Transport and redistribution of selenium in the bean plant (Phaseolus vulgaris)

After incubation for 3 h with (⁷⁵Se) selenate, the selenium distribution in the bean plant (Phaseolus vulgaris L. cv. Contender) through a 29-day period showed an uneven distribution: roots and trifoliate leaves were richer in ⁷⁵Se than stem and primary leaves. The high selenium concentration of roots resulted from the retention of selenate by the root cells: at the end of the 29-day period about 60° of the radioactivity was always ethanol-soluble, and when analysed by paper chromatography, proved to be selenate. By contrast, much of the radioactivity of the leaves was ethanol-insoluble, ⁷⁵Se being quickly captured in metabolic processes which immobilize it. During plant development, a portion of the total selenium remains mobile and is continually mobilized to the younger organs which display a rapid growth rate. This delivery results from a progressive liberation of selenate retained by mature organs, especially the roots, and from turnover in older leaf tissues, especially the trifoliate leaves.     

Stress-Induced Alterations in Metabolism of Γ-Aminobutyric Acid in Rat Brain

The effect of a stressful manipulation on the metabolism of gamma-aminobutyric acid (GABA) in the rat brain was studied. Application of an immobilized stress to animals induced a significant increase in the striatal and hypothalamic GABA contents without affecting those in other central structures examined. It was also found that the increase in striatal GABA level preceded that in the hypothalamus. This increase in steady-state levels of GABA in the striatum and hypothalamus disappeared at 12 h after the termination of the application of stress for 3 h, which exhibited a maximal stimulatory action on the GABA contents in both central areas. The activity of L-glutamic acid decarboxylase was found to be significantly elevated in the striatum and hypothalamus following the stress application with a concomitant decrease in the content of L-glutamic acid, which is converted to GABA by the catalytic action of the latter enzyme. The in vivo turnover of GABA in the brain was estimated by taking advantages of the postmortem accumulation of GABA following decapitation and of the selective inhibitory action of a low dose of aminooxyacetic acid on the GABA degrading system, respectively. Analysis using these two different methods revealed that the cerebral turnover of GABA in vivo was not significantly altered under stressful situations despite of the increase in its steady-state level. These results suggest that central GABA system may respond to the input of painful stimuli resulting from the application of a severe physical and psychological stressor, in addition to the well-known functional alterations in catecholamine neurons. The functional significance of these alterations in the central GABA neurons is also discussed.     

Turnover rates of amino acid neurotransmitters in regions of rat cerebellum

The turnover rates of aspartate, gamma-aminobutyric acid (GABA), glutamate, glutamine, alanine, serine, and glycine were measured in five regions of rat cerebellum. Turnover rates of the putative neurotransmitters (aspartate, glutamate, and GABA) were 2-20-fold higher than those of alanine and serine, and generally consistent with the proposed neurotransmitter functions for these amino acids. However, glutamate turnover was high and similar in magnitude in the deep nuclei and granule layer, suggesting possible release, not only from parallel fibers, but from mossy fibers as well. The differential distribution of turnover rates for GABA supports its neuronal release by Purkinje, stellate, basket, and Golgi cells, whereas aspartate may be released by both climbing and mossy fibers. The distribution of glycine turnover rates is consistent with release from Golgi cells, whereas alanine may be released from granule cell parallel fibers. Turnover rates measured in two other motor areas, the striatum and motor cortex, indicated that utilization of these amino acid neurotransmitters is differentially distributed in brain motor regions. The data indicate that turnover rate measurements may be useful in identifying neurotransmitter function where content measurements alone are insufficient.     

Substrate inactivation of enzymes in vitro and in vivo

Some enzymes are inactivated by their natural substrates during catalytic turnover, limiting the ultimate extent of reaction. These enzymes can be separated into three broad classes, depending on the mechanism of the inactivation process. The first type is enzymes which use molecular oxygen as a substrate. The second type is inactivated by hydrogen peroxide, which is present either as a substrate or a product, and are stabilized by high catalase activity. The oxidation of both types of enzymes shares common features with oxidation of other enzymes and proteins. The third type of enzyme is inactivated by non-oxidative processes, mainly reversible loss of cofactors or attached groups. Sub classes are defined within each broad classification based on kinetics and stoichiometry. Reaction-inactivation is in part a regulatory mechanism in vivo, because specific proteolytic systems give rapid turnover of such labelled enzymes. The methods for enhancing the stability of these enzymes under reaction conditions depends on the enzyme type. The kinetics of these inactivation reactions can be used to optimize bioreactor design and operation.     

