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31.
Summary The binding of saxitoxin, a specific inhibitor of the sodium conductance in excitable membranes, has been measured in giant axons from the squid,Loligo pealei. Binding was studied by labeling saxitoxin with tritium, using a solvent-exchange technique, and measuring the toxin uptake by liquid scintillation counting. Total toxin binding is the sum of a saturable, hyperbolic binding component, with a dissociation constant at 2–4°C of 4.3±1.7nm (meanse), and a linear, nonsaturable component. The density of saturable binding sites is 166±20.4 m–2. From this density and published values of the maximum sodium conductance, the conductance per toxin site is estimated to be about 7 pS, assuming sequential activation and inactivation processes (F. Bezanilla & C.M. Armstrong, 1977,J. Gen. Physiol.
70: 549). This single site conductance value of 7 pS is in close agreement with estimates of the conductance of one open sodium channel from measurements of gating currents and of noise on squid giant axons, and is consistent with the hypothesis that one saxitoxin molecule binds to one sodium channel. 相似文献
32.
The composition of intracellular granules from the metal-accumulating cells of the common garden snail (Helix aspersa).
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Certain cells in the hepatopancreas of the common garden snail (Helix aspersa) contain intracellular granules that are sites of metal-ion accumulation. These granules have been extracted and investigated by u.v. and i.r. spectroscopy, atomic-absorption spectroscopy, X-ray microanalysis, thermogravimetric analysis, enzymic assay and microanalysis. The deposits contain about 18% (w/w) water, 5% (w/w) organic matter and 76% (w/w) inorganic material of which the main components are Ca2+, Mg2+ and P2O7(4)-. The possible origin of these granules is discussed, as is their role in detoxifying heavy-metal ions. 相似文献
33.
A J Ritchie D R Johnson B M Ewenstein J S Pober 《Journal of immunology (Baltimore, Md. : 1950)》1991,146(9):3056-3062
We have investigated whether TNF-induced changes in human endothelial cell (EC) surface Ag expression are mediated by protein kinase C (PKC). This suggestion arose from the observations that PMA, a potent PKC activator, can mimic TNF by inducing expression of endothelial leukocyte adhesion molecule 1, intercellular adhesion molecule 1 (ICAM-1), and class I MHC molecules on human EC. However, in contrast to the actions of PMA, TNF neither causes membrane translocation of PKC nor induces the phosphorylation of the myristoylated alanine-rich C kinase substrate, two measures of PKC activation. Moreover, the PKC inhibitor staurosporine can block PMA-induced endothelial leukocyte adhesion molecule 1 expression at 4 h, but does not inhibit the actions of TNF. At 24 h, staurosporine itself induces intercellular adhesion molecule 1 and class I MHC, and acts additively with TNF. Twenty four hour treatment with PMA causes loss of PKC. We propose that at 24 h, staurosporine and PMA share a mechanism of action, namely diminution of PKC activity. However, 24 h treatment with TNF does not reduce the amount of PKC nor does it prevent activation of PKC by PMA. We conclude that TNF effects in EC are not mediated by PKC activation or inactivation. 相似文献
34.
A procedure has been developed for transforming protoplasts of the novobiocin producing strain Streptomyces niveus at high frequency. This required the isolation of strains LH13 and LH20 defective in DNA restriction from the wild type (ATCC 19793) which is transformed at very low frequencies. The LH13 and LH20 derivatives were obtained by curing pIJ702 DNA from the few S. niveus transformed protoplasts obtained by transformation of the wild type with high concentrations of pIJ702 DNA. Protoplasts of S. niveus strains LH13 and LH20 produced about 10(6) transformants/micrograms DNA with modified pIJ702 DNA derived by replication in S. niveus. Unmodified DNA (derived from replication in S: lividans) from a series of pIJ101, SCP2 and pSN2-based derivatives, gave transformation frequencies in the range of 10(2)-10(3) transformants/micrograms DNA. Optimal conditions for the formation and transformation of S. niveus protoplasts are described. 相似文献
35.
I Wilmut W A Ritchie C S Haley C J Ashworth R P Aitken 《Journal of reproduction and fertility》1992,95(1):45-56
A comparison was made of the rate and uniformity of development of embryos recovered from Meishan and European white sows. The time of ovulation was estimated to be 34.3 and 49.0 h after the onset of oestrus in large white and Meishan sows, respectively. Embryos were recovered from a total of 38 Meishan and 37 European pigs between 18 and 219 h after the estimated time of ovulation. Embryos recovered after 18-59 or 44-82 h were classified into one of 11 stages (from early fertilization to early blastocyst), and the maximum blastocyst diameter was measured for embryos recovered 140-219 h after ovulation. There was no evidence of a difference between the genotypes in the stage or size of embryos at these times or of large differences between the genotypes in the extent of variation in embryo stage within females, although a minority of European white females had very variable embryos. As the differences between the embryos of the Meishan and the European white were small, it seems unlikely that greater uniformity of Meishan embryo development is a major cause of the higher prenatal survival in that breed. 相似文献
36.
A. Balmain L. Frew G. Cole R. Krumlauf A. Ritchie G.D. Birnie 《Journal of molecular biology》1982,160(2):163-179
37.
