Calcium and EDTA fluxes in dialyzed squid axons

Engineering Human Epidermal Growth Receptor 2-Targeting Hepatitis B Virus Core Nanoparticles for siRNA Delivery in Vitro and in Vivo

Hepatitis B virus core (HBc) particles acquire the capacity to disassemble and reassemble in a controlled manner, allowing entrapment and delivery of drugs and macromolecules to cells. HBc particles are made of 180-240 copies of 21 kDa protein monomers, assembled into 30-34 nm diameter icosahedral particles. In this study, we aimed at formulating HBc particles for the delivery of siRNA for gene silencing in vitro and in vivo. We have previously reported recombinant HBc particles expressing Z_HER2 affibodies, specifically targeting human epidermal growth receptor 2 (HER2)-expressing cancer cells (Z_HER2-ΔHBc). siRNA was encapsulated within the Z_HER2-ΔHBc particles following disassembly and reassembly. The Z_HER2-ΔHBc-siRNA hybrids were able to secure the encapsulated siRNA from serum and nucleases in vitro. Enhanced siRNA uptake in HER2-expressing cancer cells treated with Z_HER2-ΔHBc-siRNA hybrids was observed compared to the nontargeted HBc-siRNA hybrids in a time- and dose-dependent manner. A successful in vitro polo-like kinase 1 (PLK1) gene knockdown was demonstrated in cancer cells treated with Z_HER2-ΔHBc-siPLK1 hybrids, to levels comparable to commercial transfecting reagents. Interestingly, Z_HER2-ΔHBc particles exhibit intrinsic capability of reducing the solid tumor mass, independent of siPLK1 therapy, in an intraperitoneal tumor model following intraperitoneal injection.

Encapsulation and delivery of plasmid DNA by virus-like nanoparticles engineered from Macrobrachium rosenbergii nodavirus

Virus-like particles (VLPs) are potential candidates in developing biological containers for packaging therapeutic or biologically active agents. Here, we expressed Macrobrachium rosenbergii nodavirus (MrNv) capsid protein (encoding amino acids M1-N371 with 6 histidine residuals) in an Escherichia coli BL21(DE3). These easily purified capsid protein self-assembled into VLPs, and disassembly/reassembly could be controlled in a calcium-dependent manner. Physically, MrNv VLPs resisted to digestive enzymes, a property that should be advantageous for protection of active compounds against harsh conditions. We also proved that MrNv VLPs were capable of encapsulating plasmid DNA in the range of 0.035-0.042 mol ratio (DNA/protein) or 2-3 plasmids/VLP (assuming that MrNV VLPs is T=1, i made up of 60 capsid monomers). These VLPs interacted with cultured insect cells and delivered loaded plasmid DNA into the cells as shown by green fluorescent protein (GFP) reporter. With many advantageous properties including self-encapsulation, MrNv VLPs are good candidates for delivery of therapeutic agents.

In the squid axon Na+/Ca2+ exchanger the state of the Ca i-regulatory site influences the affinities of the intra- and extracellular transport sites for Na+ and Ca2+

In squid axons, intracellular Mg2+ reduces the activity of the Na+/Ca2+ exchanger by competing with Ca2+ i for its regulatory site. The state of the Ca i-regulatory site (active-inactive) also alters the apparent affinity of intra- and extracellular transport sites. Conditions that hinder the binding of Ca2+ i (low pH i, low [Ca2+]i, high [Mg2+]i) diminish the apparent affinity of intracellular transport sites, in particular for Na i due to its synergism with H+ inhibition, but less noticeably for Ca2+ i because of its antagonism towards (Ha i + Na+ i) and Mg2+ i inhibitions. These are kinetic effects unrelated to the true affinity of the sites. With the Ca i-regulatory site saturated, the intracellular transporting sites are insensitive to [H+]i and to ATP. Likewise, the state of the Ca i-regulatory site (activated or inactivated) influences the affinity of the extracellular Ca o and Na o-transport sites (trans effects). In this case, the effects are opposite to those predicted by any of the transport schemes proposed for the Na+/Ca2+exchanger; i.e. its mechanism remains unexplained. In addition to their intrinsic importance for a full understanding of the properties of the Na+/Ca2+ exchanger, these findings show a new way by which the state of the Ca i-regulatory site may determine net movements of Ca2+ through this system.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions

The Na(+)/Ca(2+) exchanger's family of membrane transporters is widely distributed in cells and tissues of the animal kingdom and constitutes one of the most important mechanisms for extruding Ca(2+) from the cell. Two basic properties characterize them. 1) Their activity is not predicted by thermodynamic parameters of classical electrogenic countertransporters (dependence on ionic gradients and membrane potential), but is markedly regulated by transported (Na(+) and Ca(2+)) and nontransported ionic species (protons and other monovalent cations). These modulations take place at specific sites in the exchanger protein located at extra-, intra-, and transmembrane protein domains. 2) Exchange activity is also regulated by the metabolic state of the cell. The mammalian and invertebrate preparations share MgATP in that role; the squid has an additional compound, phosphoarginine. This review emphasizes the interrelationships between ionic and metabolic modulations of Na(+)/Ca(2+) exchange, focusing mainly in two preparations where most of the studies have been carried out: the mammalian heart and the squid giant axon. A surprising fact that emerges when comparing the MgATP-related pathways in these two systems is that although they are different (phosphatidylinositol bisphosphate in the cardiac and a soluble cytosolic regulatory protein in the squid), their final target effects are essentially similar: Na(+)-Ca(2+)-H(+) interactions with the exchanger. A model integrating both ionic and metabolic interactions in the regulation of the exchanger is discussed in detail as well as its relevance in cellular Ca(i)(2+) homeostasis.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

