Showing papers by "University of Alberta published in 1988"
TL;DR: This article investigated the relationship between management ownership and market valuation of the firm, as measured by Tobin's Q. In a 1980 cross-section of 371 Fortune 500 firms, they found evidence of a significant nonmonotonic relationship.
7,523 citations
TL;DR: This paper discusses measurement of the local thickness t of a transmission microscope specimen from the log-ratio formula t = lambda ln (It/I0) where It and I0 are the total and zero-loss areas under the electron-energy loss spectrum.
Abstract: We discuss measurement of the local thickness t of a transmission microscope specimen from the log-ratio formula t = lambda ln (It/I0) where It and I0 are the total and zero-loss areas under the electron-energy loss spectrum. We have measured the total inelastic mean free path lambda in 11 materials of varying atomic number Z and have parameterized the results in the form lambda = 106F (E0/Em)/ln (2 beta E0/Em) where F = (1 + E0/1,022)/(1 + E0/511)2, the incident energy E0 is in keV, the spectrum collection semiangle beta is in mrad, and Em = 7.6Z0.36. This formulation should allow absolute thickness to be determined to an accuracy of +/- 20% in most inorganic specimens.
757 citations
TL;DR: It is shown that anti-bodies directed against synthetic peptides and fusion proteins derived from the N-terminal region of human DMD cDNA strongly react with an antigen present in skeletal muscle sar-colemma on cryostat sections of normal human muscle biopsies and concludes that the product of the DMD gene is associated with the sar colemma rather than with the triads.
Abstract: Duchenne muscular dystrophy (DMD) and its milder form, Becker muscular dystrophy (BMD), are allelic X-linked muscle disorders in man. The gene responsible for the disease has been cloned from knowledge of its map location at band Xp21 on the short arm of the X chromosome. The product of the DMD gene, a protein of relative molecular mass 400,000 (Mr 400K) recently named dystrophin, has been reported to co-purify with triads of mouse and rabbit skeletal muscle when assayed using polyclonal antibodies raised against fusion proteins encoded by regions of mouse DMD complementary DNA. Here we show that antibodies directed against synthetic peptides and fusion proteins derived from the N-terminal region of human DMD cDNA strongly react with an antigen present in skeletal muscle sarcolemma on cryostat sections of normal human muscle biopsies. This immunoreactivity is reduced or absent in muscle fibres from DMD patients but appears normal in muscle fibres from patients with other myopathic diseases. The same antibodies specifically react with a 400K protein in sodium dodecyl sulphate (SDS) extracts of normal human muscle subjected to Western blot analysis. We conclude that the product of the DMD gene is associated with the sarcolemma rather than with the triads and speculate that it strengthens the sarcolemma by anchoring elements of the internal cytoskeleton to the surface membrane.
628 citations
TL;DR: Results support an earlier report that thigmotaxis may be a useful test for anxiolytic activity in rats and show some degree of drug-class specificity.
Abstract: It has been suggested that "phylogenetically prepared fear reactions" may be useful behavioral assays of the effects of anxiolytic agents. In the present experiments, rats' natural proclivity to stay near the perimeters of a novel environment (i.e., thigmotaxis) was suppressed by anxiolytic agents (diazepam 1-5 mg/kg; chlordiazepoxide 1-10 mg/kg; pentobarbital 1-10 mg/kg), with a relative potency that was similar to their relative potency in the treatment of human anxiety. Furthermore, when effects on general activity were factored out using analysis of covariance, the test also showed some degree of drug-class specificity, since neither d-amphetamine, morphine, nor chlorpromazine produced this anti-thigmotaxic effect. These results support an earlier report that thigmotaxis may be a useful test for anxiolytic activity in rats.
621 citations
TL;DR: It is suggested that hepatic secretion of VLDL, but not HDL, requires active phosphatidylcholine biosynthesis, and the inhibitory effect of choline deficiency on V LDL secretion can be compensated by the methylation of phosphatodylethanolamine.
