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1.
Sodium-rich frog muscles are found to extrude sodium and reaccumulate potassium at 0 degrees C. The uptake of potassium by these muscles is studied at three different temperatures as a function of external potassium concentration, K(ex). The steady-state potassium content of the tissue is related to K(ex). by a sigmoidal cooperative curve at all temperatures. These results are compared with findings on a mammalian smooth muscle. 相似文献
2.
Xiang XD Hou JG Briceño G Vareka WA Mostovoy R Zettl A Crespi VH Cohen ML 《Science (New York, N.Y.)》1992,256(5060):1190-1191
Sizable single crystals of C(6O) have been synthesized and doped with potassium. Above the superconducting transition temperature T(c), the electrical resistivity p(T) displays a classic metal-like temperature dependence. The transition to the superconducting state at T(c) = 19.8 K is extremely sharp, with a transition width DeltaT < 200 mK. In contrast to transport behavior of doped polycrystalline and granular thin films, no anomalous fluctuations are observed near T(c) in single crystal specimens. 相似文献
3.
Iqbal Z Baughman RH Ramakrishna BL Khare S Murthy NS Bornemann HJ Morris DE 《Science (New York, N.Y.)》1991,254(5033):826-829
The appearance of superconductivity at relatively high temperatures in alkali metal-doped C(60) fullerene provides the challenge to both understand the nature and origin of the superconductivity and to determine the upper limit of the superconducting transition temperature (T(c)). Towards the latter goal, it is shown that doping with potassium-thallium and rubidium-thallium alloys in the 400 to 430 degrees C temperature range increases the T(c) of C(60)/C(70) mixtures to 25.6 K and above 45 K, respectively. Similar increases in T(c) were also observed upon analogous doping of pure C(60). Partial substitution of potassium with thallium in interstitial sites between C(60) molecules is suggested by larger observed unit cell parameters than for the K(3)C(60) and K(4)C(60) phases. Contrary to previous results for C(60) doped with different alkali metals, such expansion does not alone account for the changes in critical temperature. 相似文献
4.
Squid giant axon possesses a hyperpolarizing electrogenic sodium pump which is stimulated by internal sodium and by external potassium. This conclusion is based on the following observations: strophanthidin depolarizes the membrane and enhances the depolarizing effect of 5 or 10 millimolar external potassium; the magnitude of these effects is directly related to the internal sodium concentration; both effects are abolished by cyanide. 相似文献
5.
Solute concentration gradients in frog muscles at 0 degree C: active transport or adsorption? 总被引：2，自引：0，他引：2
M C Neville 《Science (New York, N.Y.)》1972,176(32):302-303
In isolated frog muscle that has been incubated for 7 days at 0 degrees C invitro K(+) and Na(+) remain at normal concentrations. Amino acids are accumulated against a concentration gradient at this temperature; for example, glycine accumulates in muscle to a concentration ten times that in the external solution. The amount of cycloleucine accumulated is greater at 0 degrees C than at 25 degrees C. These findings, which are difficult to explain on the basis of metabolically linked active transport, are Consiststent with the view that solute accumulation by cell is the result of adsorption on spesific sites. 相似文献
6.
Tarascon JM Greene LH McKinnon WR Hull GW Geballe TH 《Science (New York, N.Y.)》1987,235(4794):1373-1376
Structural, magnetic, and electronic properties of compounds in the series La2-xSrx CuO4-y for 0.05 相似文献
7.
The study of magnetic phase transitions in insulating molecular solids provides new insights into mechanisms of magnetic coupling in the solid state and into critical phenomena associated with these transitions. Only a few such materials are known to display cooperative magnetic properties. The use of high-spin molecular components would enhance intermolecular spin-spin interactions and thus a series of chargetransfer (CT) salts have been synthesized that utilize the spin S = 1 molecular cation, [Mn(C(5)(CH(3))(5))(2)](+) (decamethylmanganocenium). The structure and cooperative magnetic behavior of [Mn(C(5)(CH(3))(5))(2)](+)[TCNQ(-) (decamethylmanganocenium 7,7,8,8-tetracyano-p-quinodimethanide) are reported. This salt is a bulk molecular ferromagnet with the highest critical (Curie) temperature (T(c) = 6.2 K) and coercive field (3.6 x 10(3) gauss), yet reported for such a material. 相似文献
8.