A minimum mechanism for Na+-Ca++ exchange: net and unidirectional Ca++ fluxes as functions of ion composition and membrane potential

Both simultaneous and consecutive mechanisms for Na+-Ca++ exchange are formulated and the associated systems of steady-state equations are solved numerically, and the net and unidirectional Ca++ fluxes computed for a variety of ionic and electrical boundary conditions. A simultaneous mechanism is shown to be consistent with a broad range of experimental data from the squid giant axon, cardiac muscle and isolated sarcolemmal vesicles. In this mechanism, random binding of three Na+ ions and one Ca++ on apposing sides of a membrane are required before a conformational change can occur, translocating the binding sites to the opposite sides of the membranes. A similar (return) translocation step is also permitted if all the sites are empty. None of the other states of binding can undergo such translocating conformational changes. The resulting reaction scheme has 22 reaction steps involving 16 ion-binding intermediates. The voltage dependence of the equilibrium constant for the overall reaction, required by the 3:1 Na+: Ca++ stoichiometry was obtained by multiplying and dividing, respectively, the forward and reverse rate constants of one of the translocational steps by exp(-FV/2RT). With reasonable values for the membrane density of the enzyme (approximately 120 sites micron 2) and an upper limit for the rate constants of both translocational steps of 10(5) . sec-1, satisfactory behavior was obtainable with identical binding constants for Ca++ on the two sides of the membrane (10(6) M-1), similar symmetry also being assumed for the Na+ binding constant (12 to 60 M-1). Introduction of order into the ion-binding process eliminates behavior that is consistent with experimental findings.

Changes in intracellular free Ca ion concentration evoked by electrical activity in cat spinal neurons in situ

In cats under allobarbitone anaesthesia, Ca2+-sensitive microelectrodes were inserted into the lumbosacral spinal neurons to measure intracellular free Ca2+ concentration [Ca]i. In 72 resting motoneurons, the global mean [Ca]i was 7.9 microM (SD +/- 25.9). In the 36 "best" cells (with resting and action potentials better than 60 mV), mean [Ca]i was 1.6 microM (SD +/- 1.64). Activation of motoneurons by antidromic or direct stimulation evoked mean increases in [Ca]i of about 90 nM when stimulating for 30 s at 10 Hz, and 170 nM at 20 Hz. The mean time to half-recovery was 23 s (SD +/- 14.5). Orthodromic stimulation consistently produced smaller increases in [Ca]i. Measurements in motor axons showed a comparable resting level of [Ca]i, but only minimal changes during stimulation, even at 100 Hz. Sensory axons (also recorded within the spinal cord) similarly failed to show any increase in [Ca]i during high frequency stimulation. In some interneurons, however, particularly large and rapid increases in [Ca]i could be evoked by dorsal root stimulation at 1-5 Hz. Unresponsive cells (presumably neuroglia), with a typically high and stable resting potential, had a variable [Ca]i giving a mean of 32 microM (SD +/- 63.0). A tentative theoretical analysis of the magnitude and time course of delta [Ca]i evoked in motoneurons by tetanic stimulation is consistent with remarkably slow apparent diffusion of intracellular Ca2+ (1/250 of rate of diffusion in water), such as might be expected in the presence of very efficient mechanisms of Ca2+ sequestration.

Interactions of physiological ligands with the Ca pump and Na/Ca exchange in squid axons

We have studied the interaction of physiological ligands other than Nai and Cai with the Ca pump and Na/Ca exchange in internally dialyzed squid axons. The results show the following. (a) Internal Mg2+ is an inhibitor of the Nao-dependent Ca efflux. At physiological Mg2+i (4 mM), the inhibition amounts to approximately 50%. The inhibition is partial and noncompetitive with Cai, and is not affected by Nai or ATP. The ATP-dependent uncoupled efflux is unaffected by Mgi up to 20 mM. Both components of the Ca efflux require Mg2+i for their activation by ATP. (b) At constant membrane potential, Ki is an important cofactor for the uncoupled Ca efflux. (c) Orthophosphate (Pi) activates the Nao-dependent Ca efflux without affecting the uncoupled component. Activation by Pi occurs only in the presence of Mg-ATP or hydrolyzable ATP analogues. Pi under physiological conditions has no effect on the uncoupled component; nevertheless, at alkaline pH, it inhibits the Ca pump, probably by product inhibition. (d) ADP is a potent inhibitor of the uncoupled Ca efflux. The Nao-dependent component is inhibited by ADP only at much higher ADP concentrations. These results indicate that (a) depending on the concentration of Ca2+i, Na+i Mg2+i, and Pi, the Na/Ca carrier can operate under a low- or high-rate regime; (b) the interactions of Mg2+i, Pi, Na+i, and ATP with the carrier are not interdependent; (c) the effect of Pi on the carrier-mediated Ca efflux resembles the stimulation of the Nao-dependent Ca efflux by internal vanadate; (d) the ligand effects on the uncoupled Ca efflux are of the type seen in the Ca pump in red cells and the sarcoplasmic reticulum.

