Effect of equilibration period on the viability of frozen-thawed mouse morulae after rapid freezing

Mouse morulae were frozen with 1.5-4.0 M glycerol + 0.25 M lactose solution by direct plunging into liquid nitrogen vapor 0.5-30 min after equilibration at room temperature. After thawing, embryos were cultured in vitro, and the highest survival rates were obtained after exposure for 3 min at 3.0 and 4.0 M and for 5 min at 1.5 and 2.0 M glycerol levels. Significant reductions in the survival rates (P less than 0.05) were observed when equilibration periods were extended for 3-5 min at 3.0 and 4.0 M and for 5-10 min at 1.5 and 2.0 M glycerol levels. These results clearly demonstrate that the equilibration time of embryos in glycerol-lactose mixture is one of the most important factors in the present rapid freezing conditions. To clarify the factors that lower embryo viability after prolonged equilibration, we performed further experiments on the effects of exposure to glycerol-lactose mixture on the developmental potential of embryos without freezing and on the volume changes of embryos during the exposure to glycerol solution with or without lactose. It was suggested that the detrimental effects of prolonged equilibration are due not only to the toxicity and osmotic injury of higher concentrations of cryoprotectant solution but also to the influx of water into embryonic cells caused by the hypotonic salt concentration of the extracellular (freezing) solution.     

Ovarian effects of a high lactose diet in the female rat

Young women with galactosemia experience ovarian failure at a very early age raising concern about the ovarian toxicity of galactose. While galactose may be present in the diet as a monosaccharide, it is predominantly derived from cleavage of the disaccharide lactose within the intestine. Our previous studies in animals have shown that high galactose diets inhibit ovarian follicular development and long-term exposure to high lactose diets retards growth of rats. The objective of the present study was to determine whether galactose exposure in the form of dietary lactose mimics the effects found previously with diets rich in galactose. Sixty female Long-Evans rats (25-day-old) were randomly assigned to two groups and fed a control diet (41.9% glucose in AIN93G [American Institute of Nutrition], CON) before lactose treatment. Unilateral ovariectomy (uOVX) was performed on half of the rats in each group to determine baseline ovarian follicle numbers. The study diet was a high lactose diet (HLD) containing 41.9% lactose in AIN93G. Study diet exposure started 1 month after uOVX (3 months old) and continued for 7 months in the treatment group. The control group remained on the 41.9% glucose diet throughout. Vaginal cytology, ovarian morphometric analyses, and serum concentrations of estradiol and progesterone were examined. Long-term exposure to the HLD decreased the body weights of animals and progesterone concentrations in the serum but produced no harmful effects on ovarian morphology or function. Beginning at 5 months of age (two months of lactose treatment) increasing numbers of females began to cycle irregularly but there was no difference between the glucose and lactose diet groups. These negative findings imply that administration of galactose in the form of lactose seems to be much less toxic than when galactose is fed to animals. From a human health perspective, these results are somewhat reassuring, since in general, women eat lactose-containing foods rather than foods that contain large amounts of free galactose.     

Differential expression of oxidative stress and extracellular matrix remodeling genes in low- or high-dose-rate photon-irradiated skin

Changes in the expression of genes implicated in oxidative stress and in extracellular matrix (ECM) remodeling and selected protein expression profiles in mouse skin were examined after exposure to low-dose-rate or high-dose-rate photon irradiation. ICR mice received whole-body γ rays to total doses of 0, 0.25, 0.5 and 1 Gy at dose rates of 50 cGy/h or 50 cGy/min. Skin tissues were harvested for characterization at 4 h after irradiation. For oxidative stress after low-dose-rate exposure, 0.25, 0.5 and 1 Gy significantly altered 27, 23 and 25 genes, respectively, among 84 genes assessed (P < 0.05). At doses as low as 0.25 Gy, many genes responsible for regulating the production of reactive oxygen species (ROS) were significantly altered, with changes >2-fold compared to 0 Gy. For an ECM profile, 18-20 out of 84 genes were significantly up- or downregulated after low-dose-rate exposure. After high-dose-rate irradiation, of 84 genes associated with oxidative stress, 16, 22 and 22 genes were significantly affected after 0.25, 0.5 and 1 Gy, respectively. Compared to low-dose-rate radiation, high-dose-rate exposure resulted in different ECM gene expression profiles. The most striking changes after low-dose-rate or high-dose-rate exposure on ECM profiles were on genes encoding matrix metalloproteinases (MMPs), e.g., Mmp2 and Mmp15 for low dose rate and Mmp9 and Mmp11 for high dose rate. Immunostaining for MMP-2 and MMP-9 proteins showed radiation dose rate-dependent differences. These data revealed that exposure to low total doses with low-dose-rate or high-dose-rate photon radiation induced oxidative stress and ECM-associated alterations in gene expression profiles. The expression of many genes was differentially regulated by different total dose and/or dose-rate regimens.     

