Use of the true absorption coefficient as a measure of bioavailability of radiocaesium in ruminants

Limitations of existing methods to describe the bioavailability of dietary radionuclides to ruminants (the transfer coefficient and apparent absorption coefficient) have led to the alternative suggestion of using the true absorption coefficient (A _t). Various approaches to estimatingA _t for radiocaesium, involving the intravenous administration of a second isotope, are presented and discussed with reference to results from studies in which a range of radiocaesium sources were examined in sheep. Although estimates ofA _t differed between the sources, they were reasonably consistent between measurement techniques. Those methods which involved the estimation of endogenous faecal excretion of radiocaesium could be used with previously contaminated animals and did not require continuous administrations of radiocaesium isotopes, but gave unreliable results for sources of low bioavailability. Methods based on estimating the turnover rate of dietary radiocaesium through blood plasma were sufficiently sensitive to measureA _t for the range of sources studied. However, they require previously uncontaminated animals and continuous administration of both isotopes for approximately 7 days. Bioavailability is more effectively measured asA _t than as the transfer or apparent absorption coefficients sinceA _t does not incorporate factors relating to the metabolism of radiocaesium in the tissues of the animal. The results of these studies show that differences in transfer coefficients between sheep and cattle and between sheep of differing ages are not due to variation in absorption across the gut. The potential for applying these approaches to other radioactive elements is discussed.     

Variation in the metabolism of radiocaesium between individual sheep

Considerable variability has been recorded in the radiocaesium activity concentration of muscle between individual sheep in the same flocks in upland areas that received fallout from the Chernobyl accident. In a previous paper we demonstrated that there is a propensity for certain sheep within a flock to be always amongst the most contaminated and others to be consistently the least contaminated. Here we report a study to determine the extent to which variation in the metabolism of radiocaesium by individual sheep may contribute to the observed variability within sheep flocks. The transfer coefficient and biological half-life of orally administered ionic radiocaesium in muscle were determined under controlled conditions in 22 ewes from an upland farm in an area of the UK which received comparatively high levels of Chernobyl fallout. There was considerable variation between individuals in both the transfer coefficient (0.19–0.56 day⋅kg^–1; mean 0.34 day⋅kg^–1) and biological half-life in muscle (5.2– 18.7 days; mean 9.8 days). Changes in liveweight during the study and feed intake together accounted for 72% of the variation in the derived transfer coefficients; liveweight change also accounted for 56% of the observed variation in biological half-life. In a subsequent study, the true absorption coefficient of radiocaesium was determined in 12 of the ewes. There was a positive correlation between transfer and true absorption coefficients (R=0.57). We conclude that differences in the metabolism of radiocaesium will contribute to the observed variability in radiocaesium activity concentrations within sheep flocks in areas which were contaminated by Chernobyl fallout. We also suggest that for growing animals, the influence of liveweight change and feed intake on radiocaesium transfer may be greater than observed here. Similarly, in dairy cattle, for which feed intake changes considerably during the course of a lactation, large temporal variation in radiocaesium transfer to milk could be expected. Received: 10 August 1998 / Accepted in revised form: 20 October 1998     

Prediction of utilizable true protein of mixed rations for sheep using an in vitro incubation technique

The objective of the present experiment was to study the relationship between in vitro utilizable true protein (uTP) and in vivo-uTP of sheep rations by regression analysis. A further aim was to analyse if in vivo-uTP of mixed rations could be predicted by regression analysis between in vitro-uTP and in vivo-uTP, using N-retention of sheep as important evaluation criteria of protein value. Three adult male sheep (body weight [BW] 46 + 1.3 kg) fitted with rumen cannulas and simple T-type duodenal cannulas were fed with twelve typical rations with graded levels of crude protein and true protein in four experiments according a 3 x 3 Latin square design. Each experimental period included an adaptation (7 days), a N balance trial (4 days) and a collection of duodenal digesta (3 days). During collection of duodenal digesta, polyethylene glycol and chromium oxide were used as dual markers for the measurement of duodenal digesta flow and calculation of the in vivo-uTP of duodenal digesta. The in vitro-uTP of the rations was determined using the in vitro incubation technique of Zhao and Lebzien (2000). It was found that both in vitro-uTP intake and in vivo-uTP intake were significantly correlated with N-retention (p < 0.001) and that there was a significant linear relationship between the content of in vitro-uTP and in vivo-uTP in rations (p < 0.001). Therefore, it was concluded that the used in vitro incubation technique is suitable for the determination of in vitro-uTP of mixed rations for sheep, and that the amount of in vivo-uTP can be predicted by regression between in vitro-uTP and in vivo-uTP.     

