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941.
Settler agriculture and the dynamics of resource allocation in frontier environments 总被引:1,自引:0,他引:1
Francisco J. Pichón 《Human ecology: an interdisciplinary journal》1996,24(3):341-371
This article attempts to conceptualize the dynamics of resource allocation by colonist farmers under the unique conditions of land abundance and labor scarcity which characterize frontier environments, such as the smallholder agricultural settlement areas in the Amazon basin. In contrast, most previous theoretical literature on household agricultural decision making and land-use change in rural areas considers conditions of high population density and land scarcity, and is not, therefore, adequate for understanding critical land-use changes which may be occurring in frontier regions. This article first discusses the appropriateness and inadequacies of the analytical frameworks commonly used to explain the expansion of settler agriculture into remote forest regions and the unsustainable land-use practices observed in these areas. This review serves as the basis for characterizing resource allocation under the particular conditions of frontier environments. A conceptual advance in the analysis is its consideration of the way institutional/policy factors and farm-level characteristics can interact to produce land-use outcomes. This knowledge is essential to understand not only the social and economic factors affecting present land use and choice of technology, but also those factors influencing farmers' demand for more optimal systems of land use which are consistent with varying agro-ecological potentials, demographic situations, and their own management capacity. 相似文献
942.
J. A. Chowen L. M. García-Segura S. González-Parra J. Argente 《Cellular and molecular neurobiology》1996,16(3):297-310
Summary 1. The secretory pattern of growth hormone (GH) is sexually dimorphic in the adult rat. However, this difference between the sexes does not become apparent until after the onset of puberty, suggesting that pubertal sex steroids play an important role in the manifestation of this phenomenon.2. We have addressed the question as to whether there exists a sexual dimorphism in the hypothalamic neuropeptides that regulate GH release from the anterior pituitary,i.e., somatostatin (SS) and growth hormone-releasing hormone (GHRH). In addition, we have investigated whether the developmental changes in the GH secretory pattern are correlated with changes in these neuropeptides. The effect of testosterone treatment on SS and GHRH neurons during both the neonatal period and adulthood have also been studied.3. We have found that the synthetic capacity, as reflected in relative messenger RNA (mRNA) levels, of both SS and GHRH neurons changes throughout development in both male and female rats. These mRNA levels are sexually dimorphic at certain times during maturation and can be modulated by changes in testosterone levels, suggesting that sex steroid modulation of these two neuropeptide systems could at least partially account for the sexual dimorphism seen in the adult GH secretory pattern.4. The neonatal steroid environment has also been suggested to be involved in the generation of the final adult GH secretory pattern, although the mechanisms underlying this effect are even less well understood. In support of the hypothesis that the neonatal steroid environment plays an important role in organizing the GH axis, we have found that the number of GHRH neurons in the adult brain, as well as their sensitivity to adult steroids, is modulated by neotatal testosterone treatment. The number of SS neurons in the periventricular and paraventricular nuclei were not modulated by neonatal steroids; however, the synthetic capacity of these neurons does appear to be influenced by the neonatal steroid environment.5. These studies suggest that both the neonatal and adult sex steroid environments influence the adult GH secretory pattern by modulating GHRH and SS neurons. 相似文献
943.
Neural control of the expression of a Ca2+-activated K+ channel involved in the induction of myotonic-like characteristics 总被引:2,自引:0,他引:2
Beatriz U. Ramírez Maria Isabel Behrens Cecilia Vergara 《Cellular and molecular neurobiology》1996,16(1):39-49
Summary 1. Expression of the apamin-sensitive K+ channel (SK+) in rat skeletal muscle is neurally regulated. The regulatory effect of the nerve over the expression of some muscle ion
channels has been attributed to the electrical activity triggered by the nerve and/or to a trophic effect of some molecules
transported from the soma to the axonal endings.
2. SK+ channels apparently are involved in myotonic dystrophy (MD), therefore understanding the factors that regulate their expression
may ultimately have important clinical relevance.
3. To establish if axoplasmic transport is involved in this process, we used two experimental approaches in adult rats: (a)
Both sciatic nerves were severed, leaving a short or a long nerve stump attached to the anterior tibialis (AT). (b) Colchicine
or vinblastine (VBL), two axonal transport blockers of different potencies, was applied on one leg to the sciatic nerve. To
determine whether electrical activity affects the expression of SK+ channels, denervated AT were directly stimulated. The corresponding contralateral muscles were used as controls.
4. With these experimental conditions we measured (a) apamin binding to muscle membranes, (b) muscle contractile characteristics,
and (c) electromyographic activity.
5. In the short- and long-nerve stump experiments, 5 days after denervation125I-apamin binding to AT membranes was 2.0 times higher in the short-stump side. This difference disappeared at longer times.
The delayed expression of SK+ channels in the muscle left with a longer nerve stump can be attributed to the extra axoplasm contained in the longer stump,
which maintains a normally repressive signal for a longer period of time. Ten to 15 days after application of axonal transport
blockers we found that the muscle half-relaxation time increased in the drug-treated side and apamin partially reverted the
prolonged relaxation. Myotonic-like discharges specifically blockable by apamin were always present in the drug-treated leg.125I-Apamin binding, which is undetectable in a microsomal preparation from hind leg control muscles, was increased in the drug-treated
preparations. Apamin binding to denervated and stimulated AT muscles was lower than in the contralateral unstimulated muscles
[3.3±1.0 vs 6.8±0.8 (n=4) fmol/mg protein].