Mutations affecting internal TEA blockade identify the probable pore-forming region of a K+ channel 总被引：41，自引：0，他引：41
The active site of voltage-activated potassium channels is a transmembrane aqueous pore that permits ions to permeate the cell membrane in a rapid yet highly selective manner. A useful probe for the pore of potassium-selective channels is the organic ion tetraethylammonium (TEA), which binds with millimolar affinity to the intracellular opening of the pore and blocks potassium current. In the potassium channel encoded by the Drosophila Shaker gene, an amino acid residue that specifically affects the affinity for intracellular TEA has now been identified by site-directed mutagenesis. This residue is in the middle of a conserved stretch of 18 amino acids that separates two locations that are both near the external opening of the pore. These findings suggest that this conserved region is intimately involved in the formation of the ion conduction pore of voltage-activated potassium channels. Further, a stretch of only eight amino acid residues must traverse 80 percent of the transmembrane electric potential difference. 相似文献
9.
Regulation of a heart potassium channel by protein kinase A and C 总被引：28，自引：0，他引：28
The enzymes adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase (protein kinase A) and protein kinase C regulate the activity of a diverse group of cellular proteins including membrane ion channel proteins. When protein kinase A was stimulated in cardiac ventricular myocytes with the membrane-soluble cAMP analog 8-chlorphenylthio cAMP (8-CPT cAMP), the amplitude of the delayed-rectifier potassium current (IK) doubled when recorded at 32 degrees C but was not affected at 22 degrees C. In contrast, modulation of the calcium current (ICa) by 8-CPT cAMP was independent of temperature with similar increases in ICa occurring at both temperatures. Stimulation of protein kinase C by phorbol 12,13-dibutyrate also enhanced IK in a temperature-dependent manner but failed to increase ICa at either temperature. Thus, cardiac delayed-rectifier potassium but not calcium channels are regulated by two distinct protein kinases in a similar temperature-dependent fashion. 相似文献
10.
Hersh LS 《Science (New York, N.Y.)》1969,164(3876):179-181
The adsorption of dodecyl, tetradecyl, hexadecyl, and octadecyl trimethylammonium chlorides at an interface between porous glass and potassium chloride solution has been characterized by measurements of membrane potentials. The specific potential Phi is 0.97 kT per methylene group (where k is the Boltzmann constant and T is the absolute temperature) or 580 calories per mole at 23 degrees C. 相似文献
11.
Yannas I 《Science (New York, N.Y.)》1968,160(3825):298-299
The vitrification temperature (T(g)) of water is an elusive quantity because of the great difficulty in supercooling water sufficiently without initiating its crystallization. Water depresses the vitrification temperature of glycerol (184 degrees K) monotonously, and this property has been used for derivation of an estimate of its T(g). The result, 127 degrees +/- 4 degrees K (-146 degrees +/- 4 degrees C), is the temperature below which water presumably exists as a brittle, glassy substance. 相似文献
12.
E C Crawford 《Science (New York, N.Y.)》1972,177(47):431-433
Panting in Sauromalus obesus is effective enough to keep deep body temperature (T(C)) and brain temperature (T(B)) below an ambient temperature of 45 degrees C for extended periods of time and has a greater cooling effect on the brain than on the remainder of the body. Six animals maintained T(C) and T(B) 0.9 degrees C (+/- 0.08 standard error) and 2.7 degrees C (+/- 0.2 standard error) respectively lower than the ambient temperature of 45 degrees C. It is possible that intracranial vascular shunts play a role in cranial cooling during panting. 相似文献
13.