Diffusion of lactose in kappa-carrageenan/locust bean gum gel beads with or without entrapped growing lactic acid bacteria

Effective diffusion coefficients (De) of lactose in kappa-carrageenan (2.75% wt/wt)/locust bean gum (0.25% wt/wt) (LBG) gel beads (1.5-2.0-mm diameter)with or without entrapped lactic acid bacteria (LAB) were determined at 40 degrees C. The effects of lactose concentration, bacteria strain (Streptococcus salivarius subsp. thermophilus and Lactobacillus casei subsp. casei) and cell content at various steps of the fermentation process (after immobilization, pre-incubation of the beads and successive fermentations) were measured on De as a first step for process modelling. Results were obtained from transiend concentration changes n well-stirred lactose solutions in which the beads were suspended. A mathematical model of unsteady-state diffusion in a sphere was used, and De was obtained from the best fit of the experimental data. Diffusivity of lactose in cell-tree beads was significantly lower than in pure water mainly because of the obstruction effect of the polymer chains and the hydration region. Furthermore, effective diffusivity and equilibrium partition factor were independent of lactose concentration in the range from 12.5 to 50 g/L. No significant difference was found for De (effective diffusivity) and Kp (partition) coefficients between beads entrapping S. thermophilus (approximately 5 x 10(9) CFU/mL) and cell-free beads. On the other hand higher cell counts obtained with L. casei (close to 1.8 x 10(11) CFU/mL) increased mass transfer resistance resulting in lower effective diffusivities and Kp. Finally, the effects of the type of bacteria and their distribution in the beads on the diffusivity were also discussed.     

Protection from radiation-induced apoptosis by the radioprotector amifostine (WR-2721) is radiation dose dependent

The radioprotective agent amifostine is a free radical scavenger that can protect cells from the damaging effects of ionising radiation when administered prior to radiation exposure. However, amifostine has also been shown to protect cells from chromosomal mutations when administered after radiation exposure. As apoptosis is a common mechanism by which cells with mutations are removed from the cell population, we investigated whether amifostine stimulates apoptosis when administered after radiation exposure. We chose to study a relatively low dose which is the maximum radiation dose for radiation emergency workers (0.25 Gy) and a high dose relevant to radiotherapy exposures (6 Gy). Mice were administered 400 mg/kg amifostine 30 min before, or 3 h after, whole-body irradiation with 0.25 or 6 Gy X-rays and apoptosis was analysed 3 or 7 h later in spleen and bone marrow. We observed a significant increase in radiation-induced apoptosis in the spleen of mice when amifostine was administered before or after 0.25 Gy X-rays. In contrast, when a high dose of radiation was used (6 Gy), amifostine caused a reduction in radiation-induced apoptosis 3 h post-irradiation in spleen and bone marrow similar to previously published studies. This is the first study to investigate the effect of amifostine on radiation-induced apoptosis at a relatively low radiation dose and the first to demonstrate that while amifostine can reduce apoptosis from high doses of radiation, it does not mediate the same effect in response to low-dose exposures. These results suggest that there may be a dose threshold at which amifostine protects from radiation-induced apoptosis and highlight the importance of examining a range of radiation doses and timepoints.     

Effects of fenvalerate on the net-spinning behaviour of Hydropsych siltalai (Döhler) (Trichoptera: Hydropsychidae)