581 citations
TL;DR: The particular track followed by an organization will be a function of the degree of alignment or compatibility between structures and contingency constraints, the pattern of commitment to prevailing and alternative interpretive schemes and the incidence of interest dissatisfaction of powerful groups.
Abstract: Change and stability in organizations is to be understood through the twin concepts of design archetypes and tracks. Organizations operate with structural designs which are given meaning and coherence by underlying interpretive schemes. Particular interpre tive schemes coupled with associated structural arrangements constitute a design archetype. The temporal relationship between an organization and one or more archetypes defines an organization's track. Prototypical tracks include inertia, aborted excursions, re-orientations and unresolved excursions. The particular track followed by an organization will be a function of the degree of alignment or compatibility between structures and contingency constraints, the pattern of commitment to prevailing and alternative interpretive schemes and the incidence of interest dissatisfaction of powerful groups.
562 citations
TL;DR: The alanine detection limit corresponds to fewer than 6000 molecules injected onto the column and represents an improvement of four orders of magnitude in the state of the art for fluorescent detection of amino acids and an improvement for the detection limit for isothiocyanate derivatives of amino amino acids.
Abstract: Subattomole analysis of fluorescein isothiocyanate (FITC) derivatives of amino acids is accomplished by combining capillary zone electrophoresis for high-efficiency separation with laser-induced fluorescence for high-sensitivity detection. Concentration detection limits range from 5 x 10(-12) molar for alanine to 9 x 10(-11) molar for lysine, injected in the column; 9 x 10(-21) mole of alanine is contained within the approximately 1-nanoliter injection volume at the detection limit. The alanine detection limit corresponds to fewer than 6000 molecules injected onto the column and represents an improvement of four orders of magnitude in the state of the art for fluorescent detection of amino acids and an improvement of six orders of magnitude in the state of the art for the detection limit for isothiocyanate derivatives of amino acids.
470 citations
TL;DR: Nimodipine treatment in poor-grade patients with SAH results in an increase in the number of good outcomes and a reduction in the incidence of delayed neurological deterioration due to vasospasm.
Abstract: A multicenter, randomized placebo-controlled double-blind trial of nimodipine in poor-grade aneurysm patients was carried out in 17 Canadian hospitals Of 188 patients enrolled in the trial, 32 were excluded for protocol violations and two were excluded due to statistical considerations, leaving 154 patients for valid outcome analysis Nimodipine treatment was associated with a significantly better outcome (p less than 0001): 21 (292%) of 72 nimodipine-treated patients had a good outcome at 3 months after subarachnoid hemorrhage (SAH) compared to eight (98%) of 82 placebo-treated patients Delayed ischemic deficits from vasospasm alone were significantly less frequent in the nimodipine group (p less than 005) with permanent deficits occurring in five nimodipine-treated patients (69%) and in 22 placebo-treated patients (268%) Improvement in the good outcome rate and reduction in delayed ischemic deficits from vasospasm alone occurred in both Grade 3 and 4 patients, with no difference between nimodipine- and placebo-treated patients being found in Grade 5 patients Repeat angiography after Day 4 was carried out in 124 patients There was no significant difference in the incidence of moderate or severe diffuse spasm, which was seen in 643% of nimodipine-treated patients and 662% of placebo-treated patients The authors conclude that nimodipine treatment in poor-grade patients with SAH results in an increase in the number of good outcomes and a reduction in the incidence of delayed neurological deterioration due to vasospasm This effect occurs by a mechanism other than prevention of large-vessel spasm as visualized on angiography
465 citations
TL;DR: In this paper, the authors show that the generic strategies are not mutually exclusive and that each strategy may be linked to a variety of strategic means, and the implications that these results have for structuring organizations are discussed.
Abstract: According to Porter, cost leadership and product differentiation can be pursued simultaneously only under rare conditions: It is also unclear how these strategies can be implemented. In this article Porter's generic strategies are linked to external preconditions. This approach shows that the generic strategies are not mutually exclusive and that each strategy may be linked to a variety of strategic means. The implications that these results have for structuring organizations are discussed.
