Persistent Inhibitory Effect Research Articles

Luteinizing hormone and growth hormone secretion in ewes infused intracerebroventricularly with neuropeptide Y

Neuropeptide Y (NPY) provides an important hypothalamic link between nutritional status and neuroendocrine mechanisms regulating growth and reproduction. The objective of the following series of experiments was to determine the effects of single or continuous administration of NPY on secretion of luteinizing hormone (LH) and (or) growth hormone (GH). In experiment 1, four ovariectomized (OVX) ewes and four OVX+estrogen-treated ewes each received, in a 4×4 Latin Square arrangement of treatments, a single injection of 0, 0.5, 5, or 50 μg NPY via an intracerebroventricular (i.c.v.) cannulae to determine the effects on secretion of GH. NPY significantly elevated serum GH at the 50 μg dose regardless of estrogen exposure ( P=0.003). In experiment 2, eight OVX ewes were infused i.c.v. with NPY or saline ( n=4/trmt) continuously for 20 h in a linearly increasing dose, ending at 50 μg/h NPY. Blood samples were collected via jugular cannulae every 10 min during hour –4–0 (interval 1, pre-treatment), hour 6–10 (interval 2) and hour 16–20 (interval 3) relative to the initiation of infusion (0 h). Mean LH and LH pulse frequency were lower in NPY- versus saline-infused ewes during intervals 2 and 3 ( P<0.01), but NPY had no discernable effect on serum GH ( P>0.10). In experiment 3, four OVX ewes were continuously infused with NPY as in experiment 2, except that the maximum 50 μg/h dose was achieved after only 10 h of infusion. Blood samples were collected every 10 min, beginning 4 h before and continuing until 4 h after the NPY infusion. Mean serum LH changed significantly over time ( P=0.0001), decreasing below pre-treatment levels by hour 3 of NPY infusion ( P<0.01), and returning to pre-treatment concentrations following the end of infusion ( P>0.15). Serum GH also changed significantly over time ( P<0.001). Mean GH levels tended to be greater than pre-treatment levels by hour 2 of infusion ( P<0.08), but thereafter returned to basal levels. Serum GH also increased following the end of NPY infusion ( P<0.03). From these data we conclude that NPY exerts a persistent inhibitory effect on secretion of LH, and may stimulate the secretion of GH during the initiation and cessation of infusion of NPY. These observations support a role for NPY in mediating the effects of undernutrition on both LH and GH, and also provide evidence for potential mechanisms by which leptin, acting through NPY, may stimulate the secretion of GH.

Prevention of LDL α-tocopherol consumption, cholesterol oxidation, and vascular endothelium dysfunction by polyphenolic compounds from red wine

Red wine polyphenolic compounds (RWPCs) have been demonstrated to possess antioxidant properties, and several studies have suggested that they might constitute a relevant dietary factor in the protection from coronary heart disease. The aim of the present study was to determine further the mechanism by which RWPCs can prevent the formation of vasoactive compounds in oxidized LDL. RWPCs were obtained from the Cabernet-Sauvignon grape variety. Human LDL was oxidized in the presence of CuSO 4 (ox-LDL). Vascular reactivity studies were conducted on rabbit aortic rings. RWPCs significantly reduced the formation of 7β-hydroxycholesterol and 7-ketocholesterol and in a lower extent the emergence of lysophosphatidylcholine in ox-LDL. The ability of RWPCs to prevent cholesterol oxide formation was directly dependent on the LDL α-tocopherol content. Once the LDL α-tocopherol has been consumed, RWPCs were no longer effective, indicating that RWPCs act by sparing endogenous α-tocopherol. As a consequence of the preservation of the endogenous α-tocopherol content of LDL, RWPCs could prevent the inhibition of the acetylcholine-mediated endothelium-dependent relaxation of rabbit aorta which was linked to a direct effect on NO release. Independently of a treatment with ox-LDL, RWPC exerted a concentration-dependent and persistent inhibitory effect on the norepinephrine-induced contraction of rabbit aorta. In conclusion, RWPCs can preserve a normal vascular reactivity by acting at different stages of the cascade that leads to lipid oxidation, endothelium dysfunction and vasospasm.