Diffusibility of selenate, selenite, seleno-methionine, and seleno-cystine during simulated gastrointestinal digestion

The present study was undertaken to evaluate the in vitro availability of chemically varying forms of selenium (Se), supplemented in cow's milk. Two inorganic (selenite and selenate) and two organic (seleno-methionine [Se-Met] and seleno-cystine [Se-Cys]) Se sources were evaluated. The in vitro availability was estimated by the diffusibility of Se during simulated gastrointestinal digestion. First, the diffusibility was compared after adding a constant amount of Se as either selenate, selenite, seleno-methionine, or Se-Cys in milk samples. Se-Met and selenate were found to be significantly more diffusible than selenocystine and selenite under the simulated gastrointestinal conditions. The tendency for superior in vitro availability of selenate and Se-Met compared to selenite and Se-Cys was confirmed for a supplementation range of 5–40 ng/g of Se. This study suggests that the high diffusibility of selenate and Se-Met in a simulated gastrointestinal environment may contribute to their high absorption in vivo.     

Study on Zn Relative Concentration and State in Sheep Duodenum by XAFS

Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy is not widely used in animal science. The objective of this study was to employ the XAFS technique to determine changes in zinc absorption and concentrations in the sheep interstinal sac using different zinc sources. Forty-eight sheep were slaughtered and their duodena were dissected. The duodena were randomly assigned to six zinc sources (ZnO, ZnSO₄, ZnMet, ZnLys, ZnSO₄ + methionine, ZnSO₄ + lysine). Ten centimeters of duodenal sac midpiece was incubated for 40 min in vitro using the everted intestinal sac technique in culture medium containing zinc from different sources. The amount of zinc present was normalized to 4 mg. XAFS was used to analyze the relative concentration and oxidation state of zinc, and atomic absorption spectrometry (AAS) was used to verify zinc concentration. The results showed that, for increasing zinc concentrations, organic zinc was a better source than inorganic zinc. In addition, the zinc concentration achieved using ZnMet was higher than that for ZnO and ZnSO₄ (P < 0.05) as measured by atomic absorption spectroscopy. The results using XAFS were consistent with that of AAS. The states of organic zinc and inorganic zinc were identical after being incubated for 40 min. As observed in these experiments, organic zinc was more easily absorbed than inorganic zinc. Our data demonstrate that organic zinc was dissociated into ions and then absorbed as inorganic zinc. In our experiments, we are the first investigators to use XAFS spectroscopy to determine zinc absorption in the sheep duodenal wall. We observed reduced absorption of inorganic zinc in the presence of methionine or lysine. Taken together, we postulate that the optimal molar ratio of inorganic zinc and ligand requires further study.     

Effect of dietary phosphate on intestinal calcium and phosphate absorption

Intestinal Ca and P absorption was investigated on rachitic chicks raised on diets with a 1% Ca and 0.3% or 1% P contents. 45Ca and 32P absorption was determined by the technique of the isolated gut sac in vivo. In addition, 32P transport was also measured by the everted gut sac procedure in vitro. Treatment with vit. D3 during 7 days increased the 45Ca absorption in animals fed diets containing 0.3% or 1% P. 32P absorption showed an increase after 2 days of treatment and a decrease afterwards. The reduction of 32P absorption was larger in animals fed diet with 1% P. Study of 32P transport with the everted gut sac technique showed an increase after vit. D3 and a loss of intracellular P, regardless the duration of treatment.     