J V Howarth J M Ritchie D Stagg 《Proceedings of the Royal Society of London. Series B, Containing papers of a Biological character. Royal Society (Great Britain)》1979,205(1160):347-367
A study has been made of the temperature changes associated with the passage of a single impulse in the non-myelinated fibres of the garfish olfactory nerve: and the time course of these temperature changes has been compared with the time course of the electrical events during the action potential. As in other non-myelinated nerves studied the observed temperature changes result from a biphasic initial heat production consisting of a transient evolution of heat (the positive heat) followed by a rapid heat reabsorption (referred to as the negative heat). There is no evidence of any additional phases of initial heat production. At 0 degrees C the measured positive initial heat is 224 mucal/g impulse (937 muJ/g impulse); and the corresponding negative initial heat is 230 mucal/g impulse (962 muJ/g impulse). The residual initial heat is very small, being about -6 mucal/g impulse (-25 muJ/g impulse). In the range 0-10 degrees C there is no significant effect of temperature on the magnitude of either the positive or the negative phases of heat production. The experimental thermal records were analysed to determine the true time course of the temperature changes in the nerve undistorted by the recording system. The time course of the temperature changes does not fit with that of the transmembrane voltage change as represented by the monophasic compound action potential recorded externally from the same point on the nerve. A better fit is obtained if the temperature changes are compared with the square of the voltage change in accordance with the view that the heat derives almost wholly from free energy changes and entropy changes in the membrane capacity. The best fit is obtained if it is assumed that the membrane potential does not discharge to zero during the action potential but that at the peak of the action potential the charge (and hence the p.d.) across the membrane capacity retains about 24% of its resting value. 相似文献
38.
The acetylcholine reversal potential (Er) of cultured rat myotubes is -3mV. When activated, the receptor is permeable to K+ and Na+, but not to Cl- ions. Measurement of Er in Tris+-substituted, Na-free medium also indicated a permeability to Tris+ ions. Unlike adult frog muscle the magnitude of Er was insensitive to change in external Ca++ (up to 30 mM) or to changes in external pH (between 6.4 and 8.9). The equivalent circuit equation describing the electrical circuit composed of two parallel ionic batteries (EK and ENa) and their respective conductances (gK and gNa), which has been generally useful in describing the Er of adult rat and frog muscle, could also be applied to rat myotubes when Er was measured over a wide range of external Na+ concentrations. The equivalent circuit equation could not be applied to myotubes bathed in media of different external K+ concentrations. In this case, the Er was more closely described by the Goldman constant field equation. Under certain circumstances, it is known that the receptor in adult rat and frog muscle can be induced to reversibly shift from behavior described by the equivalent circuit equation to that described by the Goldman equation. Attempts to similarly manipulate the responses of cultured rat myotubes were unsussessful. These trials included a reduction in temperature (15 degress C), partial alpha-bungarotoxin blodkade, and activation of responses with the cholinergic agonist, decamethonium. 相似文献
39.
1. The interaction of a variety of fluorescent probes with the membranes of adrenal medullary chromaffin granules is described. 2. Changes in the motional properties of the bound probes with temperature were investigated and evidence is presented which indicates that ordering of the membrane lipids occurs below 33 degrees C. 3. The ordering is characteristics of the membrane lipids and is retained by sonicated aqueous dispersions of the total lipid extracted from chromaffin granule membranes. 4. The ATPase and NADH:acceptor oxidoreductase activities of the chromaffin granule membrane have discontinuous Arrhenius temperature versus activity relationships with 'transitions' at 33 degrees C. 5. The ATPase has a second transition at 36.5 degrees C. 6. The 33 degrees C 'transition' for the NADH:acceptor oxidoreductase is removed by treatment with the detergent Triton X-100. 7. The correlation between the onset of lipid ordering and the change in activation energy of the membrane-bound enzyme activities is discussed in terms of the co-operative interactions of the different membrane components. The possible role of lipid ordering in exocytosis is discussed. 相似文献
40.
The structure of the chromaffin granule membrane has been probed using a number of different spin labels. Both the effect of temperature and high levels of calcium have been studied. 1. The results from three positional isomers of the stearic acid spin label demonstrate that a substantial part of the membrane lipid (that is sensed by the probe) is in a bilayer structure which undergoes a structural transition at 32-36 degrees C, characterized by an increase in the population of gauche isomers in the lipid chains. A possible mechanism for this transition would be the preferential segregation of cholesterol. 2. The covalently bound iodoacetamide spin label reveals a transition within the protein component of the membrane or its immediate lipid environment at 32 degrees C. This transition corresponds to an increased degree of motional freedom of the spin label above the transition temperature. 3. The lipid-soluble spin label 2,2,6,6-tetramethyl-piperidine-1-oxyl exhibits a break at 34 degrees C in the temperature-dependence of its partitioning into the membrane. This could correspond to the onset of a lateral separation in the membrane lipid, again possible involving a re-distribution of cholesterol. 4. Calcium abolishes, diminishes or shifts the transition observed by the spin label and decreases the amplitude of motion of the stearic acid spin labels, again possibly involving a redistribution of cholesterol and also lysolecithin. The temperatures of the structural transition agree well with the changes in the enzymic activity of the membrane ATPase and NADH oxidase functions and also with the results from fluorescent probes [Bashford et al., Eur. J. Biochem. 67, 105-114(1976)]. It is possible that triggering of the transition either by calcium or some other stimulus may play a role in catecholamine release and membrane fusion. 相似文献