Transmembrane 45Ca fluxes in isolated smooth muscle cells: basal Ca2+ fluxes

Methods are described for monitoring unidirectional fluxes of 45Ca in smooth muscle cells in suspension. Compartmental analysis of 45Ca influx data indicate that 45Ca exchange consists of two kinetically distinguishable components in the toad stomach muscle cells: a relatively small (approximately 100-pmol X cm-2) component with a rapid rate of exchange (t1/2 approximately 1.4 min) and a larger (approximately 930-pmol X cm-2) component, which exchanges more slowly (t1/2 congruent to 87 min). The rate of exchange of the latter but not the former Ca2+ pool is increased on exposure of the cells to elevated K+ levels; thus the "slow" component of uptake appears to reflect transmembrane Ca2+ flux, whereas the "rapid" component may reflect exchange of surface-bound label. Consistent with this interpretation is the finding that under conditions in which surface 45Ca is rapidly and completely displaced, 45Ca efflux occurs as a simple monoexponential process with a slow rate of exchange (k = 7.80 X 10(-5) s-1). The apparent rate of transmembrane Ca2+ flux in smooth muscle cells at rest is approximately 0.1 pmol X cm-2 X s-1. Cellular processes that could give rise to an apparent transmembrane flux rate of this magnitude are discussed, and a model is presented which appears to describe cellular 45Ca exchange in the isolated smooth muscle cells.

The spindle potential in the frog muscle spindle does not require external Na+

Spindle potential recorded from the sensory nerve terminal of isolated frog muscle spindles disappeared within 20-30 min after the spindle receptor was perfused with Na+-free (Li, Tris or choline) Ringer's solution, whereas the amplitude of spindle potential was not attenuated for periods up to 60 min when the spindles were perfused in a Na+-free Ringer's solution containing both 10 mM TEA and 0.1 mM 4-aminopyridine after being washed with a normal Ringer's solution containing both the K+-channel blockers. It is concluded that the time-dependent decrease in the amplitude of spindle potential during the application of Na+-free solution is not ascribable to a decrease in the inward current carried by Na+, but is due to an increase in an outward current carried by K+.

Numerical analysis of the method of internal dialysis of giant axons

This paper presents a numerical analysis of the method of internal dialysis used for studies of membrane transport in giant axons. Account is taken of the complete geometry, end effects, and finite dialyzate flow rates. Both influx and efflux experimental conditions are considered. Results place quantitative limits on system performance that are sufficiently general for use in experimental design. The completeness of solute equilibration and the uniformity of solute concentration at the axon membrane are assessed, as well as the sensitivity to dialysis solution flow rate. The effects of undialyzed axon ends on the equilibration rate and on the uniformity of concentration are determined, and the contribution of undialyzed ends to influx measurements is evaluated. Equilibration times are correlated with physical properties of axoplasm and dialysis tubing, and with dialysis solution flow rate. Numerical results confirm the general qualitative assessments of the method that are based on years of successful application.

Effects of internal sodium and hydrogen ions and of external calcium ions and membrane potential on calcium entry in squid axons

Squid giant axons were impaled with electrodes to measure pNai, pHi, Em, and were injected with either aequorin or arsenazo III to measure [Ca]i or with phenol red to measure [H]i. Depolarization of such axons with elevated [K] in sea water leads to a Ca entry that is a function of [Ca]o, [Na]i, and [H]i. With saturating [Na]i half-maximal Ca entry is produced by a [Ca]o of 0.58 mM. With saturating [Ca]o, depolarization produced by 450 mM-K+ leads to half-maximal Ca entry when [Na]i is 25 mM; entry is virtually undetectable if [Na]i is 18 mM. If [Ca]o is 50 mM, Ca entry upon depolarization as measured with aequorin is phasic with a rapid phase of light emission and a plateau; Ca entry as measured with arsenazo III shows no such phasic behaviour, absorbance vs. time is a square wave that closely follows the depolarization vs. time trace. Both detectors of [Ca]i show a square-wave response if [Ca]o is 3 mM. The introduction of 2 mM-CN into the sea water bathing the axon does not affect the response to depolarization nor does the destruction of most of the ATP in the axon following the injection of apyrase. If axons are microinjected with phenol red rather than arsenazo, the entry of Ca produces an acidification in the peripheral parts of the axoplasm. Other experiments measuring [Ca]i show that Ca entry is strongly inhibited by a decrease in pHi. Making sea water alkaline with pH buffers scarcely affects the Ca entry induced by depolarization; making axoplasm alkaline by adding NH4+ to sea water greatly enhances Ca entry by Na/Ca exchange and also enhances the ability of axoplasmic buffers to absorb Ca.