Absorption,retention and tissue distribution of radiocaesium in reindeer: effects of diet and radiocaesium source

Radiocaesium absorption and retention in reindeer (Rangifer tarandus) calves was compared in groups fed diets containing different proportions of lichen and concentrates, and different chemical forms of radiocaesium ((134)CsCl in solution or fallout from the Chernobyl accident). Daily intakes of fallout radiocaesium were 15-23 kBq, while daily intakes of (134)CsCl ranged from 70 kBq to 1,160 kBq. The half-life for radiocaesium in red blood cells (RBC) in animals fed with a pure lichen diet exceeded that in animals fed with a combined diet of lichen and concentrates by 40% (17.8+/-0.7 days vs. 12.7+/-0.4 days). Corresponding differences in the half-lives for urinary and faecal excretion were about 60% and 40%, respectively. Transfer coefficients (F(f)) to reindeer meat were estimated to be 0.25+/-0.01 days kg(-1) for fallout radiocaesium and 1.04+/-0.03 days kg(-1) for (134)CsCl, reflecting differences in both radiocaesium bioavailability and retention. The bioavailability of the Chernobyl radiocaesium in lichen in 1988 was estimated at ca. 35% compared to (134)CsCl.     

Magnesium movement in hypothyroid sheep.

We described magnesium transport in hypothyroid and normal sheep with an eight-compartment biomodel fitted by a least-squares technique to data from multiple compartments and routes of entry of 28-Mg. Isotopic equilibrium was not attained during this period of observation in hypothyroid or control sheep. Hypothyroidism reduced transfer coefficients for absorption (P less than 0.01) of 28-Mg from the GI tract but did not change the rate coefficient for endogenous 28-Mg losses to feces (P less than 0.05). Nutritional balance data indicated higher absorption and retention of Mg in hypothyroid sheep, and the observed decrease in rate of passage of digestive residues suggested that mean retention time of GI tract contents had a definite effect on the availability of Mg.     

Monte-Carlo prediction of changes in areas of west Cumbria requiring restrictions on sheep following the Chernobyl accident

Following the 1986 Chernobyl accident radiocaesium levels in sheep meat in some upland areas of the United Kingdom were above the national intervention limit. West Cumbria was one of these areas and restrictions are currently still in place. In addition to deposition from the Chernobyl accident, Cumbria has been subject to radiocaesium deposition from atmospheric nuclear weapons tests, the 1957 Windscale accident and routine releases from the Sellafield nuclear reprocessing plant. A Monte-Carlo approach has been used to try to predict areas in west Cumbria where radiocaesium activity concentrations in lamb meat would require the imposition of restrictions at different times after the Chernobyl accident. The approach models the transfer of radiocaesium from soil to vegetation, based upon soil organic matter, and from vegetation to lamb meat. Spatial inputs are soil organic matter and total post-Chernobyl (137)Cs and (134)Cs deposition; a ratio of Chernobyl (137)Cs to (134)Cs deposition has been used to differentiate Chernobyl and pre-Chernobyl (137)Cs deposition. Comparisons of predicted radiocaesium transfer from soil-vegetation and the spatial variation in lamb (137)Cs activity concentrations are good and predicted restricted areas with time after Chernobyl compare well to the restricted areas set by UK government. We predict that restrictions may be required until 2024 and that in some areas the contribution of pre-Chernobyl (137)Cs to predicted lamb radiocaesium activity concentrations is significant, such that restrictions may only have been required until 1994 as a consequence of Chernobyl radiocaesium deposition alone. This work represents a novel implementation of a spatial radioecological model using a Monte-Carlo approach.     

Modelling of radiocaesium soil-plant transfer

Existing methods of predicting of radiocaesium transfer from soil to plants was critcally reviewed. The analysis of radiocaesium behavior in the system "soil solid phase/pore solution/plant" was carried out. Equations for calculation of radiocaesium uptake by plants as a function of soil properties were obtained and tested using the reported experimental data. Key soil parameters which natural variability and estimation difficulty are the main sources of prediction uncertainty were identified.     

Radiocaesium in an organic soil and the effect of treatment with the fungicide ‘Captan’

