Uncoupling of an epithelial cell membrane junction by calcium-ion removal

Calcium takes part in maintaining ion communication between salivary gland cells (Chironomus thummi). Its withdrawal from the cell systems results in virtual disconnection of ion communication, at Ca(++) concentrations which do not noticeably affect cell adhesion. The junctional membrane surfaces. which are normally quite freely permeable to ions, become as impermeable as the nonjunctional membrane surfaces; each cell seals itself off irreversibly as a unit. In maintaining ion communication Mg(++) substitutes for Ca(++)     

Gap junctional conductance and permeability are linearly related

The permeability of gap junctions to tetraethylammonium ions was measured in isolated pairs of blastomeres from Rana pipiens L. and compared to the junctional conductance. In this system, the junctional conductance is voltage-dependent and decreases with moderate transjunctional voltage of either sign. The permeability to tetraethylammonium ions was determined by injecting one cell of a pair with tetraethylammonium and monitoring its changing concentration in the prejunctional and postjunctional cells with ion-selective electrodes. Junctional conductance was determined by current-clamp and voltage-clamp techniques. For different cell pairs in which the transjunctional voltage was small and the junctional conductance at its maximum value, the permeability to tetraethylammonium ions was proportional to the junctional conductance. In individual cell pairs, a reduction in the junctional conductance induced by voltage was accompanied by a proportional reduction in the permeability of the gap junction over a wide range. The diameter of the tetraethylammonium ion (8.0 to 8.5 A, unhydrated) is larger than that of the potassium ion (4.6 A, hydrated), the predominant current-carrying species. The proportionality between the permeability to tetraethylammonium ions and the junctional conductance, measured here with exceptionally fine time resolution, indicates that a common gap junctional pathway mediates both electrical and chemical fluxes between cells, and that closure of single gap junction channels by voltage is all or none.     

Phosphate Glass Electrode with Good Selectivity for Alkaline-Earth Cations

A phosphate glass has been found to have a significant electrode specificity toward alkaline-earth ions. The order of selectivity is 2H(+) > Ba(++) > Sr(++) > Ca(++) > 2K(+) > 2Na(+) > Mg(++). Exchange properties are discussed in relation to possible structure. Its use to determine activity of Ca(++) in natural systems containing Mg(++) is sug gested.     

Tetrodotoxin and manganese ions: effects on electrical activity and tension in taenia coli of guinea pig

Tetrodotoxin, at concentrations up to 5 x 10(-6) gram per milliliter, has no effect on the spontaneous discharge in the smooth muscle of taenia coli. However, the spontaneous discharge is abolished by Mn(++) at a concentration of 0.5 millimole per liter. The contraction induced by immersing the muscle in isotonic KCl solution is also suppressed in the presence of Mn(++). Because Mn(++) is a specific suppressor of the spike induced by Ca(++) and tetrodotoxin is an inhibitor of the spike induced by Na(+), we suggest that Ca(++) is a charge carrier in the production of spike potential in the smooth muscle and that the entry of intervening Ca(++) through the membrane acts as a trigger for the contraction of smooth muscle.     

Measurement of single channel currents from cardiac gap junctions

Cardiac gap junctions consist of arrays of integral membrane proteins joined across the intercellular cleft at points of cell-to-cell contact. These junctional proteins are thought to form pores through which ions can diffuse from cytosol to cytosol. By monitoring whole-cell currents in pairs of embryonic heart cells with two independent patch-clamp circuits, the properties of single gap junction channels have been investigated. These channels had a conductance of about 165 picosiemens and underwent spontaneous openings and closings that were independent of voltage. Channel activity and macroscopic junctional conductance were both decreased by the uncoupling agent 1-octanol.     

Effects of veratrine and cocaine on cerebral carbohydrate-amino acid interrelations

The rate of aerobic transformation of glucose-U-C(14) into radioactive amino acids by slices of rat-brain cortex is greatly influenced by the K(+)/Ca(++) ratio in the incubating medium. Protoveratrine has effects on the amino acid pattern resembling that due to an increase in the K(+)/Ca(++) ratio. These effects are antagonized by cocaine and may be correlated with the neurophysiological activities of these drugs.     