Interleukins 2 and 12 Produce Recovery of Cytotoxic Function in Tributyltin-Exposed Human Natural Killer Cells

Cytotoxic function of human natural killer (NK) cells is modulated by a variety of cytokines. Interleukins (IL) 2 and 12 are both potent stimulators of NK cell cytotoxic function. Tributyltin (TBT) is used in a variety of consumer products and industrial applications. TBT is found in dairy products, meat, and fish. We and others have shown that there are measurable levels of TBT in human blood. Butyltins appear to increase the risk of cancer and viral infections in exposed individuals. We have demonstrated that the ability of NK cells to kill tumor cells is greatly diminished after a l-h exposure to TBT and that this inhibition persists even after removal of the compound. In the current study we examine the effects of the NK-stimulatory ILs, IL2 and IL12, on the ability of NK cells to recover from the persistent inhibitory effects of a 1-h TBT treatment. Highly purified NK cells (>95% CD16+) or a lymphocyte preparation containing both T lymphocytes and NK cells were treated with 300 nM TBT and then allowed to recover for 24 h, 48 h, 4 days, and 6 days in TBT-free media containing no interleukin, 1000 U/mL IL2, 20 ng/mL IL l2, or a combination of IL2 plus IL12. Tumor killing function was then tested using a radioactive chromium release assay. As seen in our previous studies there is no recovery of NK cell cytotoxic function even after a 6-day recovery period when no interleukin is present in the medium. However, there is significant recovery of NK cytotoxic function when IL2, IL12, or the combination of IL2 plus IL12 is present in the medium during the recovery period.

Effect of maternal antibodies on influenza virus-specific immune response elicited by inactivated virus and naked DNA.

While vaccines are effective in adults, they are less successful in newborns and infants. Neonatal unresponsiveness to vaccines could be owing to immaturity of lymphocytes and/or to inhibition by maternal antibodies. Unresponsiveness of newborn to vaccines can be overcame by genetic immunization. In the present study we investigated the effect of maternal antibodies on the anti-influenza virus protective response in progeny born to dams immunized with plasmid containing the hemagglutinin gene or UV-inactivated virus. The effect of maternal antibodies was studied in plasmid immunized F1 mice born to BALB/c dams, previously immunized with virus or plasmid and crossed with C57BL/6 males, as well as in offspring born to BALB/c dams immunized with plasmid and then immunized with UV-inactivated WSN virus. We have found that the inhibition period of the anti-HA antibody response in offspring born to dams immunized with DNA is shorter than that of offspring born to dams immunized with virus. Furthermore, there is a persistent inhibitory effect on B cells from offspring born to dams immunized with virus or injected with antiviral monoclonal antibodies (MoAb), after the decline of maternal antibody titers. The analysis of the haemagglutinin-specific clonotype reactivity pattern of offspring born to dams immunized with inactivated influenza virus or with a plasmid showed that clonotypes producing antibodies specific for the immunizing virus strain were predominant in offspring born to dams immunized with DNA compared to those born to dams immunized with virus. Maternal antibodies do not affect cell-mediated immunity. These findings might be used to design efficient vaccination schedules for newborns and infants.

