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Summary Brink (1982) characterizes the distribution of standing crop of nectar for Delphinium nelsonii as bonanzablank, based on comparison with a Poisson. He then discusses possible effects of standing crop variability on pollinator foraging behavior. We disagree with the use of the Poisson and the resulting conclusions. The expected distribution should not be based on doling out random amounts of nectar to flowers, but based on random return times to flowers by pollinators (elapsed time=nectar accumulated). When this model is used, standing crop variance does not differ markedly from expectation. What differences do exist can be accounted for by variability in nectar production rates of individual plants. We also take issue with the use of the bonanza-blank terminology. As originally formulated this refers to nectar production differences within a plant rather than standing crop differences among plants. 相似文献
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Meylin partially purified from spinal cords of dysmyelinating mutant (shiverer) mice had almost three-fold the specific activity of 5′-nucleotidase found in the respective myelin fraction from normal mice. The specific activities of two other normally myelin-associated enzymes, 2′,3′-cyclic nucleotide-3′-phosphohydrolase and carbonic anhydrase, were only slightly higher in the myelin membranes from shiveres, compared to those from controls. In the mutants, the three enzymes probably occur in oligodendrocyte processes. Hhypothetically, the 5′-nucleotidase in the myelin sheath in shiverer and normal mice may be localized in specialized structures. 相似文献
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R Parrilla K Okawa K O Lindros U J Zimmerman K Kobayashi J R Williamson 《The Journal of biological chemistry》1974,249(15):4926-4933
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The effects of Colcemid, the deacetyl-N-methyl derivative of colchicine, on the eggs of Arbacia punctulata were investigated. Colcemid in concentrations of 2.7 x 10-5
M or greater blocks syngamy (the fusion of the pronuclei) in these eggs. Although a tenfold decrease in concentration of Colcemid usually permits the pronuclei to fuse, the subsequent division is blocked. In the sea urchin egg, the duration of presyngamy is about 15 min during which time there is no DNA synthesis. However, DNA synthesis is recorded in Colcemid-blocked cells prior to syngamy. Radioautographs of Colcemid-blocked cells which were immersed into thymidine-3H exhibited silver grains above each of the pronuclei. The action of Colcemid on Arbacia eggs is reversible. Nevertheless, exposures to 2.7 x 10-5
M Colcemid for only 3 min, initiated 5 min after insemination, caused delays of 70 min in subsequent division. In general, cells are more sensitive to Colcemid prior to the time when the mitotic spindle is being assembled than at presyngamy stages. The results are discussed in terms of Colcemid action on pronuclear fusion and cell division. 相似文献
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Rajender K. Kamboj Darryle D. Schoepp Stephen Nutt Lee Shekter Bozena Korczak Rebecca A. True Vikarna Rampersad Dennis M. Zimmerman Michael A. Wosnick 《Journal of neurochemistry》1994,62(1):1-9
Abstract: Kainate is a potent neuroexcitatory agent; its neurotoxicity is thought to be mediated by an ionotropic receptor with a nanomolar affinity for kainate. In this report, we describe the cloning of a cDNA encoding a human glutamate ionotropic receptor subunit protein from a human hippocampal library. This cDNA, termed humEAA1, is most closely related to rat and human cDNAs for kainate receptor proteins and, when expressed in COS or Chinese hamster ovary cells, is associated with high-affinity kainate receptor binding. We have successfully established cell lines stably expressing humEAA1. This is the first report of establishment of stable cell lines expressing a glutamate receptor subunit. The relative potency of compounds for displacing [3 H] kainate binding of humEAA1 receptors expressed in these stable cell lines was kainate > quisqualate > domoate > L-glutamate > ( RS )-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid > dihydro-kainate > 6, 7-dinitroquinoxaline-2, 3-dione > 6-cyano-7-nitroquinoxaline-2, 3-dione. Homooligomeric expression of humEAA1 does not appear to elicit ligand-gated ion channel activity. Nevertheless, the molecular structure and pharmacological characterization of high-affinity kainate binding of the humEAA1 expressed in the stable cell line (ppEAA1–16) suggest that the humEAA1 is a subunit protein of a human kainate receptor complex. 相似文献
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Leckband D Chen YL Israelachvili J Wickman HH Fletcher M Zimmerman R 《Biotechnology and bioengineering》1993,42(2):167-177
The adhesion forces between various surfaces were measured using the "surface forces apparatus" technique. This technique allows for the thickness of surface layers and the adhesion force between them to be directly measured in controlled vapor or liquid environments. Three types of biological surfaces were prepared by depositing various lipid-protein monolayers (with thicknesses ranging from 1 to 4 nm) on the inert, molecularly smooth mica surface: (i) hydrophobic lipid monolayers; (ii) amphiphilic polyelectrolyte surfaces of adsorbed polylysine; and (iii) deposited bacterial S-layer proteins. The adhesion, swelling, and wetting properties of these surfaces was measured as a function of relative humidity and time. Initial adhesion is due mainly to the van der Waals forces arising from nonpolar (hydrophobic) contacts. Following adhesive contact, significant molecular rearrangements can occur which alter their hydrophobic-hydrophilic balance and increase their adhesion with time. Increased adhesion is generally enhanced by (i) increased relative humidity (or degree of hydration); (ii) increased contact time; and (iii) increased rates of separation. The results are likely to be applicable to the adhesion of many other biosurfaces, and show that the hydrophobicity of a lipid or protein surface is not an intrinsic property of that surface but depends on its environment (e.g., on whether it is in aqueous solution or exposed to the atmosphere), and on the relative humidity of the atmosphere. It also depends on whether the surface is in adhesive contact with another surface and-when considering dynamic (nonequilibrium) conditions-on the time and previous history of its interaction with that surface. (c) 1993 John Wiley & Sons, Inc. 相似文献
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R Utili M F Tripodi C O Abernathy H J Zimmerman J Gillespie 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1992,199(1):49-53
The present study has demonstrated that tauroursodeoxycholate (TUDC), but not taurocholate, can reverse chlorpromazine (CPZ)-induced cholestasis in the isolated perfused rat liver. At an infusion rate of 1.5 mumol/min, TUDC led to restoration of bile flow in the perfused rat liver made cholestatic by the addition of 250 microM CPZ. This reversal was accompanied by an increased excretion of CPZ and its metabolites. A higher infusion rate of 5.0 mumols TUDC/min, however, led to only a transient increase in bile flow and to no increase in CPZ excretion. In contrast to the effects of TUDC, infusion of taurocholate led to an exacerbation of CPZ-induced cholestasis. The differences in the efficacy of the two bile salts may be due to their relative detergent (hydrophobic) properties. 相似文献