The effects of fenvalerate exposure on the net-spinning behaviour of Hydropsyche siltalai were examined in a laboratory study. The larvae were exposed to nominal pulse-doses of 0.25 and 0.5 μg fenvalerate l-1. Nets were collected and examined for anomalies after four days of exposure to fenvalerate. Additional nets were collected after another four day of exposure. The fenvalerate dissipated rapidly from the water column, and since only two doses of fenvalerate were given, the larvae were exposed to two pulse-doses of fenvalerate rather than to a constant concentration. In the 0.5 μg l-1-treatment the net-spinning behaviour was significantly affected, expressed as an increased mesh-opening and a decreased symmetry of the nets. No significant effects of fenvalerate exposure on the net-spinning behaviour were detected in the 0.25 μg l-1-treatment. Thus, with the conditions given in this experiment, exposure to fenvalerate starts to affect the net-spinning behaviour of Hydropsyche siltalai at a concentration between 0.25 and 0.5 μg l-1. The use of net-anomalies and Hydropsyche as bioindicators for monitoring pollutants in stream ecosystems are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Changes in alpha-lactalbumin, total lactose, UDP-galactose hydrolase and other factors in tammar wallaby (Macropus eugenii) milk during lactation

alpha-Lactalbumin was isolated from milk of M. eugenii and its concentration in milk samples taken at various times during lactation (0-40 weeks post partum) was determined by single radial immunodiffusion using rabbit antiserum to the purified protein. The alpha-lactalbumin concentration remained almost constant throughout lactation even though the concentration of total lactose (free lactose plus lactose contained in oligosaccharides) fell to zero after 34 weeks post partum. This fall in lactose was accompanied by a rise in the free galactose and glucose concentrations and marked increases in UDP-galactose hydrolase, nucleotide pyrophosphatase, alkaline phosphatase and acid beta-galactosidase activities. It is suggested that the in vitro hydrolysis of UDP-galactose was due to nucleotide pyrophosphatase and that this enzyme may also play a role in vivo late in lactation by making UDP-galactose unavailable for the synthesis of lactose. Alternatively, lactose and lactose-containing oligosaccharides might be degraded by the acid beta-galactosidase during or after secretion.     

Binding of Lactose and Galactose to Native and Iodinated Ricin D

The binding of lactose and galactose to native and iodinated ricin D was investigated by equilibrium dialysis and ultraviolet difference spectroscopy. The results provided direct evidence that native ricin D has two independent saccharide binding sites with different affinities, of which the high-affinity (HA-) binding site is able to bind with both lactose and galactose while the low-affinity (LA-) binding site binds only with lactose. In contrast, the iodinated ricin D possesses only one binding site both for lactose and galactose with high affinity.

By UV-difference spectroscopic analysis we found that there is one tyrosyl residue at or near the HA-binding site in ricin D which may be involvled in binding with saccharide. This tyrosyl residue was not iodinated in the presence of lactose but was iodinated in the absence of lactose and was perturbed by an addition of lactose even after iodination.

From these results, it was inferred that the binding site abolished by the iodination is the LA-binding site and this may be due to the conformational alteration of the LA-binding site caused by the iodination of the tyrosyl residue(s) present near the LA-binding site.     

Counter-diffusion of lactose and lactic acid in kappa-carrageenan/locust bean gum gel beads with or without entrapped lactic acid bacteria.

The effective diffusion coefficient (De) and equilibrium partition factor (Kp) for lactose and lactic acid in k-carrageenan (2.75% w/w)/locust bean gum (0.25% w/w) (LBG) gel beads (1.5-2.0 mm diameter), with or without entrapped Lactobacillus casei subsp. casei (L. casei), were determined at 40 degrees C. Results were obtained from transient concentration changes in well-stirred solutions of finite volume in which the beads were suspended. Mathematical models of unsteady-state diffusion into and/or from a sphere and appropriate boundary conditions were used to calculate effective diffusion coefficients of lactose and lactic acid from the best fit of the experimental solute concentration changes. The effective diffusivities of lactose and lactic acid were 5.73 x 10(-10) and 9.96 x 10(-10) m2 s-1, respectively. Furthermore, lactic acid was found to modify gel structure since lactose diffusion characteristics (De and Kp) differed significantly from an earlier study and in the literature. In gel beads heavily colonized with L. casei, the effective diffusion coefficients of lactose and lactic acid were respectively 17% and 24% lower than for cell-free beads. Partition coefficients also confirmed the obstruction effect due to the cells, and decreased from 0.89 to 0.79, and from 0.98 to 0.87, for lactose and lactic acid, respectively. External mass transfer was estimated by an unsteady-state model in infinite volume using the Biot number. The effect of external mass transfer resistance on De results and the data reported in the literature are discussed.     