PROGESTERONE INDUCED MODULATIONS OF SERUM HORMONAL PROFILES IN ADULT MALE AND FEMALE RATS

The impact of progesterone on serum hormonal profiles in the presence and absence of gonads was studied in adult male and female albino rats. Progesterone was administered intramuscularly for 30 days at a dose of 1 mg/100 g body weight/day. Serum testosterone, estradiol and prolactin titres decreased in male and female rats with intact gonads given progesterone. While the levels of both luteinizing hormone (LH) and follicle stimulating hormone (FSH) decreased in male rats with intact gonads, only FSH decreased in female rats. The inhibitory effect of progesterone on serum estradiol, LH, FSH and prolactin persisted even after gonadectomy in male rats. This persistent inhibitory effect of progesterone was also seen on serum testosterone, FSH and prolactin levels of female rats. Ovariectomy modified progesterone action on LH, as is evident from the decreased levels of LH observed only in ovariectomized rats given progesterone. While progesterone had no effect on serum T3 and T4 in male rats, gonadectomy altered the levels of T3 and T4 in male and female rats. Progesterone increased the levels of T3 and decreased the levels of T4 in ovariectomized rats. Growth hormone (GH) and thyroid stimulating hormone (TSH) levels seem to be resistant to changes in progesterone titre, irrespective of the sex and gonadal status. The present data suggest the existence of a sex specific effect of progesterone on gonadotrophins. The data on T3, T4 and TSH reveals that progesterone has no effect on the pituitary thyroid axis in the presence of gonads.

Candesartan Causes Long-lasting Antagonism of the Angiotensin II Receptor-mediated Contractile Effects in Isolated Vascular Preparations: a Comparison with Irbesartan, Losartan and its Active Metabolite (EXP-3174)

Candesartan is a new angiotensin II type 1 (AT 1 ) receptor blocker. It displays insurmountable antagonism of angiotensin II responses, binding tightly to and dissociating slowly from the AT 1 -receptor. The purpose of this study was to compare the duration of angiotensin II antagonism by the AT 1 -receptor blockers candesartan, irbesartan, losartan and its active metabolite EXP-3174 in an isolated tissue preparation. The contractile response to angiotensin II was studied in the isolated portal vein of the rat, during incubation with AT1-receptor blockers and after an extensive washout period. The portal vein preparation was pre-stretched to a passive force of 5 mN in an organ bath filled with oxygenated Krebs' buffer at 37°C. The contractile tension developed by the vascular smooth muscles was monitored using a force-displacement transducer. The contractile response to repeated administration of angiotensin II was recorded before, during and following exposure (for 30-180 min) to candesartan, 0.1-1 nmol/l, irbesartan, 1-50 nmol/l, losartan, 30-100 nmol/l, and EXP-3174, 1-10 nmol/l. Drug exposure was followed by washing for up to 2 h. Candesartan produced a long-lasting blockade of the vascular contractile response to angiotensin II, as shown by maintenance of inhibition during the washout period. This effect of candesartan was independent of drug concentration and exposure time prior to washing. Irbesartan, losartan and EXP-3174 also blocked the angiotensin II-mediated contraction. However, in contrast to candesartan, the responses to angiotensin II rapidly returned towards baseline values during the washout period. The relatively short-lasting blockade by irbesartan, losartan and EXP-3174 was also independent of drug concentration and exposure time prior to washout. It is concluded that the AT 1 -receptor blockers differ in their ability to inhibit angiotensin II-mediated vascular contraction, with candesartan producing longer-lasting blockade than irbesartan, losartan and EXP-3174. The mechanism of the persistent inhibitory effect of candesartan is at present unclear. Possible explanations include tight binding and slow dissociation from the AT 1 -receptor, tissue accumulation resulting in 'local( dissociation and reassociation to the AT 1 -receptor, and stimulation of internalization of the AT 1 -receptor.

Methane oxidation in arable soil as inhibited by ammonium, nitrite, and organic manure with respect to soil pH