Regulation of Histamine Turnover via Muscarinic and Nicotinic Receptors in the Brain

To clarify the regulation of central histaminergic (HAergic) activity by cholinergic receptors, the effects of drugs that stimulate the cholinergic system on brain histamine (HA) turnover were examined, in vivo, in mice and rats. The HA turnover was estimated from the accumulation of tele-methylhistamine (t-MH) during the 90-min period after administration of pargyline (65 mg/kg, i.p.). In the whole brain of mice, oxotremorine, at doses higher than 0.05 mg/kg, s.c., significantly inhibited the HA turnover, this effect being completely antagonized by atropine but not by methylatropine. A large dose of nicotine (10 mg/kg, s.c.) also significantly inhibited the HA turnover. This inhibitory effect was antagonized by mecamylamine but not by atropine or hexamethonium. A cholinesterase inhibitor, physostigmine, at doses higher than 0.1 mg/kg, s.c., significantly inhibited the HA turnover. This effect was antagonized by atropine but not at all by mecamylamine. None of these cholinergic antagonists used affected the steady-state t-MH level or HA turnover by themselves. In the rat brain, physostigmine (0.1 and 0.3 mg/kg, s.c.) also decreased the HA turnover. This inhibitory effect of physostigmine was especially marked in the striatum and cerebral cortex where muscarinic receptors are present in high density. Oxotremorine (0.2 mg/kg, s.c.) and nicotine (1 mg/kg, s.c.) also decreased the HA turnover in the rat brain. However, these effects showed no marked regional differences. These results suggest that the stimulation of central muscarinic receptors potently inhibits the HAergic activity in the brain and that strong stimulation of central nicotinic receptors can also induce a similar effect.     

Effects of Oxygen Depletion on Norepinephrine- and Carbachol-Stimulated Phosphoinositide Turnover in Rat Brain Slices

We examined the effects of in vitro anoxia and in vivo hypoxia (8% O2/92% N2) on norepinephrine (NE)- and carbachol-stimulated phosphoinositide (PI) turnover in rat brain slices. The formation of 3H-labeled polyPI in cortical slices was impaired by in vitro anoxia and fully restored by reoxygenation. Accumulation of 3H-labeled myo-inositol phosphates (3H-IPs) stimulated by 10(-5) M NE was significantly reduced by anoxia (control at 60 min, 1,217 +/- 86 cpm/mg of protein; anoxia for 60 min, 651 +/- 82 cpm/mg; mean +/- SEM; n = 5; p less than 0.01), and reoxygenation following anoxia resulted in overshooting of the accumulation (control at 120 min, 1,302 +/- 70 cpm/mg; anoxia for 50 min plus oxygenation for 70 min, 1,790 +/- 126 cpm/mg; n = 5; p less than 0.01). The underlying mechanisms for the two phenomena--the decrease caused by anoxia and the overshooting caused by reoxygenation following anoxia--seemed to be completely different because of the following observations. (a) Although the suppression of NE-stimulated accumulation at low O2 tensions was also observed in Ca2+-free medium, the overshooting in response to reoxygenation was not. (b) Carbachol-stimulated accumulation was significantly reduced by anoxia and was restored by reoxygenation only to control levels. Thus, the postanoxic overshooting in accumulation of 3H-IPs seems to be a specific response to NE. (c) The decrease observed at low O2 tensions was due to a decrease in Emax value, whereas the postanoxic overshooting was due to a decrease in EC50 value.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of the Histamine H3-Agonist (R)-α-Methylhistamine and the Antagonist Thioperamide on Histamine Metabolism in the Mouse and Rat Brain

To study the feedback control by histamine (HA) H3-receptors on the synthesis and release of HA at nerve endings in the brain, the effects of a potent and selective H3-agonist, (R)-alpha-methylhistamine, and an H3-antagonist, thioperamide, on the pargyline-induced accumulation of tele-methylhistamine (t-MH) in the brain of mice and rats were examined in vivo. (R)-alpha-Methylhistamine dihydrochloride (6.3 mg free base/kg, i.p.) and thioperamide (2 mg/kg, i.p.), respectively, significantly decreased and increased the steady-state t-MH level in the mouse brain, whereas these compounds produced no significant changes in the HA level. When administered to mice immediately after pargyline (65 mg/kg, i.p.), (R)-alpha-methylhistamine (3.2 mg/kg, i.p.) inhibited the pargyline-induced increase in the t-MH level almost completely during the first 2 h after treatment. Thioperamide (2 mg/kg, i.p.) enhanced the pargyline-induced t-MH accumulation by approximately 70% 1 and 2 h after treatment. Lower doses of (R)-alpha-methylhistamine (1.3 mg/kg) and thioperamide (1 mg/kg) induced significant changes in the pargyline-induced t-MH accumulation in the mouse brain. In the rat, (R)-alpha-methylhistamine (3.2 mg/kg, i.p.) and thioperamide (2 mg/kg, i.p.) also affected the pargyline-induced t-MH accumulation in eight brain regions and the effects were especially marked in the cerebral cortex and amygdala. These results indicate that these compounds have potent effects on HA turnover in vivo in the brain.     

Fine root demography in alfalfa (Medicago sativa L.)