438 citations
TL;DR: A brief review of past research on leisure constraints, paying particular attention to conceptual and analytical issues and to the practical applications of investigating constraints on leisure behavior, is provided in this article.
Abstract: This paper provides a brief review of past research on leisure constraints, paying particular attention to conceptual and analytical issues and to the practical applications of investigating constraints on leisure behavior. Specific findings are not reviewed; the intention is to clarify concepts by focusing upon broadly‐defined methodological aspects of past research. These include the nature and role of constraints on leisure participation, the distinction between barriers and reasons (e.g., for non‐participation), and empirical and conceptual methods of classifying constraints. It is suggested that future investigations of leisure constraints will give rise to findings that can be generalized across time, space, and social groups only if they are based upon clearly‐defined and standardized concepts.
TL;DR: Infarct-related major complications were less frequent in the NG than the control groups: infarct expansion syndrome and cardiogenic shock, and mortality was less in NG than in control groups in-hospital.
Abstract: To determine 1) whether the effect of intravenous nitroglycerin (NG) therapy during acute myocardial infarction on creatine kinase infarct size is influenced by infarct location (anterior vs. inferior), timing (therapy less than 4 hours vs. greater than or equal to 4 hours after onset of pain), and dose response (mean blood pressure greater than or equal to 80 mm Hg vs. less than 80 mm Hg during the first 12 hours) and 2) whether NG therapy modifies infarct expansion, 310 patients were randomly allocated to NG (n = 154) and control (n = 156) groups. NG infusion was titrated to lower mean blood pressure by 10% in normotensive and 30% in hypertensive patients, but not below 80 mm Hg, and was maintained for 39 hours. Measurements included clinical variables, creatine kinase infarct size (geq) as well as left ventricular (LV) asynergy, LV ejection fraction, expansion index, and thinning ratio on serial two-dimensional echocardiography. Compared with controls, creatine kinase infarct size was less in the NG group (41 vs. 55 geq, p less than 0.001), in anterior (44 vs. 58 geq, p less than 0.05), and inferior (39 vs. 53 geq, p less than 0.025) NG subgroups, and in early than late NG subgroups (43% vs. 22% decrease). Other indexes of infarct size also improved (p less than or equal to 0.05) with NG compared with controls. Thus, by 10 days, LV asynergy was 40% less, LV ejection fraction was 22% more, and Killip class score was 41% less. A negative effect of mean blood pressure less than 80 mm Hg with NG was reflected in these indexes. In addition, expansion index increased (p less than 0.001) by 31% and thinning ratio decreased (p less than 0.001) by 17% in controls by 10 days but remained unchanged with NG. Infarct-related major complications were less frequent in the NG than the control groups: infarct expansion syndrome (2% vs. 15%, p less than 0.0005), LV thrombus (5% vs. 22%, p less than 0.0005), cardiogenic shock (5% vs. 15%, p less than 0.005), and infarct extension (11% vs. 22%, p less than 0.025). Mortality was less in NG than in control groups in-hospital (14% vs. 26%, p less than 0.01), at 3 months (16% vs. 28%, p less than 0.025) and 12 months (21% vs. 31%, p less than 0.05), but this advantage was only found in the anterior subgroups.(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: Although cubs in western Hudson Bay were weaned at a younger age and a lighter weight than their counterparts in more northern populations, cub mortality rates were similar and it is speculated that sea-ice conditions may be sufficiently different to allow weaned bears at a lighter body weight to hunt seals more successfully there than further north.