The effects of inorganic N and organic manure, applied to a loamy arable soil, on CH4 oxidation were investigated in laboratory incubation experiments. Applications (40 mg N kg–1) of NH4Cl, (NH4)2SO4, and urea caused strong instantaneous inhibition of CH4 oxidation by 96%, 80%, and 84%, respectively. After nitrification of the added N the inhibitory effect was not fully reversible, resulting in an residual inhibition of 21%, 16%, and 25% in the NH4Cl, (NH4)2SO4, and urea treatments, respectively. With large NH4 + applications [240 mg N kg–1 as (NH4)2SO4] the residual inhibition was as high as 64%. Exogenous NO2 – (40 mg NO2 –-N kg–1) initially inhibited CH4 oxidation by 84%, decreasing to 41% after its oxidation. Therefore, applied NO2 – was a more effective inhibitor of CH4 consumption than NH4 +. Temporary accumulation of NO2 – during nitrification of added N was small (maximum: 1.9 mg NO2 –-N kg–1) and thus of minor importance with respect to the persistent inhibition after NH4 + or urea application. CH4 oxidation after NaNO3 (40 mg N kg–1) and NaCl addition did not differ to that of the untreated soil. The effect of organic manures on CH4 oxidation depended on their C/N ratio: fresh sugar beet leaves enhanced mineralization, which caused an instantaneous 20% inhibition, whereas after wheat straw application available soil N was rapidly immobilized and no effect on CH4 oxidation was found. The 28% increase in CH4 oxidation after biowaste compost application was not related to its C/N ratio and was probably the result of an inoculation with methanotrophic bacteria. Only with high NH4 + application rates (240 mg N kg–1) could the persistent inhibitory effect partly be attributed to a pH decrease during nitrification. The exact reason for the observed persistent inhibition after a single, moderate NH4 + or urea application is still unknown and merits further study.

Persistence of effects of nitric oxide synthase inhibitors: Comparisons on blood flow and plasma exudation in guinea pig skin

Plasma protein extravasation has been measured in guinea pig skin using 125 I -albumin and blood flow using 133 X enon ( 133 X e) clearance. The nitric oxide (NO) synthase inhibitors N G-nitro- l-arginine methyl ester ( l-NAME), N G-monomethyl- l-arginine (l -NMMA) and N G-nitro- l-arginine ( l-NOArg) and the α-adrenoceptor agonist, phenylephrine, inhibited bradykinin induced plasma protein extravasation when co-injected with the peptide. The inhibitory effects of l-NAME and l-NOArg lasted for up to 8 and 4 h, respectively, whereas phenylephrine and l-NMMA had no persistent inhibitory effects. When co-injected with 133 X e, l-NAME, l-NMMA, l-NOArg and phenylephrine, but not d-NAME, produced significant reductions in skin blood flow. When injected prior to 133 X e, l-NAME and l-NOArg, but not phenylephrine or l-NMMA, significantly reduced flow. The effect of l-NAME on flow was not significant at 8 h. Thus, although the inhibitory effects of the NO synthase inhibitors on mediator induced plasma protein extravasation show correlations with their effects on blood flow, the persistent effect of l-NAME on exudation appears to extend beyond its effect on flow.

Intense peripheral electrical stimulation evokes brief and persistent inhibition of the nociceptive tail withdrawal reflex in the rat

In a study of modulation of nociception by sensory inputs, electrical stimulation was applied to specific sites in the hindlimb and effects on the nociceptive tail withdrawal reflex were monitored in the lightly anaesthetized rat. Stimulation was applied to previously defined sites in the hindlimb, meridian points femur-futu (ST-32), fengshi (GB-31) and zusanli (ST-36). It consisted of a 4 Hz train of 2 ms square pulses given for 20 min at 20 × the threshold intensity required for muscle twitch. Tail withdrawal was provoked by application of a noxious heat stimulus applied to the tip of the tail. Results were expressed as a percentage of the maximal possible inhibition which is achieved when the post-treatment latency is 2 × the pre-treatment latency otherwise known as the cut off. During stimulation, the latency of the withdrawal increased to ≈ 70% of the maximal possible inhibition. Following stimulation, the inhibition persisted for > 1 h. Stimulation at 2 or 6 Hz elicited similar effects but stimulation at 8 Hz evoked inhibition during the stimulation only. Stimulation applied to sites away from defined meridian points inhibited tail withdrawal during the stimulation; no post-stimulation effect was produced. In acutely transected animals (≤ 48 h), stimulation of meridian points elicited a small, brief increase in latency but during stimulation only. At 7 and 14 days after spinal transection, this response during stimulation was greater in magnitude and a brief post-stimulation increase was also observed. The return of the of this latter effect was coincident with the return of bladder function. These data suggest that high intensity, low frequency electrical stimulation of hindlimb meridian points in the lightly anaesthetized rat produces both brief and persistent inhibitory effects on the nociceptive tail withdrawal reflex. These effects appear to be elicited by different mechanisms. The persistent effect may represent a plastic change in central inhibitory mechanisms. Data from spinal animals indicate a major participation of supraspinal structures but that spinal mechanisms are also capable of sustaining both types of effect.

