicine has two effects leading to suppression of exocytosis in the mammary cell: one involves early interference with capacity of secretory vesicle membranes to fuse and a further effect, related to higher concentrations of colchicine, causes intracellular disorganization and loss of polarity. Microtubules were not seen as directly involved in the mechanisms of exocytosis. The secretion of milk fat globules is coupled to exocytosis and thereby is also inhibited by colchicine.

Inhibition of mammary gland lactose secretion by colchicine and vincristine

The possible role of microtubules in milk production was examined in mammary gland slices from lactating guinea pigs. Colchicine, 10(-5)-10(-4) M, depressed lactose secretion within 15 min, maintaining maximal inhibition over 2.5 h, accompanied by retention of lactose within the slices. In colchicine dose-response studies (2 h, 10(-8)-10(-5) M), secretion was depressed 22% by 10(-5) M, whereas tissue lactose increased with dose up to +25% at 10(-5) M. Lactose synthesis was inhibited 3-19% without correlation to colchicine concentration. In another study, incubation with 10(-5) M lumicolchicine yielded one-third less inhibition of secretion than 10(-5) M colchicine with no increase in tissue lactose. Both drugs depressed synthesis 31%. Lactose secretion showed a negative correlation with 10(-7)-10(-4) M vincristine yielding a maximal 66% inhibition at 10(-4) M, whereas tissue retention showed a linear increase with concentration up to 151% of control at 10(-4) M. Effects on synthesis were sporadic. These data suggest that microtubules have a role in facilitating the transport and/or secretion of lactose and perhaps other milk components.

Milk secretion at the cellular level: a unique approach to the mechanism of exocytosis

Drugs which interfere with the mechanism of exocytosis such as colchicine and vincristine, so-called microtubule antagonists, are providing a fruitful approach to the study of milk secretion at the cellular level. Intramammary infusions of a milligram or less of these substances into lactating goats produce dramatic drops in milk yields in 24 to 36 h. These depressions are reversed substantially by 48 h. In vitro experiments and tissue observations confirm that these drugs are blocking secretion at the level of the lactating cell and that secretion of all the major milk components (fat globules, casein micelles, lactose, and water) is restrained. Mammary infusion of the plant lectin concanavalin A, a protein which binds to cell surface receptors, produces similar changes in milk flow to those of the microtubule antagonists. This indicates that cell surface membrane components perturbed by concanavalin also must be involved in the secretory mechanism. One of the known receptors for concanavalin A in the apical (secretory) plasma membrane of the lactating cell is the enzyme 5'nucleotidase. The possibility must be considered that this enzyme (glycoprotein), inactivated by concanavalin A, is involved in milk secretion.

The supression of milk fat globule secretion by clochicine: an effect coupled to inhibition of exocytosis

The effects of colchicine on release of milk lipids from mammary tissue were evaluated by biochemical analysis of milk and morphological study of the tissue following intramammary infusions of the alkaloid into lactating goats. Colchicine produces a reversible drop in milk yield. As the flow of milk resumes, 36--48 h after infusion, the fat content of the milk increases, phospholipid per g of total globule lipid falls, mean size of milk fat globules increases and diameters of fat droplets (presecretory milk fat globules) within lactating cells approximately double. These observations are consistent with the conclusion that colchicine suppresses milk fat globule secretion but that globules continue to grow in size wihtin cells during the suppression period. These findings indicate that secretion of milk fat globules and the skim milk phase are coupled.

Mechanisms of secretion: effects of colchicine and vincristine on composition and flow of milk in the goat

Colchicine, the plant alkaloid, produced a dramatic decrease in milk flow when infused into the udder of the goat. The compound (1 to 5 mg) dissolved in 5 ml of water was inserted into one side of the under via the teat canal. Such treatments consistently caused a depression in milk yield from the infused side with maximum at 36 h and substantial reversal by 72 to 96 h. Milks from both the infused and uninfused sides of the udder were essentially normal in composition (fat, protein, and lactose). However, globulins and riboflavin were elevated in milks from the infused side. The plant alkaloid, vincristine, produced effects on milk secretion similar to those of colchicine but at dosages roughly one-tenth the latter. The two substances had no effect on the amount of milk from the uninfused side of the udder. Experiments employing [carbon-14] colchicine revealed that less than 20% of the infused colchicine is secreted in the milk. Both the secretion of fat globules and the emptying of secretory vesicles by the lactating cell are inhibited by colchicine indicating that a portion of the cell population is turned off from secretion. Plant substances such as colchicine and vincristine may at times limit yields of milk, especially in grazing ruminants.

Lipid transport across the intestinal epithelial cell. Effect of colchicine

Rats injected with colchicine (0.5 mg/100 g of body weight) 1 h before ingestion of a margarine emulsion (1 g in 2 ml of saline) do not show the rise in plasma triacylglycerol concentration found in controls during the subsequent hours. The effect of colchicine is more dramatic when the experiment is performed after prior administration of Triton WR-1339, a substance known to inhibit the catabolism of lipoproteins. Colchicine-treated rats also showed a five-fold increase in the content of triacylglycerol in proximal jejunum, when compared to controls. These results are consistent with the idea that colchicine interferes with the intracellular phase of fat absorption, suggesting that the microtubular-microfilamentous system could be involved in the release of chylomicrons from the intestinal cell into the circulation.

Involvement of vesicle coat material in casein secretion and surface regeneration

The ultrastructure of the apical zone of lactating rat mammary epithelial cells was studied with emphasis on vesicle coat structures. Typical 40-60 nm ID "coated vesicles" were abundant, frequently associated with the internal filamentous plasma membrane coat or in direct continuity with secretory vesicles (SV) or plasma membrane proper. Bristle coats partially or totally covered membranes of secretory vesicles identified by their casein micelle content. This coat survived SV isolation. Exocytotic fusion of SV membranes and release of the casein micelles was observed. Frequently, regularly arranged bristle coat structures were identified in those regions of the plasma membrane that were involved in exocytotic processes. Both coated and uncoated surfaces of the casein-containing vesicles, as well as typical "coated vesicles", were frequently associated with microtubules and/or microfilaments. We suggest that coat materials of vesicles are related or identical to components of the internal coat of the surface membrane and that new plasma membrane and associated internal coat is produced concomitantly by fusion and integration of bristle coat moieties. Postexocytotic association of secreted casein micelles with the cell surface, mediated by finely filamentous extensions, provided a marker for the integrated vesicle membrane. An arrangement of SV with the inner surface of the plasma membrane is described which is characterized by regularly spaced, heabily stained membrane to membrane cross-bridges (pre-exocytotic attachment plaques). Such membrane-interconnecting elements may represent a form of coat structure important to recognition and interaction of membrane surfaces.