The effects of sodium replacement on the responses of toad rods

1. We have investigated the effects of Na(+) substitution on the membrane potential and light responses of rods in the superfused retina of the toad, Bufo marinus.2. When all of the Na(+) in the Ringer was replaced with Li(+), the effects on the rods depended upon the external free Ca(2+) concentration ([Ca(2+)](o)). At [Ca(2+)](o) >/= 10(-6) M, the membrane potential (E(m)) hyperpolarized and light responses were greatly diminished or abolished. At [Ca(2+)](o) </= 10(-7) M, Li(+) replacement had little effect on E(m) or response amplitude.3. We interpret these results as revealing a Na(+)-Ca(+) counter-transport in rods. At high [Ca(2+)](o), replacing Na(+) with Li(+) would have produced an increase in the rod cytosol free Ca(2+) concentration ([Ca(2+)](i)) and the blockage of the light-dependent conductance, leading directly to the suppression of light responses. At [Ca(2+)](o) </= 10(-7) M, this presumably would not have occurred.4. Since at these low Ca(2+) concentrations we observed light responses of nearly normal amplitude in Li(+), our results suggest that the light-dependent conductance is permeable to Li(+).5. Substitution of Na(+) with K(+) in low Ca(2+) produced a complete suppression of the responses. However, it was still possible to measure large light-induced changes in rod input resistance.6. Substitution of Na(+) with tetramethylammonium, tetraethylammonium, Tris, or choline in low Ca(2+) produced a large hyperpolarization of the membrane potential and a diminution of response amplitude. However, we were unable to observe a complete suppression of the responses for these cations.7. Substitution of Na(+) with tetrapropylammonium or with an uncharged substance (glucose or urea) in low Ca(2+) produced a large hyperpolarization of membrane potential and a considerable decrease in the light responses. In about half our attempts, the responses were observed to decline reversibly to less than 20% of their peak amplitude in Na(+).8. Results with tetrapropylammonium were indistinguishable from those of glucose or urea, indicating that the light-dependent conductance probably is not permeable to TPA. The resistance changes measured with K(+) substitution and the responses observed in the presence of the organic ions TMA, TEA, Tris and choline suggest that these species may be permeable, but we are unable to discount alternative explanations.

Mechanisms of transmembrane calcium movement in cultured chick embryo ventricular cells

1. Uptake of calcium was studied in spontaneously contracting monolayers of cultured chick embryo ventricular cells. Ca exchange could be separated into two components: a rapid phase with a rate constant of 3.91/min, accounting for 1.6 nmol/mg protein; and a slower phase with a rate constant of 0.069/min, accounting for 2.7 nmol/mg protein.2. Negatively inotropic concentrations of the slow Ca channel blocker verapamil inhibited the rapid phase of Ca uptake partially, with a maximum inhibition of 30-40% observed at concentrations of verapamil which completely inhibited contraction.3. The component of Ca uptake not inhibited by verapamil could be stimulated up to 25-fold by elevation of intracellular Na concentration and reduction of extracellular Na concentration, and thus appeared to represent at least in part Ca uptake via Na-Ca exchange.4. Ca uptake by cultured cells could be almost completely inhibited by exposure to LaCl(3), 1 mmol/l, within 5 s. This same concentration of La completely inhibited contraction within 5 s.5. During efflux of (45)Ca from cells, exposure to La (1 mmol/l) slightly inhibited efflux initially with more marked inhibition of Ca influx after 3 min of La exposure. There was no evidence for a component of superficial La-displaceable Ca, and thus the rapid phase of Ca uptake probably is due to an intracellular rapidly exchanging Ca pool. Efflux of Ca from this rapidly exchanging intracellular Ca pool was not significantly altered by exposure to Na-free choline chloride solutions.6. We conclude that rapid Ca uptake in these cultured myocardial cells is due primarily to Ca influx via the slow Ca channel and via Na-Ca exchange. In the presence of physiological [Ca(2+)](i), efflux of Ca from this intracellular Ca pool does not appear to be due to Na-Ca exchange.