Sleep-Induced Breathing Instability

We present a view of the neuromechanical regulation of breathing and causes of breathing instability during sleep. First, we would expect transient increases in upper airway resistance to be a major cause of transient hypopnea. This occurs in sleep because a hypotonic upper airway is more susceptible to narrowing and because the immediate excitatory increase in respiratory motor output in response to increased loads is absent in non-REM sleep. Secondly, sleep predisposes to an increased occurrence of ventilatory “overshoots”, in part because abruptly changing sleep states cause transient changes in upper airway resistance and in the gain of the respiratory controller. Following these ventilatory overshoots, breathing stability will be maintained if excitatory short-term potentiation is the prevailing influence. On the other hand, apnea and hypopnea will occur if inhibitory mechanisms dominate following the ventilatory overshoot. These inhibitory mechanisms include: a) hypocapnia—if transient, will inhibit carotid chemoreceptors and cause hypopnea, but if prolonged will inhibit medullary chemoreceptors and cause apnea; b) a persistent inhibitory effect from lung stretch; c) baroreceptor stimulation, from a transient rise in systemic blood pressure immediately following termination of apnea or hypopnea may partially suppress the accompanying hyperpnea; d) depression of central respiratory motor output via prolonged brain hypoxia. Once apneas are initiated, reinitiation of inspiration is delayed even though excitatory stimuli have risen well above their apneic thresholds, and these prolonged apneas are commonly accompanied by tonic EMG activation of expiratory muscles of the chest wall and upper airway.

Effects of dihydrotestosterone and hydroxyflutamide on androgen receptors in cultured human breast cancer cells (EVSA-T)

The purpose of our study was to evaluate the effects of 5α-dihydrotestosterone (DHT) and hydroxyflutamide (HF), alone or in combination, on androgen receptor (AR) dynamics and on cellular growth in cultured breast cancer cells (EVSA-T). The incubation of cells with DHT increased the concentration of nuclear AR after 24 and 48 h. HF was also able to promote the nuclear accumulation of AR after 24 and 48 h of treatment. When HF-treated cells are incubated with DHT, the nuclear AR concentration is lower than that of found in cells treated with DHT alone. We conclude that HF acts by increasing nuclear accumulation of receptor-antiandrogen complexes. Moreover, DHT stimulates cell growth while HF has an inhibitory effect. Thymidine incorporation in cells also increased after DHT treatment and decreased after HF incubation. The HF-induced inhibition of cell growth persisted both after renewal of the medium and after the addition of DHT to cultures. It may be hypothesized that either DHT is converted to inactive metabolites or that HF exerts a persistent inhibitory effect. In the latter case, the antiandrogen action of HF could be exerted by retention of high levels of antiandrogen in cells or by such a depressed protein synthesis that the renewal of growth is slower than the 48 h period studied.

The use of block copolymers to inhibit bacterial adhesion and biofilm formation on hydrophobic surfaces in marine habitats

Inhibition of adhesion of marine bacterial cultures to polystyrene surfaces treated with tri‐block copolymers of polyethylene oxide and polypropylene oxide ('Synperonic’ PE series) ranged from 21 to 100% depending on the properties of the copolymer and of the test organism. After 4 h submergence in natural marine habitats, copolymer treatment resulted in 70–100% reduction in colonisation. After 5d, the inhibitory activity was maintained in terms of colonisation by bacterial aggregates and eukaryotes, but single bacteria were present in equivalent numbers on both treated and untreated surfaces. After 2 months, no difference in biofouling between treated and untreated substrata could be detected. Reasons for the lack of a persistent inhibitory effect of the copolymer are considered.