In perennial forages like alfalfa (Medicago sativa L.), repeated herbage removal may alter root production and mortality which, in turn, could affect deposition of fixed N in soil. Our objective was to determine the extent and patterns of fine-diameter root production and loss during the year of alfalfa stand establishment. The experiment was conducted on a loamy sand soil (Udorthentic Haploboroll) in Minnesota, USA, using horizontally installed minirhizotrons placed directly under the seeded rows at 10, 20, and 40 cm depths in four replicate blocks. We seeded four alfalfa germplasms that differed in N₂ fixation capacity and root system architecture: Agate alfalfa, a winter hardy commercially-available cultivar; Ineffective Agate, which is a non-N₂-fixing near isoline of Agate; a new germplasm that has few fibrous roots and strong tap-rooted traits; and a new germplasm that has many fibrous roots and a strongly branched root system architecture. Video images collected biweekly throughout the initial growing season were processed using C-MAP-ROOTS software.More than one-half of all fine roots in the upper 20 cm were produced during the first 7 weeks of growth. Root production was similar among germplasms, except that the highly fibrous, branch-rooted germplasm produced 29% more fine roots at 20 cm than other germplasms. In all germplasms, about 7% of the fine roots at each depth developed into secondarily thickened roots. By the end of the first growing season, greatest fine root mortality had occurred in the uppermost depth (48%), and least occurred at 40 cm (36%). Survival of contemporaneous root cohorts was not related to soil depth in a simple fashion, although all survivorship curves could be described using only five rates of exponential decline. There was a significant reduction in fine root mortality before the first herbage harvest, followed by a pronounced loss (average 22%) of fine roots at the 10- and 20-cm depths in the 2-week period following herbage removal. Median life spans of these early-season cohorts ranged from 58 to 131 days, based on fitted exponential equations. At all depths, fine roots produced in the 4 weeks before harvest (early- to mid-August) tended to have shorter median life spans than early-season cohorts. Similar patterns of fine root mortality did not occur at the second harvest. Germplasms differed in the pattern, but not the ultimate extent, of fine root mortality. Fine root turnover during the first year of alfalfa establishment in this experiment released an estimated 830 kg C ha^–1 and 60 kg N ha^–1, with no differences due to N₂ fixation capacity or root system architecture.     

Soil nitrogen dynamics along a gradient of long-term soil development in a Hawaiian wet montane rainforest

I analyzed the rates of net N mineralization and nitrification of soils from seven sites in a Hawaiian wet montane forest. The sites differ in age, ranging from 400 to 4,100,000 yr, but are comparable in other variables (all at 1200 miasl with 4000 mm or more mean annual rainfall), and the chronosequence simulated a development of soils from basaltic lava. Soils were incubated for 20 days at 17.5 °C, which is nearly equivalent to a mean field air temperature of the sites, and at an elevated temperature of 25.5 °C under three treatments: 1) field-wet without amendments, 2) air dried to a permanent wilting point, and 3) fertilized with phosphate (NaH₂PO₄) at the rate of 50 g P per g dry soil. Both mineralization and nitrification rates varied significantly among the sites at the field temperature (p<.00001). Fractions of the mineralized organic matter (indexed by the N produced per g organic C) increased sharply from the youngest to the 5000-yr site before declining abruptly to a near constant value from the 9000 to the 1,400,000-yr sites. Total organic C in the top soils (<15 cm deep) increased almost linearly with age across the sites. Consequently, net NH₄- and NO₃-N produced on an area basis (g m^-2 20 d^-1) increased sharply from 0.2 in the youngest site to 1.2 in the 5000-yr site, then both became depressed once but steadily increased again. The fraction of organic matter mineralized, and the net N turnover rates were outstandingly high in the oldest site where a large amount of organic matter was observed; the topsoil organic matter which was used in this analysis appeared to be highly labile, whereas the subsurface organic matter could be relatively recalcitrant. As suggested by earlier workers, the initial increase in N turnover seemed to correspond to the increasing quantity of N in the soils through atmospheric deposition and biological fixation. The later decline in fraction of organic matter mineralized seemed to relate to increasing soil C/N ratios, increasingly recalcitrant organic matter, and poorer soil drainage with age. The elevated temperature treatment produced significantly higher amounts of N mineralization, except for the youngest site where N was most limiting, and for two sites where soil waterlogging might be severe. P fertilization invariably resulted in slower N turnovers, suggesting that soil microbes responded to added P causing N immobilization. The youngest site did not significantly respond to added P. The magnitude of immobilization was higher in older than in younger soils, suggesting that P more strongly limits microbial populations in the older soils.     

Applications of minirhizotrons to understand root function in forests and other natural ecosystems

Minirhizotrons have proved useful to understand the dynamics and function of fine roots. However, they have been used comparatively infrequently in forests and other natural plant communities. Several factors have contributed to this situation, including anomalous root distributions along the minirhizotron surface and the difficulty of extracting data from minirhizotron images. Technical and methodological advances have ameliorated some of these difficulties, and minirhizotrons have considerable potential to address some questions of long standing interest. These questions include more fully understanding the role of roots in carbon and nutrient cycling, rates of root decomposition, responses to resource availability and the functional significance of interactions between plant roots and soil organisms. Maximizing the potential for minirhizotrons to help us better understand the functional importance of fine roots in natural plant communities depends upon using them to answer only those questions appropriate to both their inherent strengths and limitations.