Soil fungi accumulate radiocaesium from contaminated soil and it has been hypothesised that this may alter the plant availability and movement of the radionuclide in soil. The effect of twice-monthly addition of an aqueous suspension of the fungicide ‘Captan’ on the changes in a peaty podzol soil at 2 sites, contaminated 2 or 3 years earlier by the injection of ¹³⁴Cs, has been quantified. The sites had different soil acidity and vegetation cover. The less acid soil (pH_water 5.0) had been improved by the addition of lime and fertilizer and was reseeded with grass and clover. The more acid soil (pH_water 3.8) was under hill grasses, herbs and heather. On both sites the addition of fungicide did not alter the amount or concentration of radiocaesium in plant material sampled monthly or the depth distribution of radiocaesium in the soil profile. The concentration of the fungal constituent, ergosterol, in the soil, measured monthly, was unaffected by the fungicide treatment but evidence was obtained from a pot experiment to show that ergosterol decomposes slowly in cold, wet soils. On the more acid soil, two weeks after the last application of fungicide, there was a decline in active fungi as measured by fluorescein diacetate staining. Chloroform fumigation of the more acid soil resulted in a small increase in the amount of ¹³⁴Cs exchangeable with 1 M ammonium acetate. Radiocaesium in seven different fungi grown in pure culture was found to be almost entirely extractable (> 95%) with 1 M ammonium acetate. Another, Amanita rubescens, showed some retention and 88% was extractable. These findings do not preclude the fungal biomass as an important soil component controlling plant availability of radiocaesium from acid, organic soils by maintaining radiocaesium in a biological cycle, but make it unlikely that any fixation by fungi in a chemical sense is involved.     

Relation between in vitro and in vivo assessment of amino acid availability

Even though the availability of dietary amino acids is the result of integrated phenomena of digestion, absorption and transport, it may be mainly affected by the stage of luminal digestion. In this case, amino acid availability could be predicted by an in vitro method designed to reproduce in vivo proteolysis conditions. In order to check this hypothesis, the essential amino acid (EAA) profiles of digesta collected at 8 intervals during a 24-h in vitro enzymatic proteolysis of casein and rapeseed proteins were compared to the pattern of appearance of dietary EAA in portal vein of pigs fed the same proteins, determined at each hour over a 8-h postprandial period by coupling blood flow rate with porto-arterial differences in plasma EAA concentrations. Comparisons of in vitro and in vivo data first bore on overall EAA profiles measured at each interval, and then on the individual kinetics of each EAA. Regarding total profiles, the highest correlations for casein (r: 0.80-0.98) were found when comparing EAA patterns determined during the first half of in vitro digestion and in vivo absorption periods. Similar r values were obtained with rapeseed proteins, but over longer periods of measurement. Concerning individual kinetics, the highest correspondences were found with rapeseed proteins, with 5 out of 9 EAA (methionine, isoleucine, leucine, phenylalanine and arginine) having their in vitro sequence of release significantly correlated with their in vivo sequence of absorption. With casein, correlations were significant for threonine, valine, isoleucine and leucine. These results suggest that sequential hydrolysis in the digestive tract, as reproduced by the in vitro technique, is a key determinant of amino acid appearance in the portal blood to a degree varying with the protein source and with the nature of the amino acid.     

In vitro hepatic steatosis model based on gut–liver-on-a-chip

Hepatic steatosis, also known as fatty liver disease, occurs due to abnormal lipid accumulation in the liver. It has been known that gut absorption also plays an important role in the mechanism underlying hepatic steatosis. Conventional in vitro cell culture models have limitations in recapitulating the mechanisms of hepatic steatosis because it does not include the gut absorption process. Previously, we reported development of a microfluidic gut–liver chip that can recapitulate the gut absorption of fatty acids and subsequent lipid accumulation in liver cells. In this study, we performed a series of experiments to verify that our gut–liver chip reproduces various aspects of hepatic steatosis. The absorption of fatty acids was evaluated under various culture conditions. The anti-steatotic effect of turofexorate isopropyl (XL-335) and metformin was tested, and both drugs showed different action mechanisms. In addition, the oxidative stress induced by lipid absorption was evaluated. Our results demonstrate the potential of the gut–liver chip for use as a novel, physiologically realistic in vitro model to study fatty liver disease.     

Effect of bile/pancreatic secretions on absorption of radioactive or stable zinc

Biliary/pancreatic (B/P) secretions are a major component of endogenous secretions, and endogenously secreted Zn is a primary means of Zn homeostasis. This study examined whether B/P fluid alters the absorption/reabsorption of Zn and, in doing so, whether this contributes to homeostatic control of Zn. Animal experiments utilized rats fed 10 or 300 μg Zn/kg diet. An open-ended gut perfusion study in which⁶⁵Zn-labeled B/P fluid or⁶⁷Zn-labeled and digested diet found significantly decreased Zn absorption from B/P fluid. Although Zn absorption from both sources was less in animals fed diets higher in Zn, there was no interaction of treatment and diet. Further studies utilizing cultured human colon carcinoma cells (CACO-2) as in vitro models of gut enterocytes found that the presence of B/P fluid significantly decreased Zn retention and/or transport and resulted in a redistribution of cellular Zn after 1200 min of incubation. These studies show that a substance in B/P fluid can decrease the absorption of Zn and also suggest that dietary Zn and Zn associated with B/P secretions are absorbed from distinct pools. However, the lack of an interactive effect with diet, and the amount of time required to see differences in CACO-2 cells, suggest that differences in absorption are not a major contributor to Zn homeostasis. US Department of Agriculture, Agricultural Research Service, Northern Plains Area, is an equal opportunity/affirmative action employer and all agency services are available without discrimination.     