3-O-methylglucose transport in internally dialysed giant axons of Loligo

1. The transport of the non-metabolized sugar, 3-O-methylglucose, has been studied in the squid axon under conditions where the intracellular environment of the axon is controlled by internal dialysis. 2. Sugar transport is passive, shows saturation kinetics and is asymmetric. At 15 degrees C, the Michaelis and velocity constants for exit are approximately four times those for uptake. The asymmetry of transport is increased by raising the temperature. 3. Sugar uptake is not affected by intracellular sugar levels as high as 100 mM. Sugar exit is, however, reduced by external sugars although the apparent Km for exit is unaffected. 4. The kinetics of sugar exit under exchange conditions are determined by the kinetics of sugar uptake. These results can be accounted for by the asymmetric mobile-carrier and simultaneous-carrier models for transport. 5. Both sugar uptake and exit are reduced in the absence of ATPi. Kinetic analysis of transport under these conditions show that the capacity of the system to transport sugar is unchanged but that the affinity of the system for sugar is reduced. Internal cyclic AMP, AMP, ADP or GTP (2 mM) do not mimic this action of ATP. The hydrolysable analogue of ATP, alpha, beta-methylene-5-ATP (2 mM), (but not the nonhydrolysable analogue beta, gamma-methylene-5-ATP, 2 mM) has an ATP-like action on sugar transport. 6. Transport is unaffected by internal Ca2+ concentrations in the range 4 X 10(-8)--9 X 10(-7) M.

A (Ca2+, Mg2+)-ATPase activity in plasma membrane fragments isolated from squid nerves

A (Ca2+, Mg2+)-ATPase activity and a (Ca2+, Mg2+)-dependent phosphorylation from ATP have been found in plasma membrane fragments from squid optical nerves under conditions where contamination by intracellular organelles is unlikely. The properties of this (Ca2+, Mg2+)-ATPase activity are almost identical to those of the ATP-dependent uncoupled Ca2+ efflux observed in dialyzed squid giant axons. This gives further support to the notion that the mechanism responsible for maintaining the low levels of ionized Ca concentration in nerves at rest is not a Na+-Ca2+ exchange system but an ATP-driven uncoupled Ca2+ pump.

Calcium movement in nerve fibres

Given the existence of a difference in electrical potential between the interior of a nerve cell and the media surrounding it, where the cytoplasm is some 70 mV negative (Hodgkin, 1958), it must be expected that any positively charged ion to which the cell membrane is permeable is more concentrated in the cell interior. For monovalent cations such as Na and divalent cations such as Ca and Mg this is not the case in the majority of the cells such as the squid giant axon. In other words, nerve cells maintain a lower intracellular concentration of these ions, as compared with their concentration in the extracellular fluid. For Mg, Ca and Na ions, this lower internal concentration must, in the steady state, be effected by some membrane based mechanism which consumes energy.

Calcium content and net fluxes in squid giant axons

Axons freshly dissected from living specimens of the tropical squid Dorytheutis plei have a calcium content of 68 mumol/kg of axoplasm. Fibers stimulated at 100 impulses/s in 100 mM Ca seawater increase their Ca content by 150 mumol/kg.min; axons placed in 3 Ca (choline) seawater increase their Ca content by 12 mumol/kg.min. Axons loaded with 0.2--1.5 mmol Ca/kg of axoplasm extruded Ca with a half time of 15--30 min when allowed to recover in 3 Ca (Na) seawater. The half time for recovery of loaded axons poisoned with carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and iodoacetic acid (IAA) is about the same as control axons. Axons placed in 40 mM Na choline seawater (to reduce chemical gradient for Na) or in 40 mM Na, 410 mM K seawater to reduce the electrochemical gradient for Na to near zero either fail to lose previously loaded Ca or gain further Ca.

Calcium influx in internally dialyzed squid giant axons

A method has been developed to measure Ca influx in internally dialyzed squid axons. This was achieved by controlling the dialyzed segment of the axon exposed to the external radioactive medium. The capacity of EGTA to buffer all the Ca entering the fiber was explored by changing the free EGTA at constant [Ca++]i. At a free [EGTA]i greater than 200 microM, the measured resting Ca influx and the expected increment in Ca entry during electrical stimulation were independent of the axoplasmic free [EGTA]. To avoid Ca uptake by the mitochondrial system, cyanide, oligomycin, and FCCP were included in the perfusate. Axons dialyzed with a standard medium containing: [ATP] = 2 mM, [Ca++]i = 0.06 microM, [Ca++]o = 10 mM, [Na+]i = 70 mM, and [Na+]o = 465 mM, gave a mean Ca influx of 0.14 +/- 0.012 pmol.cm-2.s-1 (n = 12. Removal of ATP drops the Ca influx to 0.085 +/- 0.007 pmol.cm-2.s-1 (n = 12). Ca influx increased to 0.35 pmol.cm-2,s-1 when Nao was removed. The increment was completely abolished by removing Nai+ and (or) ATP from the dialysis medium. At nominal zero [Ca++]i, no Nai-dependent Ca influx was observed. In the presence of ATP and Nai [Ca++]i activates the Ca influx along a sigmoid curve without saturation up to 1 microM [Ca++]i. Removal of Nai+ always reduced the Ca influx to a value similar to that observed in the absence of [Ca++]i (0.087 +/- 0.008 pmol.cm-2.s-1; n = 11). Under the above standard conditions, 50-60% of the total Ca influx was found to be insensitive to Nai+, Cai++, and ATP, sensitive to membrane potential, and partially inhibited by external Co++.

Effect of magnesium on calcium efflux in dialyzed squid axon

The Ca efflux mechanism located in the axolemma of the tropical squid Doritheutis plei is shown to be affected by the concentration of intracellular Mg (Mgi). The removal of all of the Mg from the experimental preparation causes an increase in Ca efflux. This effect seems to be more pronounced at low levels of internal ionized calcium and high levels of internal Na.