Markers for progression to acquired immune deficiency syndrome and zidovudine treatment of asymptomatic patients

Eighteen asymptomatic men with persistent human immunodeficiency virus type I (HIV-I) p24 antigenaemia were treated with zidovudine 250-500 mg (+/- acyclovir 800 mg) 6-hourly for 4-12 weeks, and subsequently with zidovudine 500 mg (+/- acyclovir 1600 mg) 12-hourly for 36 weeks. After 24 weeks six additional HIV antigenaemic subjects were entered and treated directly with zidovudine 500 mg 12-hourly. Over the treatment period serum HIV-I p24 (HIV-Ag) levels declined in all 24 subjects; significantly so in 17, and to below cut-off values in five. Mean serum HIV-Ag levels in different treatment groups declined in 68-78%. Initial increases in CD4+ cell counts were not sustained. Over 48 weeks serum HIV-Ag levels rose in three out of five non-treated men with persistent HIV antigenaemia, and they slightly declined in two; the mean serum HIV-Ag level in this group rose 67%. Regression of enlarged lymph nodes was seen in 19 out of 19 of the zidovudine-treated subjects. In the 24 zidovudine-treated subjects no disease progression occurred during follow-up, whereas two out of five non-treated men went on to develop CDC group IV A, and IV C-2 disease, respectively. Adverse reactions to the study drugs were infrequent and mild. Anaemia caused symptoms in two, but serious leucopenia or neutropenia was not observed. An initial positive effect on thrombocyte numbers was not sustained. These data demonstrate that in asymptomatic HIV-infected subjects zidovudine 500 mg 12-hourly is well tolerated and has a persistent inhibitory effect on viral replication.

Effect of six triorganotin(IV) compounds on nitrification and ammonification in soil

AbstractThe effects of six triorganotin(IV) compounds and of Thiram on nitrification and ammonification in soil were investigated. Low concentrations of up to 50 μg g−1 of the triorganotin(IV) compounds enhanced nitrate‐nitrogen (NO3−‐N) production in soil. Except for diphenylbutyltin bromide, which inhibited nitrification at 250 μg g−1, the other triorganotin(IV) compounds were inhibitory at concentrations of 100 μg g−1 to less than 250 μg g−1. At 10 μg g−1, only triphenyltin acetate was less inhibitory towards nitrification compared with Thiram. At 250 μg g−1, Thiram exerted a strongly persistent inhibitory effect towards nitrification. The NO3−‐N level recorded 28 days after application was only 0.10 μg g−1 soil. With the triorganotin compounds NO3−‐N levels of 7.05–12.06 μg g−1 soil were recorded 28 days following their application. The deleterious effects of the triorganotin(IV) compounds were less persistent and recovery of nitrification was evident seven days after application. Low concentrations of Thiram and triorganotin(IV) compounds inhibited ammonification, whereas higher concentrations enhanced ammonification. Complete inhibition of ammonification was attained 21–28 days after application of Thiram at 50 μg g−1. On the other hand, with the triorganotin(IV) compounds, except for diphenylbutyltin bromide at 10–50 μg g−1, ammonification persisted at all concentrations 28 days after application.

The effect of diabetes mellitus on chemotactic and bactericidal activity of human polymorphonuclear leukocytes

We studied the bactericidal activity (against P. aeruginosa) and chemotactic ability of polymorphonuclear leukocytes from 26 diabetic patients in three treatment groups (oral hypoglycemic, daily insulin, and continuous insulin infusion). Patients were studied before entry into intensified management protocols, and after intensified management in 11 of the 26 patients. Diabetic serum had a persistent inhibitory effect on both diabetic and normal white cells, but normal serum was unable to fully correct diabetic white cell killing to control values. After intensified management of diabetes, there was an improvement in bactericidal function of diabetic patient white cells, but not in the effect of diabetic serum. Diabetic serum, and to a lesser extent diabetic white blood cells, are defective mediators of killing of P. aeruginosa. Chemotaxis was normal in all patient groups. These findings confirm the earlier work of others showing that some patients with diabetes mellitus have a defect in host defense against infection with bacteria.