Effects of herbivory on N-cycling and distribution of added 15NH 4 + in N-limited low-alpine grasslands

Many rangelands around the world are degraded by severe overgrazing with resulting loss of nutrients and reduced productivity. However, grazing may also increase nutrient cycling and enhance ecosystem productivity. The aim of this study was to determine effects of grazing on availability of nitrogen (N), sources of N utilized by plants and cycling and distribution of N at a low-alpine site, Southern Norway. The study was part of a sheep grazing experiment with three density levels of sheep (no sheep, 25 km^?2 and 80 km^?2) since 2001. The N-content of plants was determined in June 2008, August 2008 and August 2009. Indirect effects of herbivory on sources of N and N-cycling were assessed by δ¹⁵N natural abundance and the system’s distribution of added ¹⁵NH₄-N. We found little evidence for grazing induced effects on availability, sources or cycling of N based on N content of plants and δ¹⁵N natural abundance. The organic soil horizon was the largest sink for the added ¹⁵NH₄-N. Proportional tracer recoveries and tracer enrichments indicate a somewhat greater N cycling at grazed than at non-grazed sites. We conclude that the experimental levels of grazing have limited impact on distribution and cycling of N and thus represent sustainable ecosystem management in terms of N dynamics in the long-term.     

Physiological parameters that affect the transfer of radiocaesium to ruminants

Recently there has been a renewed interest in biological scaling relationships between parameters, such as those between, for example, body mass, dry matter intake and biological half-times of radionuclides that are useful in predicting the transfer of radiocaesium to different animal species, particularly to wild animals. However, there is still a considerable unexplained variability in transfer coefficient estimates between individuals of the same species. This paper discusses the physiological parameters that affect the transfer of radiocaesium to ruminants, and it shows how a better understanding of these parameters may help to reduce the within-species variability in radiocaesium transfer coefficients. In light of the improved understanding during the past 10-15 years of the importance of source-dependent bioavailability on absorption of radiocaesium from the gastrointestinal tract, it is concluded that further studies are required on the effects of feed digestibility and physiological factors on absorption and endogenous faecal excretion of radiocaesium to better understand the variability in transfer coefficients.     

Application and comparison of two models to study effects of calcium sources in sheep

The aim of this research was to assess and compare two compartmental models by studying effects of different Ca sources on Ca and P metabolism of sheep. Brazilian male sheep (20) were fed a basal diet supplemented with different sources of Ca, being: limestone (L), alfalfa hay (AH), shell meal (OSM) and citrus pulp (CTP). After 21 days, each sheep was given, as a single dose via the right jugular vein, 7.4 MBq of radio-calcium (⁴⁵Ca) and 7.4 MBq of radio-phosphorus (³²P). Calcium and P metabolism were evaluated by comparing the Vitti–Dias (VD) and Fernández–Lopes (FL) models [Dias, R.S., Kebreab, E., Vitti, D.M.S.S., Roque, A.P., Bueno, I.C.S., France, J., 2006. A revised model for studying phosphorus and calcium kinetics in growing sheep. J. Anim. Sci. 84, 2787–2794; Lopes, J.B., Vitti, D.M.S.S., Abdalla, A.L., Haddad, M.L., Figueredo, A.V., Moraes, R.C.B., 2001. Modelo do fluxo biológico do fósforo de fontes de fosfato em suínos, usando o ³²P como marcador. Rev. Bras. Zoot. 30, 165–173], by contrasting flows between gut and plasma, plasma and bone, and plasma and tissue. There were no differences in Ca and P intakes for the treatments. Ca flows from tissue and bone to plasma and vice versa were similar among treatments, though net bone and net tissue Ca retentions were higher for treatments L and OSM and lower, as well as negative, for AH and CTP (P<0.05). Net bone Ca results were consistent between the VD and FL models, although net tissue Ca retention was slightly higher for the VD. The presence of pectin in CTP and oxalate in AH could have affected Ca balance on these treatments. Total Ca absorption was higher (P<0.05) for L with both models. The chemical form of Ca in the different sources affected its metabolism, but did not affect P metabolism. Both models had higher P resorption than P absorption in bone, suggesting that the sheep were mobilizing P. It could be inferred that impaired digestion induced P mobilization from bone to supply P for metabolic needs. Both the VD and FL models had the same pattern for the P flows, and for net bone and tissue P retentions. Both models can be used to assess Ca and P kinetics in ruminants, and both suggest that our sheep tended to be deficient in Ca and P as well as that the inorganic sources of Ca were better utilized.     