Sparn G Thompson JD Huang SM Kaner RB Diederich F Whetten RL Grüner G Holczer K 《Science (New York, N.Y.)》1991,252(5014):1829-1831
The superconducting compound K(3)C(60) (with transition temperature T(c) = 19.3 kelvin at ambient pressure), formed as a single phase by reaction of alkali vapor with solids of the icosahedral C(60) molecule (buckminsterfullerene), shows a very large decrease of T(c) with increasing pressure. Susceptibility measurements on sintered pellets showing bulk superconductivity are reported up to 21 kilobars of pressure, where T(c) is already less than 8 kelvin. The results are consistent with a piling up of the density of states at the Fermi level. 相似文献
14.
【目的】针对辽宁省花生产地速效钾含量偏低,钾肥施用量不足的现状,探究不同钾水平对花生根系、根瘤特性、主要养分吸收速率以及产量的影响,以期明确最佳的钾肥施用量,为当地的花生生产提供理论依据和参考。【方法】本试验于2018—2019年在沈阳农业大学进行,以农花9号为试材,设0（CK）、112.5 kg·hm-2（T1）、225 kg·hm-2（T2）和337.5 kg·hm-2（T3）4个钾肥处理,采用大田随机区组试验,研究了不同施钾量对花生根系干物质积累、根系形态、根瘤特性、植株养分吸收和产量的影响。【结果】2年试验结果表明,施钾对花生根系干重影响不大,各处理的根系干物质积累量无显著差异;与未施钾肥的CK相比,T2处理的施钾量可显著增加花生的总根长、总根表面积和总根体积;根瘤对钾素较敏感,在施钾量为112.5 kg·hm-2时,根瘤数量及干重在各时期均达到最大,钾素的施入促进了根瘤在花生结荚期的衰老;增施钾肥提高了植株氮、磷、钾素的最大积累速率、平均积累速率和最大积累量,延长了氮和钾的活跃积累期;施钾225 kg·hm-2时单株荚果数等产量构成因素及产量均高于其他处理。【结论】施钾量为225 kg·hm-2（T2）时,最有利于花生根系生长和养分吸收,增产效果最好;在施钾量为112.5 kg·hm-2（T1）时,最有利于根瘤的生长,增产效果次之。 相似文献
15.
Sanidine: Predicted and Observed Monoclinic-to-Triclinic Reversible Transformations at High Pressure
Hazen RM 《Science (New York, N.Y.)》1976,194(4260):105-107
High sanidine, (K,Na)AlSi(3)O(8), transforms reversibly to a triclinic phase at high pressure. This is analogous to the high-temperature monalbite transformation. Disordered alkali feldspars of various compositions have unit-cell dimensions which are very similar at the transition (a = 8.30 A, b = 12.97 A, c = 7.14 A, and beta = 116.2 degrees ), indicating that the transformation is structurally controlled. Changes in temperature, pressure, and the sodium/potassium ratio cause similar structural variations: angles between adjacent, rigid tetrahedra vary to accommodate changing effective alkali cation sizes. 相似文献
16.
[目的]明确外源物质根系调控促进烤烟养分吸收和利用的作用机制,为进一步完善基于烤烟提质增效的外源根系调控技术提供科学依据.[方法]采用盆栽试验,设常规施肥(T1)和在常规施肥基础上添加苹果酸(T2)、生根粉(T3)、复硝酚钠与海藻酸钠混合剂(T4)、解淀粉芽孢杆菌FZB42(T5)和膨胀素(T6)6个灌根处理,同时设植烟不施肥处理(CK)用于计算肥料利用率.于团棵期、现蕾期、采收结束时分别测定烟株根、茎、叶干重及烟叶的氮、磷、钾含量,同时测定土壤速效氮、速效磷和速效钾含量.[结果]相对于T1和CK处理,T4、T5和T6处理显著提高了烤烟各生长阶段根系生长速率(P<0.05,下同),增加了根系和地上部干物质积累量,提高了烟叶对氮、磷、钾营养元素的吸收量,而各处理烟叶占地上部干物质积累的比例随着生育进程推进逐渐减小.回归分析结果显示,烤烟团棵至现蕾阶段根系干物质积累量与烟叶氮(R2=0.87)、磷(R2=0.94)、钾(R2=0.88)元素的吸收量均呈极显著正相关(P<0.01),此阶段根系生长速率的显著提高是外源物质促进烤烟叶片养分吸收和利用的关键.T4和T5处理提高团棵至现蕾阶段根系生长速率50%以上,显著增加了烟株的干物质积累量,烟叶氮素吸收量分别增加46%和66%,磷素吸收量分别增加77%和118%,钾素吸收量分别增加58%和69%.[结论]基于烤烟根系促生效果的稳定性和应用安全性,解淀粉芽孢杆菌FZB42灌根可作为玉溪红壤烟区烤烟根系调控的主要手段. 相似文献
17.