The effect of sodium, calcium and metabolic inhibitors on calcium efflux from goldfish heart ventricles

1. 45Ca efflux and tissue Ca content were examined in goldfish ventricles under conditions known to affect cellular Ca movements. 2. EGTA or Ca-EGTA was added to the washout solutions in sufficient concentration (10 mM) to avoid retardation of the apparent tissue 45Ca efflux by extracellular 45Ca binding or backflux. 3. After a variable initial increase, the cellular Ca content usually stabilizes within 60 min when ventricles are immersed in Li- or K-substituted saline containing 1.8 mM Ca0 (under these conditions the internal Ca2+ concentration is below 10(-5) M). 4. 45Ca efflux is maximally activated by external concentrations of Ca2+ as low as 10(-6) M, in both Na-containing and Na-free saline. 5. 45Ca efflux decreases in Na-free solutions. It is reactivated by Na-saline. The effect of different external Na concentration on 45Ca efflux is comparable at external Ca2+ concentrations between 10(-6) M and 2 x 10(-3) M. 6. Reactivation of Ca efflux after Na0 readmission is inhibited by metabolic poisoning, or in the presence of 10 mM-caffeine. Loading with 45Ca at very low external Ca2+ concentration prevents the inhibition of Ca efflux in Na-free medium. 7. Caffeine (10 mM) produces contractions of about equla size when K-depolarized preparations are immersed in either Na- or Li-saline. At the same time there is a similar increase in 45Ca efflux in absence of Na0 and in its presence. 8. In the virtual absence of Ca2+0 (10(-5) M-Ca, 10(-2) M-EGTA) and Na+0, the residual 45Ca efflux is reversibly inhibited by cyanide (2 mM). 9. The results are roughly compatible with the general concept of ATP-dependent Na-Ca exchange in internal Ca2+ homeostasis. However, this hypothesis should probably be modified to account for the fact that under physiological concentrations Na+0 and Ca2+0 do not compete for activating 45Ca efflux. Metabolic products may be involved in Na0- and Ca0-dependent Ca efflux. It is therefore not excluded that a Na-independent active mechanism co-operates with Na-Ca exchange in Ca extrusion.

Mitochondria and other calcium buffers of squid axon studied in situ

Continuous nondestructive monitoring of intracellular ionized calcium in isolated squid axons by differential absorption spectroscopy (using arsenazo III and antipyrylazo III) was used to study uptake of calcium by carbonyl cyanide, p-trifluoromethoxy-phenylhydrazone (FCCP)- and (or) cyanide (CN)-sensitive and insensitive constituents of axoplasm. Known calcium loads imposed on the axon by stimulation produced proportional increments of free axoplasmic calcium. Measurement of increments in ionized calcium as a function of load confirmed earlier reports of buffering in normal and FCCP- and (or) CN-poisoned axons. Measurement of rates of calcium uptake by presumed mitochondria showed little uptake at ambient Ca below 200--400 nM, with sigmoidal rise to about 20--30 mumol/kg axoplasm per min (calculated to be about 200 mmol/kg mitochondrial protein per min) at 50 micrometer, indicating a functional threshold for presumed mitochondrial uptake well above physiological ionized calcium concentration. Treatment of stimulated axons with cyanide, to release calcium from presumed mitochondria, showed that the sensitivity to cyanide decreased progressively with time after stimulation (t 1/2 = 3--10 min) implying transfer of sequestered calcium into a less metabolically labile form.

The influence of external cations and membrane potential on Ca-activated Na efflux in Myxicola giant axons

In microinjected Myxicola giant axons with elevated [Na]i, Na efflux was sensitive to Cao under some conditions. In Li seawater, sensitivity to Cao was high whereas in Na seawater, sensitivity to Cao was observed only upon elevation of [Ca]o above the normal value. In choline seawater, the sensitivity of Na efflux to Cao was less than that observed in Li seawater whereas Mg seawater failed to support any detectable Cao-sensitive Na efflux. Addition of Na to Li seawater was inhibitory to Cao-sensitive Na efflux, the extent of inhibition increasing with rising values of [Na]o. The presence of 20 mM K in Li seawater resulted in about a threefold increase in the Cao-activated Na efflux. Experiments in which the membrane potential, Vm, was varied or held constant when [K]o was changed showed that the augmentation of Ca-activated Na efflux by Ko was not due to changes in Vm but resulted from a direct action of K on activation by Ca. The same experimental conditions that favored a large component of Cao-activated Na efflux also caused a large increase in Ca influx. Measurements of Ca influx in the presence of 20 mM K and comparison with values of Ca-activated Na efflux suggest that the Na:Ca coupling ratio may be altered by increasing external [K]o. Overall, the results suggest that the Cao-activated Na efflux in Myxicola giant axons requires the presence of an external monovalent cation and that the order of effectiveness at a total monovalent cation concentration of 430 mM is K + Li greater than Li greater than Choline greater than Na.