6. Our results demonstrate that electrical activity and axoplasmic transport are involved in the control of expression of
SK+ in rat skeletal muscle. However, the increased expression of this channel induces myotonic-like characteristics that are
reversed by apamin. This myotonic activity could be a model for MD. 相似文献
944.
V. Alvarez E. Coto S. González-Roces C. López-Larrea F. Setién C. López-Larrea 《Immunogenetics》1996,44(6):446-452
N-formyl peptides (FMLP) and complement fragment C5a are neutrophil chemoattractants. In humans, a single-copy gene was identified
for the C5a receptor, and the receptor for FMLP (FPR1) is encoded by a single gene that shows 53% amino acid similarity to
the C5aR. Two other humanFPR1 homologues,FPR-like 1 (FPR2/FPRL1) andFPR-like 2 (FPRL2) have been cloned. The human C5aR, FPR1, FPRL1, and FPRL2 are physically linked. By direct sequencing or by sequencing plasmid
clones we studied theC5aR andFPR genes from four non-human primates (chimpanzee, gorilla, orangutan, and macaque). The sequences showed 95%–99% similarity
to the human homologues, with the major divergences observed in macaque. In these genes, the transmembrane and the cytoplasmic
domains are highly conserved, while the highest divergence corresponded to the extracellular loops involved in ligand binding.
Additionally, we constructed a physical map of these genes in non-human primates. In all species the four genes were physically
linked and we defined the relative orientation of the four genes in primates:C5aR>FPR1>FPR2 (FPRL1)>FPRL2.
The nucleotide sequence data reported in this paper have been submitted to the EMBL/GenBank nucleotide sequence databases
and have assigned the accession numbers X97730 (PTC5aR), X97731 (MMC5aR), X97732 (PPC5aR), X97730 (GGC5aR), X97734 (MMFPR1), X97735 (PPFPR1), X97736 (GGFPR1), X97737 (MMFPRL1), X97738 (GGFPRL1), X97739 (PTFPRL1), X97740 (MMFPRL2), X97741 (PPFPRL2), X97742 (GGFPRL2), X97743 (PTFPRL2), X97744 (PPFPRL1), and X97745 (PTFPR1) 相似文献
945.
946.
Summary Fe (III) and Cu (II) each at 50 M in four commercial strains of Saccharomyces cerevisiae induced an increase of NAD(P)+ reduction in one strain (Turkish), but two others (Chilean and Brazilian), the presence of Fe(III) and/or Cu(II) diminished NAD(P)+ reduction presumably due to free radicais formation inside these living cells. Suprisingly, in the American strain, Fe(III) induced a decrease and Cu (II) an increase of NAD(P)+ reduction. 相似文献
947.
Summary Genes as POT1, HSP104 and SSA3, which are late expressed in laboratory culture conditions are expressed only during the first few days in microvinifications in wine yeast cells. This effect is probably due to the different growth conditions and leads to useless levels of enzyme activity for a reporter gene. However the ACT1 promoter, which is constitutively expressed in laboratory conditions, produces sufficient amounts of enzyme activity in late fermentation phases. 相似文献
948.
Xóchttl Castañeda Víctor Ortíz Betania Allen Cecilia García Mauricio Hernández-Avila 《Culture, medicine and psychiatry》1996,20(2):229-247
The central topic of the article is the divided world of female commercial sex workers (FCSW) in Mexico City. Fourteen focus group sessions were conducted with 133 FCSW from varying socio-economic levels and types of work site, as well as seven individual interviews.FCSW live in a constant double bind, as mother and prostitute, and come into daily contact with society's double standard for women. Reactions include justifying sex work as a better paying employment opportunity for women, as a necessary evil, and as a type of social service, while at the same time hiding their profession from their families. FCSW also live out an archetypal female ambivalence, their selves divided between the mother/saint and the traitor/prostitute.This article defines elements which should be taken into account in culturally appropriate programs for prevention of HIV/AIDS transmission, especially the importance which FCSW give to their role as mothers and promotion of the condom as a physical and symbolic barrier between professional and private life. 相似文献
949.
Jose-Emilio Mesonero María-Carmen Rodríguez Yoldi María-Jesús Rodríguez Yoldi 《Biological trace element research》1996,51(2):149-159
The element Cd is considered to have no biological function and is highly toxic to humans and animals. Toxic effects of this
metal upon cell membrane structure and function have been shown. On the other hand, Ca is an essential element in a wide variety
of cellular activities. The present study was initiated to research whether the interaction between Ca and Cd could affect
D-galactose absorption across the rabbit jejunum in vitro. In media with Ca2+, when CdCl2 was present at 0.5 or 1 mM, Cd was found to significantly reduce the sugar absorption. In Ca2+-free media, where CaCl2, was omitted and replaced isotonically with choline chloride, the sugar transport was not modified by Cd, but when CaCl2 was replaced isotonically with MgCl2, the inhibition is observed. Verapamil at 10−6
M (blocking mainly Ca2+ transport) did not modify the inhibitory effect of cadmium on D-galactose transport. When 10−6
M of A 23187 (Ca2+ specific ionophore) was added in media with/without Ca2+; CdCl2 produced no change in D-galactose transport. These results suggest that Ca and Cd could have affinity for the same chemical
groups of enterocyte membrane, which would be related with the intestinal absorption of D-galactose. 相似文献
950.