Summary. --We report here about measurements of resistance vs. temperature, alternating current (a.c.) susceptibility and X-ray diffraction performed on Bae Sr 2 Cae Cu30x superconductor doped with various amounts of Pb. The results could be explained in terms of Pb contribution in increasing both the amount of the highest To0 phase and connectivity between superconductive regions. The Pb substitution on Bi sites is also discussed.PACS 74.70 -Superconducting materials.Recently large interest has been devoted to the influence of Pb doping on BSCCOtype superconductors. This because the addition of Pb on BSCCO sintered sample has the effect of raising both zero-resistance transition temperature Tco and critical current density [l, 2, 3]. The reason why Pb produces these variations and the knowledge of the stoichiometry of the BSCCO are not yet established. Some authors [4] think that the Pb substitution of Bi sites on the Bi2 Sr2 Ca2 Cu30~ phase is the reason of Tco variation. Other authors [5] think that Pb acts like a flux favouring both the growth of higher-Tc0 superconductor phase and the improving of superconductor grain coupling.In order to better understand the role of Pb doping on BSCCO superconductor, we have detected, in the sintered bulk samples, resistive transition and alternating current (a.c.) susceptibility. X-ray diffraction pattern on powder has been obtained in dependence of various amounts of Pb referred to undoped sample. The undoped sample was obtained by a standard solid-state reaction [6]. The material was then pressed into a disk of 25 mm diameter, about 1 mm thick, and sintered in air at 865~ for a period of 120 h followed by an annealing period of 50 h at 450 ~ and final quench in air at room temperature. The doping procedure was performed by adding a fraction of PbO, ranging from 0.05 to 0.75 mole, plus a CuO mole, to the prereacted powder of BSCCO. The powdered mixture, for the lowering of melting point, was calcinated at 820 ~ for 88 -Il Nuovo Cimento D 1315
The fact that one of the major actions of insulin on muscle is to stimulate passage of glucose across the membrane makes it unlikely ~hat effects of the hormone are to be seen in cell-free systems, and indeed few observations of this sort have been reported. Likewise, if the stimulation by insulin of incorporation of amino acids into protein arises from an action of the hormone to regulate the intracellular distribution and availability of ATP we should again expect the capacity of a tissue to respond to insulin to be lost on homogenisation. If, on the other hand, the effect of insulin on amino acid incorporation were to result from a direct interaction of the hormone with the protein synthesizing system, such an effect might still be seen in a cell-flee system. It was thus thought of interest to seek to establish whether insulin could influence amino acid incorporation in homogenates or under conditions in which the membrane characteristics of muscle were varied by changes in the ion content of the supporting buffer. In general the effect of insulin is nbt seen in these broken cell preparations, from Which observations conclusions as indicated above can be drawn. However, a number of behavioural characteristics of the incorporating system lead to the belief that the optimal conditions for protein synthesis in cell-free preparations have yet to be discovered and an effect of insulin in these circumstances may be dependent on satisfying a number of conditions as yet inadequately determined. It is hoped to discuss observations bearing on these problems.
Some relationships between cation transport, carbohydrate metabolism and insulin action in muscle.T. CLAUSEI~. Inst. of Physiology, Univ. of Aarhus.Isolated rat and mouse hemidiaphragms were incubated in Krebs-Ringer bicarbonate buffer. The active transport of Na + and I~+ across cell membranes is known to be decreased in potassium-free media. Lowering of the K+-concentration from 6 to 0 mM/L was shown to cause up to 200% stimulation of 14C-glycogen synthesis in the diaphragm. This insulin-like effect was not paralleled by an increased glucose uptake. However, the increased glycogen synthesis was followed by a decreased lactate production. Ouabain (g-strophantine), which is a potent inhibitor of active cation transport was shown at normal K+-concentration to produce essentially the same effects when added to the incubation medium. Changes in the sodium concentration of the incubation medium had the following effects : the glucose uptake and lactic acid production were stimulated at concentrations of sodium higher than normal, and inhibited at concentrations lower than normal; the stimulation of glucose uptake by insulin (0.1 i. u./ml) was most pronounced at 143 mmol of sodium, being diminished both at the higher and the lower concentration of sodium; the stimulatory effect of insulin upon glycogen synthesis and lactate production was correspondingly diminished at high and low concentrations of sodium. It is suggested that the cation transport plays a central role in...
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