Effects of internal and external cations and of ATP on sodium-calcium and calcium-calcium exchange in squid axons

Calcium-45 efflux was measured in squid axons whose internal solute concentration was controlled by internal dialysis. Most of the Ca efflux requires either external Na (Na-Ca exchange) or external Ca plus in alkali metal ion (Ca-Ca exchange; cf. Blaustein & Russell, 1975). Both Na-Ca and Ca-Ca exchange are apparently mediated by a single mechanism because both are inhibited by Sr and Mn, and because addition of Na to an external medium optimal for Ca-Ca exchange inhibits Ca efflux. The transport involves simultaneous (as opposed to sequential) ion counterflow because the fractional saturation by internal Ca (Cai) does not affect the external Na (Nao) activation kinetics; also, Nao promotes Ca efflux whether or not an alkali metal ion is present inside, whereas Ca-Ca exchange requires alkali metal ions both internally and externally (i.e., internal and external sites must be appropriately loaded simultaneously). ATP increases the affinity of the transport mechanism for both Cai and Nao, but it does not affect the maximal transport rate at saturating [Ca2+]i and [Na+]o; this suggest that ATP may be acting as a catalyst of modulator, and not as an energy source. Hill plots of the Nao activation data yield slopes congruent to 3 for both ATP-depleted and ATP-fueled axons, compatible with a 3 Na+-for-1 Ca2+ exchange. With this stoichiometry, the Na electrochemical gradient alone could provide sufficient energy to maintain ionized [Ca2+]i in the physiological range (about 10(-7) M).

The control of ionized calcium in squid axons

Measurements of the Ca content, [Ca](T), of freshly isolated squid axons show a value of 60 mumol/kg axoplasm. Axons in 3 mM Ca(Na) seawater show little change in Ca content over 4 h, while axons in 3 mM Ca(Na) seawater show little change in Ca content over 4 h, while axons in 10 mM Ca(Na) seawater show gains of 18 mumol/Ca/kgxh. In 10 Ca (Choline) seawater the gain is 2,400 mumol/kgxh. Using aequorin confined to a dialysis capillary in the center of an axon, one finds that [Ca](i) is in a steady state with 3 Ca (Na) seawater, and that both 10 Ca (Na) and 3 Ca (choline) seawater cause increases in [Ca](i). In 3 Ca (Na) seawater-3 Ca (choline) seawater mixtures, 180 mM [Na](0) (40 perecent Na) is as effective as 450 mM [Na](0) (100 percent Na) in maintaining a normal [Ca](1); lower [Na] causes an increase in [Ca](i). If axons are injected with the ATP-splitting enzyme apyrase, the resulting [Ca](1) is not loading with high [Ca](0) or low [Na](0) solutions. Depolarization of an axon with 100 mM K (Na) seawater leads to an increase in the steady-state level of [Ca](1) that is reversed upon returning the axon to normal seawater. Freshly isolated axons treated with either CN or FCCP to inhibit mitochondrial Ca buffering can still maintain a normal [Ca](i) in 1 Ca (Na) seawater.

Effects of calcium and calcium-chelating agents on the inward and outward current in the membrane of mollusc neurones

1. Effects of internal and external Ca and Ca-chelating agents, EGTA and EDTA on transmembrane ionic currents were studied in isolated, internally dialysed neurones from the molluscs, Helix pomatia and Limnea stagnalis.2. The possible pharmacological effect of internally applied EGTA was investigated on the background of constant free Ca concentration (5.3 x 10(-9)M). EGTA had no effect on Ca and Na inward currents but considerably depressed the delayed K outward current. No effective removal of this action could be achieved by the elevation of intracellular free Ca.3. In the absence of divalent cations in the external medium, EGTA (as well as EDTA) applied either intra- or extracellularly caused the appearance of a very large Na inward current with kinetics similar to those of Ca inward current and with the reversal potential around 10 mV. Effective concentrations of chelating agents were 0.1 mM (extracellular) and 1.0 mM (intracellular).4. Increase in intracellular Ca in the absence of EGTA (by dialysis of the cell with Ca-saturated solutions) did not produce any significant effect on the delayed K outward current. The small change observed in this current could be evaluated as a depression of maximum slope conductance and a shift to more negative membrane potential.5. Ca inward current has been found extremely sensitive to internal Ca. 5.8 x 10(-8)M of internal free Ca produced its complete depression. This effect was reversible. Na inward current could be inhibited with 3.5 x 10(-7)M intracellular Ca.