Abstract: Data on age-specific natality rates, litter size, interbirth interval, age of first reproduction, reproductive senescence, age of weaning and cub survival were determined for a free-ranging population of polar bears inhabiting Hudson Bay, Canada, near the southern limit of the species range. Serum progesterone levels were also determined for females at different stages of their reproductive cycle to provide corroborative support for the reproductive parameters described. Animals were live captured using immobilizing drugs and each animal uniquely marked for future identification. First parturition occurred at four or five years of age and the age-specific natality rate increased with age until approximately 20 years, after which it dropped markedly. At least 40% of adult females displayed two-year interbirth intervals and 55% of cubs in their second year were independent of their mother. Mean size of cub litters in spring was 1.9 and 13% of litters had three or more cubs. The natality rate for 5–20-year-old females was estimated as 0.9, higher than that reported for any more northerly polar bear populations where two-year interbirth intervals are rare, fewer than 5% of yearling cubs are weaned and triplet litters occur with less than 1% frequency. Cub mortality was initially high and declined with age. Although cubs in western Hudson Bay were weaned at a younger age and a lighter weight than their counterparts in more northern populations, cub mortality rates were similar. The reason for the marked differences in reproductive parameters in the western Hudson Bay population is not known. We speculate that sea-ice conditions may be sufficiently different to allow weaned bears at a lighter body weight to hunt seals more successfully there than further north.
TL;DR: It is concluded that the insulin resistance of pregnancy is associated with a postbinding defect in insulin action, probably related to increasing amounts of progesterone, cortisol, PRL, and placental lactogen.
Abstract: Pregnancy is associated with insulin resistance. We studied insulin binding and postbinding function in isolated adipocytes from pregnant and nonpregnant rats. We also used a primary culture system for female virgin rat adipocytes to assess the effects of gestational hormones in vitro on insulin binding and postbinding function. Insulin binding to adipocytes was normal during pregnancy, but [14C]3-O-methylglucose transport was reduced. When hCG or estradiol was added to the culture medium, no change in maximum [14C]3-O-methylglucose transport was found; however, maximum insulin binding was increased with estradiol. Progesterone and cortisol both decreased maximum insulin binding and [14C]3-O-methylglucose transport. PRL and placental lactogen decreased maximum [14C]3-O-methylglucose transport, but did not change insulin binding. When these hormones were added concurrently no change in insulin binding was found, but maximum [14C]3-O-methylglucose transport was reduced. We conclude that the insulin resistance of pregnancy is associated with a postbinding defect in insulin action. Estradiol increased insulin receptor binding, but during pregnancy this effect may be offset by the reduction in insulin binding induced by progesterone and cortisol. The postbinding defect in insulin action during pregnancy is probably related to increasing amounts of progesterone, cortisol, PRL, and placental lactogen.
TL;DR: The most common lifetime diagnosis was alcohol abuse/dependence, followed by phobia and major depressive episode, and men were more likely to have had substance use disorders and antisocial personality disorder and women more likelyTo have had major depressiveepisode, dysthymia, agoraphobia and simple phobia.
Abstract: 3,258 randomly selected adult household residents of Edmonton were interviewed by trained lay interviewers using the Diagnostic Interview Schedule (DIS). Lifetime prevalence for 16 DIS/DSM III diagnoses are given. Overall 33.8% of the population had one or more diagnoses and, excluding substance use disorders, one fifth of the population had a diagnosis. The most common lifetime diagnosis was alcohol abuse/dependence, followed by phobia and major depressive episode. Men were more likely to have had substance use disorders and antisocial personality disorder and women more likely to have had major depressive episode, dysthymia, agoraphobia and simple phobia. Those who were married had generally lower lifetime prevalences. Those over age 65 had the lowest prevalence of any age groups.
TL;DR: Individuals of the morphologically variable, rocky intertidal gastropod Thais (or Nucella) lamellosa developed larger apertural teeth when held in the presence of the predatory crab Cancer productus than whenheld in its absence, regardless of whether snails were fed or not.