Growth ofZymomonas on lactose: Gene cloning in combination with mutagenesis

Summary Wild-type strains ofZymomonas mobilis have a limited substrate range of glucose, fructose and sucrose. In order to expand this substrate range, transconjugants ofZ. mobilis containing Lac⁺ plasmids have been constructed. Although -galactosidase is expressed in such strains, they lack the ability to grow on lactose. We now report the development ofZ. mobilis strains capable of growth on lactose. This was achieved in two stages. First, a broad host range plasmid was constructed (pRUT102) which contained the lactose operon under the control of aZ. mobilis promoter plus genes for galactose utilization.Z. mobilis CP4.45 containing pRUT102 was then subjected to mutagenesis combined with continued selection pressure for growth on lactose. One strain,Z. mobilis SB6, produced a turbid culture that yielded 0.25% ethanol from 5% lactose (plus 2% yeast extract) in 15 days.     

Aromatherapy in the Mediterranean fruit fly (Diptera: Tephritidae): sterile males exposed to ginger root oil in prerelease storage boxes display increased mating competitiveness in field-cage trials

Previous research showed that exposure to ginger root, Zingiber officinale Roscoe, oil increased the mating success of mass-reared, sterile males of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann). This work, however, involved the exposure of small groups of males (n = 25) in small containers (volume 400 ml). Several sterile male release programs use plastic adult rearing containers (so-called PARC boxes; hereafter termed storage boxes; 0.48 by 0.60 by 0.33 m) to hold mature pupae and newly emerged adults before release (approximately = 36,000 flies per box). The objective of the current study was to determine whether the application of ginger root oil to individual storage boxes increases the mating competitiveness of sterile C. capitata males. Irradiated pupae were placed in storage boxes 2 d before adult emergence, and in the initial experiment (adult exposure) ginger root oil was applied 5 d later (i.e., 3 d after peak adult emergence) for 24 h at doses of 0.0625, 0.25, 0.5, 1.0, and 2.0 ml. In a second experiment (pupal-adult exposure), ginger root oil was applied to storage boxes immediately after pupal placement and left for 6 d (i.e., 4 d after peak adult emergence) at doses of 0.25 and 1.0 ml. Using field cages, we conducted mating trials in which ginger root oil-exposed (treated) or nonexposed (control) sterile males competed against wild-like males for copulations with wild-like females. After adult exposure, treated males had significantly higher mating success than control males for all doses of ginger root oil, except 2.0 ml. After pupal-adult exposure, treated males had a significantly higher mating success than control males for the 1.0-ml but not the 0.25-ml dose of ginger root oil. The results suggest that ginger root oil can be used in conjunction with prerelease, storage boxes to increase the effectiveness of sterile insect release programs.     

Galactosidase activity of lactose-positive Neisseria

The chromogenic substrate o-nitrophenyl-beta-d-galactopyranoside (ONPG) was hydrolyzed by lactose-positive Neisseria. Eight strains of pharyngeal origin were examined. In culture reactions, seven strains resembled Neisseria meningitidis with the exception that they produced acid from 1% (w/v) lactose. An eighth strain (V8) differed in that it did not form acid from maltose or from 1% lactose. However, acid formation was observed in 10% lactose cultures of strain V8, suggesting that entry of lactose occurred by passive diffusion, rather than as a result of permease activity. The enzymes which hydrolyzed ONPG were produced constitutively by the cells of all eight strains. Thus, specific activity in these strains was not increased by prior exposure to lactose, or to two other possible inducers, isopropyl-beta-d-thiogalactoside or methyl-beta-d-thiogalactoside. Study of cell-free extracts of one strain showed that the enzyme was heat-labile, having a half-life of 10 min at 45 C. The enzyme was unstable at low protein concentrations, but it was protected completely or partially when albumin or manganous ions were added. The enzyme appeared to be a typical beta-galactosidase: alpha-galactosides (melibiose and p-nitrophenyl-alpha-d-galactopyranoside) were not hydrolyzed, activity against ONPG was not dependent upon inorganic phosphate, and galactose was released by cleavage of ONPG. ONPG hydrolysis provided a simple and rapid method for detecting lactose-positive Neisseria.     

Limited proteolysis of lactose permease from Escherichia coli

Escherichia coli lactose permease (also referred to as lactose carrier) is an integral protein of the cytoplasmic membrane. Using lactose permease either radiolabeled biosynthetically in plasmid-bearing E. coli minicells or radioalkylated post-synthetically by chemical modification, we have determined sites on the membrane-bound protein accessible to proteolytic attack and we have characterized several high-molecular-mass products. The most prominent polypeptide obtained from lactose permease radiolabeled biosynthetically is observed after digestion with different proteases. The fragment produced by thermolysin was shown to contain the intact N-terminus and to extend into the region around amino acid residue 140 which, according to secondary structure models, is presumed to be less tightly folded than the rest of the molecule. Evidence is presented that the corresponding fragments obtained after digestion with several other proteases also originate from the N-terminal part of the protein. This N-terminal segment of the lactose carrier is resistant to proteolytic digestion even in the presence of non-ionic detergents and it may represent a tightly folded domain. Additional proteolytic cleavage sites located C-terminal of the Cys148 residue can be inferred.     