Calcium carbonate concretions formed by the decomposition of organic matter

Bacterial decomposition of butterfish and smelts in small sealed jars containing seawater and other solutions, for periods ranging from 65 to 205 days, results in a large increase in concentrations of dissolved bicarbonate, carbonate, and ammonia (plus volatile amines). Accompanying this is a rise in pH and the precipitation of Ca(++) ion from solution. The Ca(++) is not precipitated as CaCO(3) but instead as a mixture of calcium fatty acid salts or soaps with from 14 to 18 carbon atoms. This can be explained by the thermodynamic instability of CaCO(3) relative to Ca soaps in the presence of excess free fatty acid. It is suggested that some ancient CaCO(3) concretions, especially those enclosing fossils of soft-bodied organisms, may have formed rapidly after death in the form of natural Ca soap (adipocere) which was later converted to CaCO(3).     

Participation of an intermediate of oxidative phosphorylation in ion accumulation by mitochondria

Heart mitochondria accumulate massive amounts of Mg(++) and phosphate or Ca(++) and phosphate when incubated under appropriate conditions. Studies with inhibitors of respiration, oligomycin, and uncouplers of oxidative phosphorylation indicate that the energy necessary for these reactions may be provided either in the form of ATP in the absence of electron transport, or in the form of a high-energy intermediate of oxidative phosphorylation, which operates even though ATP is not produced.     

Prolonged Ca2+-dependent afterhyperpolarizations in hippocampal neurons of aged rats

The possibility that calcium is elevated in brain neurons during aging was examined by quantifying afterhyperpolarizations induced by spike bursts in CAl neurons of hippocampal slices from young and aged rats. The afterhyperpolarizations result from Ca2+-dependent K+ conductance increases and are blocked in medium low in Ca2+ and prolonged in medium high in Ca2+. The afterhyperpolarization and associated conductance increases were considerably prolonged in cells from aged rats, although inhibitory postsynaptic potentials did not differ with age. Since elevated intracellular Ca2+ can exert deleterious effects on neurons, the data suggest that altered Ca2+ homeostasis may play a significant role in normal brain aging.     

Junctional membrane permeability: restoration by repolarizing current

In cells of the Chironomus salivary gland, junctional membrane conductance, depressed by various chemical treatments, is restored to its normal high level by currents passed inward through nonjunctional cell membrane.     

Coupling diurnal cytosolic Ca2+ oscillations to the CAS-IP3 pathway in Arabidopsis

Various signaling pathways rely on changes in cytosolic calcium ion concentration ([Ca2+]i). In plants, resting [Ca2+]i oscillates diurnally. We show that in Arabidopsis thaliana, [Ca2+]i oscillations are synchronized to extracellular Ca2+ concentration ([Ca2+]o) oscillations largely through the Ca2+-sensing receptor CAS. CAS regulates concentrations of inositol 1,4,5-trisphosphate (IP3), which in turn directs release of Ca2+ from internal stores. The oscillating amplitudes of [Ca2+]o and [Ca2+]i are controlled by soil Ca2+ concentrations and transpiration rates. The phase and period of oscillations are likely determined by stomatal conductance. Thus, the internal concentration of Ca2+ in plant cells is constantly being actively revised.     

Residual calcium ions depress activation of calcium-dependent current

Calcium ions enter and accumulate during depolarization of some cells, activating a potassium current, IK(Ca), that depends on the cytoplasmic concentration of calcium ions, [Ca]i. However, elevation of [Ca]i can depress IK(Ca) elicited by a subsequent membrane depolarization. The depression of IK(Ca) is ascribed here to a [Ca]i-mediated inactivation of the voltage-gated calcium conductance, which causes a net reduction in calcium ions available for the activation of IK(Ca). This suggests that other processes dependent on gated calcium entry may also be depressed by small background elevations in cytosolic free calcium ions.     

卵黄、牛奶及EDTA对冷冻过程中猪精子内Zn~(++)和Ca~(++)含量的影响

冷冻过程中,卵黄能有效地抑制精子内Zn~(++)含量的升高,而牛奶则无此作用。由于卵黄和牛奶中都含有大量的Ca~(++),因此当用作保护剂时,会使冻后精子内Ca~(++)的含量大大升高,但卵黄的影响较小。如果在卵黄中加入EDTA,则能明显阻止精子内Ca~(++)含量的升高。而且,解冻后精子的存活时间也延长。试验结果表明:(1)卵黄对精子的保护作用优于牛奶;(2)卵黄和牛奶中的Ca~(++)会影响解冻后精子的存活,但添加EDTA后可得到明显改善。     

Effect of adenosine 3',5'-monophosphate on neuronal pacemaker activity: a voltage clamp analysis

Bursting pacemaker activity in nerve cells can be modified for long periods by synaptic input of short duration. There is evidence that cyclic nucleotides may play a role in these modifications. The predominant effect of elevated levels of adenosine 3',5'-monophosphate in Aplysia neurons was an increased slope conductance to hyperpolarizing pulses, evident in voltage clamp records. A similar increase in slope conductance was seen as one component of maximum strength synaptic stimulation, which is consistent with the idea that cyclic nucleotides are important in the expression of synaptic alteration of bursting pacemaker activity.     