Abstract: Individuals of the morphologically variable, rocky intertidal gastropod Thais (or Nucella) lamellosa developed larger apertural teeth when held in the presence of the predatory crab Cancer productus than when held in its absence, regardless of whether snails were fed or not. In addition, among fed snails larger apertural teeth were produced in the presence of crabs fed conspecific snails than in the presence of crabs fed frozen fish. Because all snails were held in containers through which water flowed from physically separated aquaria holding the crabs, these results indicate that water-soluble chemical cues released by this predatory crab and by damaged conspecifics induced T. lamellosa to improve the defense effectiveness of their shells. Finally, when allowed access to food, snails exposed to these stimuli ate fewer barnacles and grew less than those in the controls.
TL;DR: All photographs courtesy of AFP, EPA, Getty Images and Reuters.
TL;DR: Evidence suggests that even short-latency, largely monosynaptic reflexes show a high degree of modulation during simple human motor activities such as walking and standing, and that the pattern of modulation can be specifically altered for the different functional requirements of each activity.
TL;DR: The crystal structure of troponin C from turkey skeletal muscle has been refined at 2.0 A resolution and polar side-chain interactions are similar to those observed in other proteins with one exception: negatively charged side-chains interact more frequently with main-chain carbonyl oxygen atoms.
TL;DR: Conformational changes in the active-site region indicate a small induced fit of enzyme and inhibitor, and some conformational differences are observed between equivalent active- site residues of subtilisin Carlsberg and alpha-chymotrypsin.
Abstract: The crystal structures of the molecular complexes between two serine proteinases and two of their protein inhibitors have been determined: subtilisin Carlsberg with the recombinant form of eglin-c from the leech Hirudo medicinalis and subtilisin Novo with chymotrypsin inhibitor 2 from barley seeds. The structures have been fully refined by restrained-parameter least-squares methods to crystallographic R factors (sigma[[Fo[ - [Fc[[/sigma[Fo[) of 0.136 at 1.8-A resolution and 0.154 at 2.1-A resolution, respectively. The 274 equivalent alpha-carbon atoms of the enzymes superpose with an rms deviation of 0.53 A. Sequence changes between the enzymes result in localized structural adjustments. Functional groups in the active sites superpose with an rms deviation of 0.19 A for 161 equivalent atoms; this close similarity in the conformation of active-site residues provides no obvious reason for known differences in catalytic activity between Carlsberg and Novo. Conformational changes in the active-site region indicate a small induced fit of enzyme and inhibitor. Some conformational differences are observed between equivalent active-site residues of subtilisin Carlsberg and alpha-chymotrypsin. Despite differences in tertiary architecture, most enzyme-substrate (inhibitor) interactions are maintained. Subtilisin Carlsberg has a rare cis-peptide bond preceding Thr211 (Gly211 in Novo). Both enzymes contain tightly bound Ca2+ ions. Site 1 is heptacoordinate with the oxygen atoms at the vertices of a pentagonal bipyramid. Site 2 in Carlsberg is probably occupied by a K+ ion in Novo. Conserved water molecules appear to play important structural roles in the enzyme interior, in the inhibitor beta-sheet, and at the enzyme-inhibitor interface. The 62 equivalent alpha-carbon atoms of the inhibitors superpose with an rms deviation of 1.68 A. Sequence changes result in somewhat different packing of the alpha-helix, beta-sheet, and reactive-site loop relative to each other. Hydrogen bonds and electrostatic interactions supporting the conformation of the reactive-site loop are conserved. The 24 main-chain plus C beta atoms of P4 to P1' overlap with an rms deviation of 0.19 A. Features contributing to the inhibitory nature of eglin-c and CI-2 are discussed.
TL;DR: In this article, the uniqueness of limit cycles in predator-prey systems of Gause type has been studied and the results of Cheng, Liou and Cheng were derived for a generalized Lienard system.
Abstract: This paper deals with the question of uniqueness of limit cycles in predator-prey systems of Gause type. By utilizing several transformations, these systems are reduced to a generalized Lienard system as discussed by Cherkas and Zhilevich and by Zhang. As a consequence, criteria for the uniqueness of limit cycles are derived, which include results of Cheng and is related to results in Liou and Cheng. Several examples are given to illustrate our results.
