The action of trypsin and neuraminidase on the synaptic membranes of brain cortex

Lectin-induced retardation of subcellular organelles during preparative density gradient electrophoresis: selective purification of plasma membranes

Plasma membranes (PM) are difficult to separate by conventional means from other cellular compartments. Using a density gradient electrophoresis (DGE) apparatus (7 cm, x 2.2 cm), mammalian subcellular organelles were separated from a total postnuclear supernatant. The sialic acid-binding lectin wheat germ agglutinin (WGA) permitted 1.5-fold electrophoretic retardation of plasma membranes lagging far behind endoplasmic reticulum, endosomes, Golgi and lysosomes (in order of increasing electrophoretic mobility). Mobilities of the latter organelles were not affected by wheat germ agglutinin. The retarded plasma membrane was monitored by surface iodination, the presence of Ca(++)- and Na+/K(+)-ATPases and by the presence of clathrin-coated pits using Western immunoblotting. In the presence of WGA two clathrin-containing compartments were detected; in the absence of WGA three clathrin populations were seen in the electropherogram: clathrin-coated vesicles, clathrin-coated pits (on the PM) and clathrin-coated structures on the trans-Golgi network (TGN). Both in the presence and absence of WGA, plasma membrane domains of different electrophoretic mobilities were detected.

High-resolution density gradient electrophoresis of proteins and subcellular organelles

Following a concept developed by Bier et al. (Electrophoresis 1993, 14, 1011-1018), binary mixtures of amphoteric buffers with low conductivity and a good buffering capacity permit rapid rate zonal separation of proteins on a density gradient electrophoresis apparatus (7 cm, x 2.2 cm). At pH 8.66 and 250 V, beta-lactoglobulin (Mr 36600) was separated into the A and B isoforms within 44 min; human transferrin (Mr 76000-81000) was separated into its sialylated glycoforms and carbonic anhydrase (Mr 30000) separated into its isoenzymes. From these results we arrive at the term high-performance density gradient electrophoresis. Compartments belonging to the endosomal system were separated by density gradient electrophoresis. Early endosomes, recycling vesicles, intermediate endosomes, late endosomes and lysomes became well-separated after 80 min at 10 mA using [125I]transferrin and horseradish peroxidase as reporter molecules in pulse-chase regimes. Mixtures of Bier buffers and standard electrophoresis media permitted very short separation times (19 min at 10 mA) for the endosomal compartments. Concommittantly, endoplasmic reticulum and proteasomes were well resolved.

High resolution density gradient electrophoresis of cellular organelles

A density gradient electrophoresis apparatus made of Perspex was constructed, with a separation column (7 x 2.2 cm) containing a 0-5% linear Ficoll gradient. The useful separation path is 6 cm. A specially designed gradient mixer is described which fits over the application cone. This cone permits precise gradient and sample introduction as well as undisturbed fractionation after electrophoresis. A bottom circular palladium cathode is separated hydrodynamically but not electrically from the density gradient by a cellophane membrane, merely secured by an O-ring. The top circular platinum anode allows for upward electrophoresis (80-100 min at 10 mA). The markedly higher resolution of subcellular organelles was compared with separations obtained earlier with a small, but much more difficult to fabricate, prototype. Moreover, ease of manipulation was greatly improved. A wide separation distance was obtained between plasma membrane, endoplasmatic reticulum as well as between two populations of lysosomes. Even early, middle, and late endosomes could be separated with high resolution. Soluble isoenzymes could be separated as well and were far away from the vesicle-enclosed enzymes.

Studies on the contracture inducing action of triphenyltin in the mouse diaphragm