Is most glucose absorbed passively in northern bobwhite?

Two hypotheses about gut function have not received general support from studies in birds. Both make assumptions about how natural selection has influenced rates of nutrient uptake. The adaptive modulation hypothesis states that rates of absorption should vary within individuals to accommodate changes in nutrient availability of the diet. The spare capacity hypothesis states that the gut's ability to absorb nutrients should slightly exceed load determined by the organism's food intake. We focus on a recent rejection of these hypotheses in northern bobwhite quail (W. Karasov, personal communication) and demonstrate that a central assumption—that carrier-mediated transport predominates—is not supported. We use a pharmacokinetic technique to show that 52–92% (depending on assumptions of metabolizability and binding) of ingested l-glucose appears in plasma. Because l-glucose is not actively transported, its appearance in plasma must be due to passive absorption. This result suggests that previous studies in birds found uptake capacity to be much less than the observed load because they failed to consider passive absorption. When both passive and carrier-mediated transport are considered, capacity and load are fairly closely matched in quail. Our results also suggest that modulation of carrier-mediated transport may not be selected for in birds, because modulation via passive absorption is faster and requires less energy. An unexplored negative consequence of passive absorption, however, may be nonselective absorption of secondary compounds and toxins.     

Ram spermatozoa cocultured with epithelial cell monolayers: An in vitro model for the study of capacitation and the acrosome reaction

The effects of different epithelial cells, namely, hamster oviduct, sheep oviduct, and pig kidney epithelial cells (IBRS-2), on the viability, percentage of progressive motility (PPM), and acrosome reactions of ejaculated ram spermatozoa were investigated. Sperm aliquots were cultured on cells, cell-conditioned medium 199, or control medium 199. The PPM of unattached spermatozoa was estimated after 0, 3, 6, 9, 12, and 24 hr of incubation at 37°C under 5% CO₂ in air. Viability and the occurrence of true acrosome reactions were assessed using a triple-stain technique. Spermatozoa started to attach within 1 hr of coculture with the hamster or sheep oviductal epithelial cell (OEC) monolayers, and these spermatozoa showed vigorous tail motion. No spermatozoa were found to attach to the IBRS-2 monolayer. The PPM of unattached spermatozoa cocultured with the various types of epithelial cell monolayers for 12 hr was significantly higher than that of spermatozoa incubated in conditioned media or medium 199 alone (54% in hamster OEC vs. 40% in conditioned; 68% in sheep OEC vs. 38% in conditioned; 36% in control medium). On the other hand, after 24 hr of incubation, there were no differences in the PPM of spermatozoa cocultured with epithelial cells or incubated in conditioned media. The percentages of cells undergoing a true acrosome reaction reached maximum values (P < 0.05) in spermatozoa incubated for 9 hr in the presence of hamster OEC (22.5%) or for 12 hr on sheep OEC (20.5%) monolayers. IBRS-2, a commercial nonreproductive cell type, had a positive influence on both PPM and sperm viability but no effect on the occurrence of the acrosome reaction. Interactions leading to the acrosome reaction were thus observed only when spermatozoa were cocultured with OEC monolayers. The values of PPM in unattached sperm cells seen after 12 hr of coculture with OEC or IBRS-2 were still at a high level (52–67%) for in vitro fertilization. The coculture with OECs provides an “in vitro” model to study the capacitation processes in a situation that may resemble that occurring in vivo. Moreover, the coculture with hamster OECs may provide a convenient and standardized in vitro system to study mechanisms underlying capacitation and the acrosome reaction. © 1993 Wiley-Liss, Inc.