Characterization of the ATP-dependent calcium efflux in dialyzed squid giant axons

The magnitude of the activating effect of ATP on the Ca efflux was explored at different [Ca++]i in squid axons previously exposed to cyanide seawater and internally dialyzed with a medium free of ATP and containing p-trifluoro methoxy carbonyl cyanide phenyl hydrazine. At the lowest [Ca++]i used (0.06 micron more than 95% of the Ca efflux depends on ATP. At high [Ca++]i (100 micron), 50-60% of the Ca efflux still depends on ATP. The apparant affinity constant for ATP was not significantly affected in the range of [Ca++]i from 0.06 to 1 micron. Axons dialyzed to reduce their internal magnesium failed to show the usual activation of the Ca efflux when the Tris or the sodium salt of ATP was used. Only in the presence of internal magnesium is ATP able to stimulate the Ca efflux. Nine naturally occurring high-energy phosphate compounds were ineffective in supporting calcium efflux. These compounds were: UTP, GTP, CTP, UDP, CDP, ADP, AMP, CAMP, and acetyl phosphate. The compounds 2' deoxy-ATP and the hydrolyzable analog alpha,beta-methylene ATP were able to activate the Ca efflux. The nonhydrolyzable analog beta,gamma-methylene ATP competes with ATP for the activating site, but is unable to activate the Ca efflux. The results are discussed in terms of the specificity of the nucleotide site responsible for the ATP-dependent Ca efflux.

Magnesium efflux in dialyzed squid axons

The efflux of Mg++ from squid axons subject to internal solute control by dialysis is a function of ionized [Mg], [Na], [ATP], and [Na]o. The efflux of Mg++ from an axon with physiological concentrations of ATP, Na, and Mg inside into seawater is of the order of 2-4 pmol/cm2s but this efflux is strongly inhibited by increases in [Na]i, by decreases in [ATP]i, or by decreases in [Na]o. The efflux of Mg++ is largely independent of [Mg]i when ATP is at physiological levels, but in the absence of ATP reaches half the value of Mg efflux in be presence of ATP when [Mg]i is about 4 mM and [Na] 40 mM. Half-maximum responses to ATP occur at about 350 micronM ATP into seawater with Na either present or absent. The Mg efflux mechanism has many similarities to the Ca efflux system in squid axons especially with respect to the effects of ATP, Nao, and Na on the flux. The concentrations of free Mg and Ca in axoplasm differ, however, by a factor of 10(5) while the observed fluxes differ by a factor of 10(2).

Proceedings of the physiological society [proceedings]

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Kinetics and energetics of calcium efflux from intact squid giant axons

The Ca efflux from intact squid axons consists of three major components: one that is activated by Cao, one that is activated by Nao and a residual flux that persists in the nominal absence of both Cao and Nao. The properties of these components have been investigated in unpoisoned axons and in axons poisoned with cyanide. 2. Under all conditions the shape of the curve relating Cao to Cao-activated Ca efflux approximates to a section of a rectangular hyperbola, consistent with simple Michaelis activation. 3. The external Ca concentration giving half-maximal activation of Cao-activated Ca efflux is about 2 muM in unpoisoned axons immersed in Na-ASW, but on poisoning changes progressively to values in the range 1-10 mM. The residual efflux from unpoisoned axons may reflect activation by traces of Ca present immediately external to the axolemma. 4. The apparent affinity for Cao of Cao-activated Ca efflux is very similar in unpoisoned axons immersed in sea waters containing Na, Li, Tris or K as major cation, whereas in poisoned axons the affinity in Na and Li is about the same but higher than that in choline and Tris. 5. In unpoisoned axons Ca influx increases linearly as Cao is increased from 2 muM to 110 mM. The absolute value of the Ca influx from 10 muM-Cao is less than 1% of the Cao-activated Ca efflux at this external Ca concentration. In poisoned axons the sizes of Cao-activated Ca efflux and Ca influx were similar at all Ca concentrations examined. 6. The shape of the curve relating Nao to Nao-activated Ca efflux approximates to a section of rectangular hyperbola in unpoisoned axons but is clearly sigmoidal in axons that have been fully poisoned with cyanide. The sigmoidal shape develops progressively during poisoning. ...

Ionized calcium concentrations in squid axons

Values for ionized [Ca] in squid axons were obtained by measuring the light emission from a 0.1-mul drop of aequorin confined to a plastic dialysis tube of 140-mum diameter located axially. Ionized Ca had a mean value of 20 x 10(-9) M as judged by the subsequent introduction of CaEGTA/EGTA buffer (ratio ca. 0.1) into the axoplasm, and light measurement on a second aequorin drop. Ionized Ca in axoplasma was also measured by introducing arsenazo dye into an axon by injection and measuring the Ca complex of such a dye by multichannel spectrophotometry. Values so obtained were ca. 50 x 10(-9) M as calibrated against CaEGTA/EGTA buffer mixtures. Wth a freshly isolated axon in 10 mM Ca seawater, the aequorin glow invariably increased with time; a seawater [Ca] of 2-3 mM allowed a steady state with respect to [Ca]. Replacement of Na+ in seawater with choline led to a large increase in light emission from aequorin. Li seawater partially reversed this change and the reintroduction of Na+ brought light levels back to their initial value. Stimulation at 60/s for 2-5 min produced an increase in aequorin glow about 0.1% of that represented by the known Ca influx, suggesting operationally the presence of substantial Ca buffering. Treatment of an axon with CN produced a very large increase in aequorin glow and in Ca arsenazo formation only if the external seawater contained Ca.