TL;DR: It is determined that waterborne prostaglandins function as potent olfactory stimulants for mature male goldfish and that hormones and their metabolites may commonly serve as reproductive pheromones in fish.
Abstract: This study establishes that ovulated female goldfish release F type prostaglandins (PGFs) to the water where they stimulate male spawning behavior and comprise the goldfish postovulatory pheromone. We first demo nstrated that ovulated and prostaglandin-injected female goldfish release immunoreactive PGFs to the water. Next, using electro-olfactogram recording (EOG), we determined that waterborne prostaglandins function as potent olfactory stimulantsfor mature male goldfish.ProstaglandinF2a (PGF2�) and its metabolite 15-keto
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TL;DR: Antibodies that are specific for DNA provide an excellent system for studying the protein‐nucleic acid interactions that allow proteins to recognize specific DNA structures or sequences.
Abstract: Antibodies that are specific for DNA provide an excellent system for studying the protein-nucleic acid interactions that allow proteins to recognize specific DNA structures or sequences.
TL;DR: A comparison of the refined crystal structures of dimeric glycogen phosphorylase b and a reveals structural changes that represent the first step in the activation of the enzyme.
Abstract: A comparison of the refined crystal structures of dimeric glycogen phosphorylase b and a reveals structural changes that represent the first step in the activation of the enzyme. On phosphorylation of serine-14, the N-terminus of each subunit assumes an ordered helical conformation and binds to the surface of the dimer. The consequent structural changes at the N- and C-terminal regions lead to strengthened interactions between subunits and alter the binding sites for allosteric effectors and substrates.
TL;DR: It is demonstrated that Etomoxir increases functional recovery of fatty acid perfused ischemic hearts, unrelated to changes in levels of long chain acylcamitines but may be due to increased glucose use by the reperfused heart, resulting in decreased oxygen consumption per unit work.
Abstract: Fatty acids are known to increase the severity of injury during acute myocardial ischemia. In this study, we determined the effects of a carnitine palmitoyltransferase I inhibitor, ethyl 2-[6-(4-chlorophenoxy)hexyl]oxirane-2-carboxylate (Etomoxir) on reperfusion recovery of fatty acid perfused hearts. Following a 25-minute period of global ischemia, isolated working hearts reperfused with 1.2 mM palmitate, 11 mM glucose exhibited depressed function compared to hearts perfused with 11 mM glucose alone. A low dose of Etomoxir (10(-9) M) decreased long chain acylcarnitine and long chain acyl-coenzyme A (CoA) levels but did not prevent depressed function. In contrast, a high dose of Etomoxir (10(-6) M) prevented the palmitate-induced depression of function but did not decrease myocardial long chain acylcarnitine or long chain acyl-CoA levels. At this high dose of Etomoxir, oxygen consumption per unit work was decreased during reperfusion recovery, and ATP and creatine-phosphate levels were significantly higher after reperfusion. In aerobic hearts not subjected to ischemia, Etomoxir (10(-6) M) increased glucose oxidation both in the presence and absence of palmitate, while 10(-9) M Etomoxir had no effect. In these aerobic hearts, only the low dose of Etomoxir decreased long chain acylcarnitine and long chain acyl-CoA levels. These data demonstrate that Etomoxir (10(-6) M) increases functional recovery of fatty acid perfused ischemic hearts. This protection is unrelated to changes in levels of long chain acylcarnitines but may be due to increased glucose use by the reperfused heart, resulting in decreased oxygen consumption per unit work.
TL;DR: A general method is described for the assay of glycosyltransferase activity, which makes use of synthetic glycoside acceptors attached to hydrophobic aglycones, which can be rapidly separated from interfering radioactivity by adsorption on to reverse-phase C-18 cartridges.