Microdetermination of Calcium by Aequorin Luminescence

A bioluminescent protein, aequorin, isolated from the jellyfish Aequorea in dilute disodium ethylenediaminetetraacetate solution, emits light on addition specifically of Ca(++) or Sr(++), thus providing the basis for a simple, quantitative micromethod for the determination of these cations, especially in biological fluids.     

Carbon tetrachloride at hepatotoxic levels blocks reversibly gap junctions between rat hepatocytes

Electrical coupling and dye coupling between pairs of rat hepatocytes were reversibly reduced by brief exposure to halogenated methanes (CBrCl3, CCl4, and CHCl3). The potency of different halomethanes in uncoupling hepatocytes was comparable to their hepatotoxicity in vivo, and the rank order was the same as that of their tendency to form free radicals. The effect of carbon tetrachloride (CCl4) on hepatocytes was substantially reduced by prior treatment with SKF 525A, an inhibitor of cytochrome P-450, and by exposure to the reducing reagent beta-mercaptoethanol. Halomethane uncoupling occurred with or without extracellular calcium and did not change intracellular concentrations of calcium and hydrogen ions or the phosphorylation state of the main gap-junctional protein. Thus the uncoupling appears to depend on cytochrome P-450 oxidative metabolism in which free radicals are generated and may result from oxidation of the gap-junctional protein or of a regulatory molecule that leads to closure of gap-junctional channels. Decreases in junctional conductance may be a rapid cellular response to injury that protects healthy cells by uncoupling them from unhealthy ones.     

CaCl2对干旱胁迫下甘蓝型油菜抗旱性相关的生理生化指标变化影响

分析了干旱诱导下抽薹期甘蓝型油菜的生化指标和蒸腾作用变化及Ca2 处理对其影响。结果表明,在干旱胁迫下,甘蓝型油菜叶片中的脯氨酸、可溶性糖以及丙二醛含量明显增加,经Ca2 预处理其脯氨酸和可溶性糖含量明显高于仅用双蒸水处理的材料,而丙二醛含量则是蒸馏水处理的材料高于Ca2 预处理的材料;气孔导度日变化分析以及气孔观察显示,Ca2 预处理可以明显促使干旱过程中叶片表皮气孔关闭,进而使叶片蒸腾速率和气孔导度降低,从而减少了干旱条件下水分的蒸发。     

Voltage-dependent calcium channels in plant vacuoles

Free calcium (Ca(2+)) in the cytoplasm of plant cells is important for the regulation of many cellular processes and the transduction of stimuli. Control of cytoplasmic Ca(2+) involves the activity of pumps, carriers, and possibly ion channels. The patch-clamp technique was used to study Ca(2+) channels in the vacuole of sugar beet cells. Vacuolar currents showed inward rectification at negative potentials, with a single-channel conductance of 40 picosiemens and an open probability dependent on potential. Channels were inhibited by verapamil and lanthanum. These channels could participate in the regulation of cytoplasmic Ca(2+) by sequestering Ca(2+) inside the vacuole.     

Tetraethylammonium and tetrodotoxin: effects on cochlear potentials

Tetraethylammonium chloride, which is believed to decrease potassium conductance, and tetrodotoxin, which apparently decreases sodium conductance in nerve fibers, were introduced iontophoretically into the organ of Corti or the scala media of guinea pig cochlea. The former depressed the direct-current endocochlear potential and also the alternating-current cochlear microphonics (the receptor potential of the ear), but tetrodotoxin was ineffective except on the nerve impulses.     

Bidirectional transmission at the rectifying electrotonic synapse: a voltage-dependent process

Rectifying properties of electrotonic synapses established by the crayfish giant motor fiber are associated with a more negative resting membrane potential in the presynaptic than in the postsynaptic side of the junction. An increased junctional conductance and bidirectional transmission are produced, with almost no delay, by inverting this polarization.