Triphenyltin induces a contracture of the mouse phrenic nerve-diaphragm preparation. This contracture was not inhibited by (+)-tubocurarine, high magnesium or the absence of electrical stimulation. Triphenyltin (0.1 mM) reduced the muscle membrane potential, the amplitude of the muscle action potential and the muscle membrane input resistance. Pretreatment with high K+ (25 mM) or veratridine (1.5 microM; a Na+ channel activator) briefly shortened the onset of the contracture and increased the peak tension of the contracture. Pretreatment with tetrodotoxin (0.3 microM; a Na+ channel blocker) or glycerol (a T tubule uncoupler) however, significantly reduced the triphenyltin-induced contracture. Removing Ca2+ from external solution and prolonged treatment with either caffeine (20 mM) or ryanodine (2 microM) inhibited the triphenyltin-induced contracture. However, a brief treatment with a lower concentration of caffeine (10 mM) potentiated the contracture. 45Ca2+ uptake studies showed that triphenyltin caused the muscle to accumulate Ca2+ which entered from external solution. Pretreatment with trypsin and dithiothreitol (a sulfhydryl-containing reducing agent) blocked the contracture induced by triphenyltin. These results suggest that triphenyltin initially interacts with the sulfhydryl groups of membrane bound proteins (possibly the Na+ channel) to cause depolarization of the muscle fibres. This depolarization triggers the release of Ca2+ from sarcoplasmic reticulum through the mechanism of Ca2+ inducing Ca2+ release, activates the contractile filaments and causes the muscle to contract.

Neuromuscular actions of sodium selenite on chick biventer cervicis nerve-muscle preparation

Sodium selenite was found to be toxic to chicks, with an LD50 of 8.5 micrograms/g, which was increased to 16.3 micrograms/g by NaCN. The major symptoms of chicks, treated with selenite, were sedation and then dyspnea and paralysis. The cause of death by selenite was apparently due to the respiratory failure. The possible mechanism of toxicity was explored in the isolated chick biventer cervicis nerve-muscle preparation. Selenite initially increased the amplitude of the twitch, reversed the suppression of the twitch caused by d-tubocurarine, Mg2+, Cd2+ or Mn2+ and significantly increased the quantal content and amplitude of endplate potentials. Subsequently, selenite depressed the amplitude of the twitch, blocked the axonal conduction and inhibited excitatory postsynaptic potentials. Both NH4+ and K+ enhanced the action of selenite in depressing the twitches. In addition, selenite induced a sustained contracture of the muscle, which was partially inhibited by removal of external Ca2+ and markedly blocked by EGTA. Entry of Ca2+ and release of the internal Ca2+ were considered to be responsible for inducing contracture by selenite. Pretreatment with trypsin, glutathione (GSH) and cyanide profoundly inhibited the effects of selenite, indicating that the site of action of selenite was on the outer membrane and the binding of selenite to the sulfhydryl groups of membrane proteins was proposed to be an essential step for selenite-induced contracture and neuromuscular action. These findings suggest that neuromuscular blockade and tetanic spasm, produced by selenite in chicks, may play a role in causing respiratory failure in vivo.

Studies on the contracture of the mouse diaphragm induced by sodium selenite

In this study, we found that sodium selenite was potent in inducing contracture of the mouse diaphragm. The possible mechanism of action of selenite was investigated. Contracture was induced by a direct action of selenite on the muscle membrane rather than that selenite enhanced transmitter release from the motor nerve terminals, since denervation, d-tubocurarine and tetrodotoxin did not inhibit the selenite-induced contracture. Although selenite decreased both the membrane potential and the amplitude of the muscle action potential, neither high K+ nor glycerol treatment, which closed the transverse tubule, reduced the selenite-induced contracture, suggesting that depolarization of the muscle membrane was not essential for the induction of the contracture. EGTA (1-50 mM) inhibited the selenite-induced contracture in a concentration-dependent manner. In contrast, varying the external Ca2+ concentrations from 10(-3) to 10 mM or raising Mg2+ concentration to 10 mM did not affect the contracture. Similarly, the contracture induced by caffeine was not affected by lowering the external Ca2+ concentration to 10(-3) mM but was completely inhibited by 30 mM EGTA. Selenite pretreatment markedly potentiated the caffeine contracture and prolonged treatment with caffeine inhibited the selenite contracture. All of these findings suggest that the selenite contracture was not dependent on external Ca2+ but was induced by the release of Ca2+ from internal membranes such as the sarcoplasmic reticulum. Pretreatment with trypsin, glutathione or cyanide blocked the selenite-evoked contracture. Therefore, we postulate that the selenite-induced contracture was induced by the initial binding of selenite to the sulfhydryl groups of the muscle membrane, which then triggered the release of Ca2+ from internal membranes such as the sarcoplasmic reticulum.