Persistent inhibitory effect of colchicine subperineurially injected to tibial nerve in chicken.

Colchicine was subperineurially injected to the tibial nerve in chickens, and it caused a persistent partial paralysis of the leg on the injected side for 60 to 90 days. The injected chickens gradually recovered. Acetylcholinesterase accumulation in the nerve after 24 hr ligation was low until day 36, but high at day 128 of post-treatment. Weight of the gastrocnemial muscle on the injected side decreased to about a half of that of the other side at day 32, then recovered considerably at day 128. The chicken seemed to be a suitable experimental animal to study the mechanism of colchicine effect on the peripheral nerve.

Effects of Temperature Fluctuation, Red and Far-Red Light and Nitrate on Seed Germination of Five Grasses

(1) The germination of seeds of five grass species which differ in their occurrence in seed banks beneath grasslands was investigated. The species were Agrostis capillaris, Holcus lanatus and Poa trivialis, which usually form large and persistent seed banks, and Cynosurus cristatus and Festuca rubra, which usually occur less frequently and more transiently. Germination was investigated in darkness at constant temperatures from 5 to 30 OC for 12 h, alternating with constant 20 OC for 12 h. In one series of experiments the effects of a short exposure to a fully unshaded situation or deep canopy shade was simulated by treatment with red or far-red light, respectively, before transfer to the different temperatures. In another series with older seed, the effects of different concentrations of nitrate were investigated. (2) The different temperature regimes markedly affected germination and the optimum appeared to be about 20/10 OC for all species. Minimum germination of C. cristatus and F. rubra occurred at the highest temperature (30/20 OC) but H. lanatus and A. capillaris germinated least well when there was little alternation of temperature (i.e. near-constant 20 0C). Although older seed, particularly of P. trivialis, were more germinable than younger ones, they showed a similar pattern of response to temperature. (3) In general, treatment with red light before transfer to the different temperature regimes had a persistent stimulatory effect and treatment with far-red light a persistent inhibitory effect on germination. The effects were largest in A. capillaris, especially with least temperature fluctuation. Far-red light slightly stimulated germination in P. trivialis, but neither far-red nor red affected germination of C. cristatus significantly. (4) Potassium nitrate solution (0.002 M) increased the germination of all species, except of C. cristatus, and 0.02 M solution further increased that of A. capillaris. (5) In most instances, exposure of ungerminated seeds to daylight and to a 20/10 OC alternating temperature regime resulted in almost full germination of all species, except of A. capillaris. Many seeds of this species, especially those treated with far-red light or maintained in darkness, failed to germinate under these conditions but required gibberellic acid additionally, indicating induction of dormancy by some initial treatments. (6) The results showed that the three species (A. capillaris, H. lanatus and P. trivialis) which tend to form persistent seed banks showed a larger response to the treatments (i.e. were more specific in their germination requirements) than did those species (C. cristatus and F. rubra) which usually occur only transiently in the seed bank.

The inhibitory effects of exogenous arachidonic acid on rabbit platelet aggregation and the release reaction