Effect of estrogen and placental lactogen on lactogenesis in pregnant rats

The removal of the corpora lutea or ovariectomy on Day 18 of pregnancy induced a rise in serum prolactin 24 h after surgery with a rapid decline to control values 4 h after the surge, only in the ovariectomized group. When hysterectomy was performed in addition to luteectomy or ovariectomy a similar rise in prolactin was obtained. Lactose synthetase activity in mammary tissue was significantly higher in the luteectomized and luteectohysterectomized rats when compared with ovariectomized, ovariohysterectomized rats and the sham-operated group. Estrogen treatment 12 h after ovariectomy increased serum prolactin and lactose synthetase activity to values similar to those measured in luteectomized rats, but this increase was significantly greater when compared with the ovariectomized-nontreated group. Treatment with Tamoxifen did not decrease serum prolactin in the luteectomized rats but lactose synthetase was reduced to values similar to that obtained in ovariectomized rats. Treatment with 2 bromo-alpha-ergocryptine-mesylate (CB-154) prevented the rise in serum prolactin in the ovariectomized, luteectomized and luteectohysterectomized groups, but lactose synthetase activity was lowered to control values (sham-operated rats) only in the luteectohysterectomized rats. According to these findings, rat placental lactogen in the absence of prolactin and progesterone induces lactose synthesis. Estrogen facilitates prolactin but not placental lactogen action on lactose synthetase activity.     

Early gene expression profile in mouse brain after exposure to ionizing radiation

Acute changes in the gene expression profile in mouse brain after exposure to ionizing radiation were studied using microarray analysis. RNA was isolated at 0.25, 1, 5 and 24 h after exposure to 20 Gy and at 5 h after exposure of the whole brain of adult mice to 2 or 10 Gy. RNA was hybridized onto 15K cDNA microarrays, and data were analyzed using GeneSpring and Significant Analysis of Microarray. Radiation modulated the expression of 128, 334, 325 and 155 genes and ESTs at 0.25, 1, 5 and 24 h after 20 Gy and 60 and 168 at 5 h after 2 and 10 Gy, respectively. The expression profiles showed dose- and time-dependent changes in both expression levels and numbers of differentially modulated genes and ESTs. Seventy-eight genes were modulated at two or more times. Differentially modulated genes were associated with 12 different classes of molecular function and 24 different biological pathways and showed time- and dose-dependent changes. The change in expression of four genes (Jak3, Dffb, Nsep1 and Terf1) after irradiation was validated using quantitative real-time PCR. Up-regulation of Jak3 was observed in another mouse strain. In mouse brain, there was an increase of Jak3 immunoreactivity after irradiation. In conclusion, changes in the gene profile in the brain after irradiation are complex and are dependent on time and dose, and genes with diverse functions and pathways are modulated.     

The induction of reciprocal translocations in rhesus monkey stem-cell spermatogonia: effects of low doses and low dose rates

The induction of reciprocal translocation in rhesus monkey spermatogonial stem cells was studied following exposure to low doses of acute X rays (0.25 Gy, 300 mGy/min) or to low-dose-rate X rays (1 Gy, 2 mGy/min) and gamma rays (1 Gy, 0.2 mGy/min). The results obtained at 0.25 Gy of X rays fitted exactly the linear extrapolation down from the 0.5 and 1.0 Gy points obtained earlier. Extension of X-ray exposure reduced the yield of translocations similar to that in the mouse by about 50%. The reduction to 40% of translocation rate after chronic gamma exposure was clearly less than the value of about 80% reported for the mouse over the same range of dose rates. Differential cell killing with ensuing differential elimination of aberration-carrying cells is the most likely explanation for the differences between mouse and monkey.     

