Ammonia Production and Assimilation in Glutamate Synthase Mutants of Arabidopsis thaliana

Ammonia production and assimilation¹ were examined in photorespiratory mutants of Arabidopsis thaliana L. lacking ferredoxin-dependent glutamate synthase (Fd-GluS) activity. Although photosynthesis was rapidly inhibited in these mutants in normal air, NH₄⁺ continued to accumulate. The accumulation of NH₄⁺ was also seen after an initial lag of 30 minutes in 2% O₂, 350 microliters per liter of CO₂ and after 90 minutes in 2% O₂, 900 microliters per liter of CO₂. The accumulation of NH₄⁺ in normal air and low O₂ was also associated with an increase in the total pool of amino acid-N and glutamine, and a decrease in the pools of glutamate, aspartate, alanine, and serine. Upon return to dark conditions, or to 21% O₂, 1% CO₂ in the light, the NH₄⁺ which had accumulated in the leaves was reassimilated into amino acids. The addition of methionine sulfoximine (MSO) resulted in higher accumulations of NH₄⁺ in glutamate synthase mutants and prevented the reassimilation of NH₄⁺ upon return to the dark. The addition of MSO also resulted in the accumulation of NH₄⁺ in glutamate synthase mutants in the light and in 21% O₂, 1% CO₂. These results indicate that glutamine synthetase is essential for the reassimilation of photorespiratory NH₄⁺ and for primary N assimilation in the leaves and strongly suggest that glutamate dehydrogenase plays only a minimal role in the assimilation of ammonia. Levels of NADH-dependent glutamate synthase (NADH-GluS) appear to be sufficient to account for the assimilation of NH₄⁺ by a GS/NADH-GluS cycle.     

Platelet-activating factor antagonists increase vascular reactivity in perfused rat lungs

Platelet-activating factor (PAF) administered to the pulmonary circulation in low dose (nanogram) has vasodilatory properties. Therefore, we investigated whether endogenous PAF plays a role in the control of tone in the pulmonary circulation. The PAF receptor antagonists, SRI 63-441 (2.6 X 10(-4) M) and L659,989 (1 X 10(-5) M), were the major investigative tools. In isolated perfused rat lungs, both agents caused a persistent increase in base-line perfusion pressure (Ppa), potentiated angiotensin II (ANG II) vasoconstriction, and potentiated hypoxic vasoconstriction (HPV). This potentiation of ANG II and HPV was found to be independent of circulating blood elements. Vasodilation in the presence of PAF blockade was also impaired. The combination of cyclooxygenase inhibition and PAF receptor blockade had an additive effect on ANG II vasoconstriction but did not cause more potentiation of HPV than achieved with PAF antagonism alone. In vivo, SRI 63-441 (10 mg/kg) caused only a transient increase in base-line Ppa without altering ANG II and hypoxic vasoconstriction. These findings support a vasodilatory role for endogenous PAF in the pulmonary circulation.     

Identification of cytokinins in young wheat spikes (Triticum aestivum cv. Chinese spring)

Cytokinins in young wheat (Triticum aestivum cv. Chinese spring) spikes (2–15 mm) were isolated by immunoaffinity chromatography. High-performance liquid chromatography-radioimmunoassay and gas chromatography-mass spectrometry indicated that major cytokinins present weretrans-zeatin, ribosyl-trans-zeatin, ribosyldihydrozeatintrans-zeatin-9-glucoside, and the glucosides oftrans-zeatin, ribosyl-trans-zeatin, andtrans-zeatin-9-glucoside. Dihydrozeatin,iso-pentenyladenosine, andiso-pentenyladenine were also present but at lower concentrations.Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the U.S. Department of Agriculture and does not imply approval to the exclusion of other products that may also be suitable. R. C. Durley's present address is Biological Sciences, Monsanto Company, St. Louis, Missouri 63167.     

Gastric exocrine “failure” in critically ill patients: incidence and associated features

Following the observation that many critically ill patients cannot maintain their gastric juice pH below 4 without treatment a study was performed to measure the gastric juice pH in such patients and relate it to other clinical data. The case notes of 64 patients who had been admitted to the intensive care unit and taken part in two trials of ranitidine treatment were reviewed. During those trials gastric juice was aspirated hourly and the pH and volume measured. In this study the values recorded during a six hour untreated control phase were used. Data on age, diagnosis, treatment, outcome, episodes of hypoxia, episodes of hypotension, and use of inotropic drugs were also reviewed. Full data were available for 61 patients: 27 had a mean baseline pH of >5 during the control phase and 34 a mean baseline pH of <5. Significantly more of those with a high pH suffered hypotension (21/27 v 13/34) and received inotropic drugs (16/27 v 8/34).These findings suggest that hypotension in critically ill patients adversely affects gastric exocrine function; prophylaxis with drugs that can improve gastric mucosal blood flow may be more effective than with antacids.     