In frog sartorius muscles immersed for 2 hours at 26 degrees C in normal Ringer solution, the intrafiber potassium concentration, C(K)(in millimoles per liter), was 123 +/- 2 (mean value plus or minus standard error), and the potassium activity, a(K) (in millimoles per liter), was 90 +/- 1.0. The corresponding sodium concentration and activity were 20 +/- 1 and 6.5 +/- 0.4, respectively. After overnight immersion in K+-free Ringer solution the values were: CK, 97 +/- 2; aK, 81.5 +/- 1.6; CNa, 47 +/- 2; and aNa, 11.2 +/- 0.6. The changes in aK and aNa during storage were not consistent with an exchange between predominantly "free" fiber K+ and external Na+. These results suggest that the Na+ taken up during overnight immersion largely replaced adsorbed or sequestered K+ in the fibers. 相似文献
18.
Isolated adrenal cells: adrenocorticotropic hormone, calcium, steroidogenesis, and cyclic adenosine monophosphate 总被引：8，自引：0，他引：8
Corticosterone production by isolated adrenal cells in response to adrenocorticotropic hormone is reduced when the cells are incubated in a medium that contains no calcium. This reduction is associated with an equal reduction of accumulation of cyclic adenosine monophosphate. Production of corticosterone and accumulation of cyclic adenosine monophosphate are increased when the calcium concentration in the medium is increased (from zero to 7.65 millimolar). This is in contrast to the situation in "subcellular membrane fragments" of adrenal tissue where high calcium in the medium (> 1.0 millimolar) inhibits cyclic adenosine monophosphate accumulation. We propose that adenyl cyclase in the intact plasma membrane is located in a compartment wherein calcium concentration is low and remains unaffected by the concentration of calcium in the extracellular space. It is proposed that, as the concentration of calcium in the incubation medium is increased from zero to 7.65 millimolar, the strength of the signal generated by the interaction of adrenocorticotropic hormone with its receptor and transmitted to the adenyl cyclase compartment is proportionately increased. 相似文献
19.
Tunneling spectroscopy has been used to characterize the magnitude and temperature dependence of the superconducting energy gap (triangle up) for K(3)C(60) and Rb(3)C(60). At low temperature the reduced energy gap, 2triangle upkappaT(c) (where T(c) is the transition temperature) has a value of 5.3 +/- 0.2 and 5.2 +/- 0.3 for K(3)C(60) and Rb(3)C(60), respectively. The magnitude of the reduced gap for these materials is significantly larger than the value of 3.53 predicted by Bardeen-Cooper-Schrieffer theory. Hence, these results show that the pair-coupling interaction is strong in the M(3)C(60) superconductors. In addition, measurements of triangle up(T) for both K(3)C(60) and Rb(3)C(60) exhibit a similar mean-field temperature dependence. The characterization of triangle up and triangle up(T) for K(3)C(60) and Rb(3)C(60) provides essential constraints for theories evolving to describe superconductivity in the M(3)C(60) materials. 相似文献
20.
Wittig J 《Science (New York, N.Y.)》1967,155(3763):685-686
The high-pressure metallic phase of aluminunm antimnonide is super conducting [critical temperature T(c) (P approximately 125 kilobars) = 2.8 degrees +/-0.2 degrees K]. This transition temperature is significantly lower than the transition temperature of metallic germanium under an equivalent high pressure. A similar result had been previously found for superconducting indiumantimonide in comparison to tin. 相似文献