Although arachidonic acid causes rabbit platelet aggregation and the release of granule contents in suspensions of washed platelets when used in concentrations of approximately 50–300 microM, higher concentrations (500 microM) cause neither aggregation nor release. Suspensions of platelets from rabbits wee exposed to arachidonic acid (250 microM) for 15 min, allowed to recover in the presence of PGE1 for 30 min, washed, and resuspended; in some experiments, the platelets were treated with aspirin before being exposed to arachidonic acid. Aggregation of platelets pretreated with arachidonic acid was inhibited in response to ADP; this effect was greater with the non-aspirin- treated platelets and persisted for at least 4 hr after resuspension. The association of 125I-fibrinogen with the platelets as a result of ADP stimulation was also inhibited. Aggregation and release of granule contents in response to collagen and low concentrations of thrombin was inhibited, but the inhibition could be overcome by higher concentrations. Thrombin induced further release of granule contents from platelets exposed to arachidonic acid without pretreatment with aspirin. Platelets that had been exposed to arachidonic acid, either with or without pretreatment with aspirin, did not aggregate or undergo further release upon stimulation with arachidonic acid after they were washed and resuspended. Inhibition of the lipoxygenase pathway with eicosatetraynoic acid (ETYA) or nordihydroguaiaretic acid (NDGA) did not affect the inhibition caused by arachidonic acid, so it is unlikely that a product of this pathway is responsible for the inhibition. Mixing experiments indicated that the pretreated platelets did not form a thromboxane-A2-like activity, and that they were unresponsive to aggregation and release induced by products formed from arachidonic acid. Experiments with 3H-arachidonic acid showed that after 45 min of incubation with platelets, only 1.1% of the 3H-arachidonic acid remained as free arachidonic acid in the platelets. Although cyclic-AMP was slightly increased 1 min after the addition of arachidonic acid, the cyclic-AMP concentration was the same as that of control platelets after the platelets were washed and resuspended, indicating that increased cyclic-AMP is not likely to be responsible for the persistent inhibitory effect. Thus, the inhibitory effect of pretreatment with arachidonic acid is a general effect on responses to a variety of aggregating agents that act through different mechanisms, and the inhibition is not related to thromboxane-A2 formation. The possibility of membrane perturbation resulting in the unavailability of receptors may explain the persistent inhibitory effect, but the responsible reactions have not been identified.

The Inhibitory Effects of Exogenous Arachidonic Acid on Rabbit Platelet Aggregation and the Release Reaction

AbstractAlthough arachidonic acid causes rabbit platelet aggregation and the release of granule contents in suspensions of washed platelets when used in concentrations of approximately 50–300 µM. higher concentrations (500 µM) cause neither aggregation nor release. Suspensions of platelets from rabbits were exposed to arachidonic acid (250 µM) for 15 min, allowed to recover in the presence of PGE1 for 30 min, washed, and resuspended; in some experiments, the platelets were treated with aspirin before being exposed to arachidonic acid. Aggregation of platelets pretreated with arachidonic acid was inhibited in response to ADP; this effect was greater with the non-aspirin-treated platelets and persisted for at least 4 hr after resuspension. The association of 125I-fibrinogen with the platelets as a result of ADP stimulation was also inhibited. Aggregation and release of granule contents in response to collagen and low concentrations of thrombin was inhibited, but the inhibition could be overcome by higher concentrations. Thrombin induced further release of granule contents from platelets exposed to arachidonic acid without pretreatment with aspirin. Platelets that had been exposed to arachidonic acid, either with or without pretreatment with aspirin, did not aggregate or undergo further release upon stimulation with arachidonic acid after they were washed and resuspended. Inhibition of the lipoxygenase pathway with eicoeatetraynoic acid (ETYA) or nordihydroguaiaretic acid (NDGA) did not affect the inhibition caused by arachidonic acid, so it is unlikely that a product of this pathway is responsible for the inhibition. Mixing experiments indicated that the pretreated platelets did not form a thromboxane-A2-like activity, and that they were unresponsive to aggregation and release induced by products formed from arachidonic acid. Experiments with 3H-arachidonic acid showed that after 45 min of incubation with platelets, only 1.1% of the 3H-arachidonic acid remained as free arachidonic acid in the platelets. Although cyclic-AMP was slightly increased 1 min after the addition of arachidonic acid, the cyclic-AMP concentration was the same as that of control platelets after the platelets were washed and resuspended, indicating that increased cyclic-AMP is not likely to be responsible for the persistent inhibitory effect. Thus, the inhibitory effect of pretreatment with arachidonic acid is a general effect on responses to a variety of aggregating agents that act through different mechanisms, and the inhibition is not related to thromboxane-A2 formation. The possibility of membrane perturbation resulting in the unavailability of receptors may explain the persistent inhibitory effect, but the responsible reactions have not been identified.