Cellulase induction by lactose in Trichoderma reesei PC-3-7

In an attempt to clarify the function of lactose in cellulase induction, experiments were carried out on cellulase formation by lactose along with other sugars in a resting cell system of Trichoderma reesei PC-3-7, a hypercellulase-producing mutant. Although lactose alone induces little cellulase under the conditions used, a synergistic effect on cellulase formation was observed following the respective addition of sophorose, cellobiose or galactose to lactose. The lactose consumption was more rapid when these sugars were added than in their absence. Furthermore, following lactose addition 10 h after the beginning of cultivation in the presence of cellobiose, cellulase formation was initiated with only a little lag, and lactose consumption started immediately, being complete in 14 h. \-Galactosidase induction experiments suggested that the rapid consumption of lactose is possibly not dependent on lactose degradation by the enzyme. From these results, it is suggested that lactose may function as an inducer for cellulase formation if it is taken up in the mycelium of T. reesei PC-3-7, and that sophorose, cellobiose or galactose may induce a putative lactose permease. *** DIRECT SUPPORT *** AG903066 00005     

Microanalysis of lactose in tissue culture medium using an enzymatic-fluorometric method

A fluoroenzymatic microdetermination of lactose in tissue culture medium is described. The method is based on the conversion of lactose to galactonic acid by the enzymes galactosidase and galactose dehydrogenase. The resultant reduction of NAD to NADH is measured fluorometrically. Before assaying for lactose the high background fluorescence of tissue culture medium is reduced by deproteinization and adsorption of fluorescent substances with either Florisil or dextran-charcoal. As little as 0.05 nmol of lactose in medium with or without phenol red can be detected after this treatment, whereas for untreated medium the limit of sensitivity of the assay is 1.0 nmol. This method should prove useful for the detection of lactose produced by small numbers of mammary epithelial cells in culture.     

Effect of 0.25 ppm ozone exposure on pulmonary damage induced by bleomycin

To study the effect of ozone in a chronically damaged lung, we used a bleomycin (BLM) induced pulmonary fibrosis model. Both endotracheal instillation of BLM and O3 exposure both produce lung inflammation and fibrosis. Oxidative stress would be a common mechanism of damage for both BLM and O3. Our aim was to assess lung injury induced by 5 and 60 days of intermittent exposure to 0.25 ppm O3 in rats with bleomycin-induced pulmonary fibrosis. Thirty-day-old Sprague Dawley rats were endotracheally instilled with BLM (1 U/100 g body weight) and, 30 days later, exposed to 0.25 ppm 03 (0.25 ppm 4 h per day, 5 days a week). Histopatology controls were instilled with saline and breathing room air. Histopathological evaluation of lungs was done 5 and 60 days after O3 exposure. BLM-induced lung damage did not change after 60 days of intermittent O3 exposure. Five days of O3 exposure increased the mean score of BLM-induced pulmonary inflammation and fibrosis (p=0.06). Frequency of bronchopneumonia increased from 1/7 to 6/6 (p <0.001), suggesting that a short-term exposure to O3 in a previously damaged lung might be a risk factor for developing further lung injury.     

Suckling-induced prolactin release potentiates mifepristone-induced lactogenesis in pregnant rats

Suckling, starting at 19:00 h on Day 18 of pregnancy, induced a significant increase in serum prolactin concentration at 20:00 h on Day 19 of pregnancy, but no increase in mammary gland casein or lactose content. Mifepristone (2 mg/kg) injection at 08:00 h on Day 19 of pregnancy induced significant increases in casein, but not in lactose, 24 h after administration. Mifepristone alone did not induce prolactin secretion, indicating that lactogenesis was induced by placental lactogen in the absence of progesterone action. When mifepristone was injected into suckling rats, serum prolactin concentrations were higher than in the untreated suckling rats. Casein in these rats increased significantly 12 h after mifepristone administration and lactose at 24 h after. If the suckling mifepristone-treated rats were given two injections of bromocriptine (1.5 mg/kg) at 12:00 h on Days 18 and 19 of pregnancy, serum prolactin concentrations were not increased by suckling, but casein and lactose concentrations in the mammary gland showed values similar to those obtained in the mifepristone-treated non-suckling rats. Mifepristone can therefore potentiate suckling-induced prolactin release in pregnant rats, demonstrating a direct central inhibitory action of progesterone on prolactin secretion. This suckling-induced prolactin secretion, unable to induce casein or lactose synthesis in the presence of progesterone, enhanced significantly synthesis of these milk components in the absence of progesterone action (rats treated with mifepristone). Fatty acid synthase, which is stimulated by the suckling stimulus in lactating rats, was not modified by mifepristone or suckling in pregnant rats.