Scales and costs of habitat selection in heterogeneous landscapes

Summary Two scales of habitat selection are likely to influence patterns of animal density in heterogeneous landscapes. At one scale, habitat selection is determined by the differential use of foraging locations within a home range. At a larger scale, habitat selection is determined by dispersal and the ability to relocate the home range. The limits of both scales must be known for accurate assessments of habitat selection and its role in effecting spatial patterns in abundance. Isodars, which specify the relationships between population density in two habitats such that the expected reproductive success of an individual is the same in both, allow us to distinguish the two scales of habitat selection because each scale has different costs. In a two-habitat environment, the cost of rejecting one of the habitats within a home range can be expressed as a devaluation of the other, because, for example, fine-grained foragers must travel through both. At the dispersal scale, the cost of accepting a new home range in a different habitat has the opposite effect of inflating the value of the original habitat to compensate for lost evolutionary potential associated with relocating the home range. These costs produce isodars at the foraging scale with a lower intercept and slope than those at the dispersal scale.Empirical data on deer mice occupying prairie and badland habitats in southern Alberta confirm the ability of isodar analysis to differentiate between foraging and dispersal scales. The data suggest a foraging range of approximately 60 m, and an effective dispersal distance near 140 m. The relatively short dispersal distance implies that recent theories may have over-emphasized the role of habitat selection on local population dynamics. But the exchange of individuals between habitats sharing irregular borders may be substantial. Dispersal distance may thus give a false impression of the inability of habitat selection to help regulate population density.     

Environmental networks,compensating life histories and habitat selection by white-footed mice

Summary Analysis of 6 years' data on a population of free-living white-footed mice documents both phenotypic and environmental control of litter size. Litter size was positively correlated with maternal body size. Maternal size depended upon both seasonal and annual variation. Paradoxically, the proportion of small versus large litters varied among habitats independently of the effects of body size. The result is an influence of habitat on life history that yields patterns of reproduction and survival opposite to the predictions of demographic theory. The habitat producing the largest litters had a relatively high ratio of adult/juvenile survival. Litter size was small in the habitat where the adult/juvenile survival ratio was smallest. All of these anomalous patterns can be explained through density-dependent habitat selection by female white-footed mice. Life-history studies that ignore habitat and habitat selection may find spurious correlations among traits that result in serious misinterpretations about life history and its evolution.     

Tunicamycin,puromycin and brefeldin A influence the subcellular distribution of neuropeptides in hypothalamic magnocellular neurones of rat

Summary Magnocellular neurones in the supraoptic nuclei of normal Long Evans and homozygous Brattleboro rats were examined electron-microscopically after intracisternal injections of tunicamycin, puromycin, or brefeldin A. Moderate (50 g) or high (200 g) doses of tunicamycin caused the formation of electron-dense filamentous accretions in the endoplasmic reticulum (ER) cisterns of vasopressin neurones, but only the high dose of tunicamycin also caused accretions to form in the ER of some oxytocin neurones. Immunogold labelling of ultrathin sections from tunicamycin-treated rats revealed that, in about 5% of vasopressin neurones, the accretions could be immunogold-labelled for vasopressin and its associated neurophysin. However, in the majority of vasopressin neurones, the sections required trypsinisation before immunolabelling of the accretions could be detected. Small accretions in the ER of oxytocin neurones did not label for oxytocin or its neurophysin without prior trypsinisation, whereas larger accretions in other oxytocin cells could be labelled without prior trypsin treatment. Administration of puromycin resulted in the formation of small ER accretions in both vasopressin and oxytocin neurones. These accretions were immunolabelled with antisera, respectively, to vasopressin and oxytocin, but neurophysin-immunoreactivity was in most cases absent and was not revealed by treatment with trypsin, suggesting that neurophysin-immunoreactive epitopes were absent from truncated peptides forming the accretions. Brefeldin A caused dilatation of ER cisterns and disruption of the Golgi apparatus in both oxytocin and vasopressin neurones, but did not cause accretions to form in the ER.