Abstract: A general method is described for the assay of glycosyltransferase activity, which makes use of synthetic glycoside acceptors attached to hydrophobic aglycones. The products formed by incubation of an enzyme with acceptor and radiolabelled sugarnucleotide can then be rapidly (one minute) separated from interfering radioactivity by adsorption on to reverse-phase C-18 cartridges. After aqueous washing, products are easily isolated by elution with methanol. The utility of the method for the assay of β(1–4)galactosyltransferase, α(1–2)fucosyltransferase andN-acetylglucosaminyltransferase I and V is demonstrated.
TL;DR: Even though the three species of primates in Santa Rosa National Park, Costa Rica were very flexible in terms of diet and range use, the behavioural variability did not correspond to changes in food availability or season.
Abstract: This paper describes the diet and range use patterns of the three species of primates in Santa Rosa National Park, Costa Rica (Ateles geoffroyi, Alouatta palliata, andCebus capucinus) and examines the variation in these variables as they relate to seasonal changes and concomitant changes in food availability. These three primate species were studied over a four-year period for a total of 24 months in the field. Santa Rosa National Park is in an area that experiences a long severe dry season in which little if any rain falls and the majority of the non-riparian trees lose their leaves. However, even though the three species were very flexible in terms of diet and range use, the behavioural variability did not correspond to changes in food availability or season.
TL;DR: Although some refinement of dosages of drugs is required, the Linpe method has proven to be a highly successful procedure for induced ovulation and/or spawning of cultured freshwater fish in China.
TL;DR: It is suggested that the decrease in prostaglandin E2, thromboxane and prostacyclin levels generally observed after fish-oil consumption may be at least partly due to inhibition of C20:4, omega 6-fatty-acid synthesis from C18:2, Omega 6 fatty acid; and consumption of fish oil prevents the further decrease by dietary cholesterol that is apparent when cholesterol is fed in combination with diets high in saturated fat.
Abstract: The effect of feeding semipurified diets enriched in linseed (rich in C18:3, omega 3 fatty acid) or fish (rich in C20:5, omega 3 and C22:6, omega 3 fatty acid) oil with and without cholesterol supplementation on the desaturation of linoleic acid (C18:2, omega 6) by rat liver microsomal fractions was investigated. Animals fed diets supplemented with beef tallow were used as equal-energy controls. Both linseed-oil and fish-oil diets, without added cholesterol, decrease conversion of C18:2, omega 6 fatty acid to gamma-linolenic acid (C18:3, omega 6). Reduction in delta 6-desaturation was significantly greater for animals fed the diet containing fish oil than with animals fed the linseed-oil diet. The major effect of cholesterol supplementation was to decrease the rate of desaturation of C18:2, omega 6, when fed in combination with the beef-tallow diet, whereas delta 6-desaturation was unaffected when cholesterol was fed along with diets high in omega 3 fatty acids (linseed oil or fish oil). The activity of the delta 6-desaturase in vitro is consistent with the fatty acid composition observed for the microsomal membranes on which this enzyme is localized. Dietary linseed oil and fish oil lowered the arachidonic (C20:4, omega 6) acid content of rat liver microsomes, with an accompanying increase in membrane eicosapentaenoic (C20:5, omega 3) and docosahexaenoic (C22:6, omega 3) acid content, in comparison with the group fed beef tallow. Inclusion of cholesterol into the beef-tallow or linseed-oil diets resulted in decreased membrane C20:4, omega 6-fatty-acid content, with concomitant increase in C18:2, omega 6-fatty-acid content. However, addition of cholesterol to the fish-oil diet did not alter the microsomal membrane content of C20:4, omega 6 fatty acid. Thus it is suggested that (1) the decrease in prostaglandin E2, thromboxane and prostacyclin levels generally observed after fish-oil consumption may be at least partly due to inhibition of C20:4, omega 6-fatty-acid synthesis from C18:2, omega 6 fatty acid; and (2) consumption of fish oil prevents the further decrease in C20:4, omega 6-fatty-acid levels by dietary cholesterol that is apparent when cholesterol is fed in combination with diets high in saturated fat or C18:3, omega 3 fatty acid.