Axoplasm chemical composition in Myxicola and solubility properties of its structural proteins

Electromagnetic modeling and simulation of the biophoton propagation in myelinated axon waveguide

Biophotons in the nervous system are a potential carrier of neural signals. Previous experiments and studies indicated that biophotons are closely related to the neuronal activity and can propagate along myelinated axons. We establish a multilayer electromagnetic simulation model and demonstrate that the myelinated axon waveguide has low attenuation and low dispersion and operates in a narrow bandwidth on the order of 10 nm. We also find that the operating wavelength of the waveguide is almost linearly related to the axon diameter and the number of myelin layers. Each additional layer of the myelin sheath causes the operating wavelength of the myelinated axon waveguide to shift 52.3 nm to the long-wave direction, while an increase in the axon diameter of 1.0 µm causes the operating wavelength to shift 94.5 nm to the short-wave direction. These findings well explain the tendency of the spectral redshift among different species and the spectral blueshift during the aging process of mice. Via the analysis method in this paper, we can predict the wavelength of the propagating biophotons based on the neural structure.

Simulating optical coherence tomography for observing nerve activity: A finite difference time domain bi-dimensional model

We present a finite difference time domain (FDTD) model for computation of A line scans in time domain optical coherence tomography (OCT). The OCT output signal is created using two different simulations for the reference and sample arms, with a successive computation of the interference signal with external software. In this paper we present the model applied to two different samples: a glass rod filled with water-sucrose solution at different concentrations and a peripheral nerve. This work aims to understand to what extent time domain OCT can be used for non-invasive, direct optical monitoring of peripheral nerve activity.

Hodgkin-Huxley model based on ionic transport in axoplasmic fluid

Hodgkin-Huxley model has been reframed to incorporate the physical parameters of fluid inside the axon. The reframed model comprises of set of partial differential equations containing the physical parameters: density, mass fraction of sodium, potassium and chlorine ions, longitudinal diffusivity of ions and rate of additions of ions along with the temperature. Obtained conduction velocity of 19.5m/sec at a temperature of 18.5 degree celcius and conduction velocity dependency on temperature within the range 5 to 25 degree celcius are two important results that strongly validate the proposed model. The behavior of all the physical parameters has been characterized with respect to the action potential. Action potential conduction velocity along with axoplasmic fluid viscosity has been characterized with respect to different temperatures. Longitudinal diffusivity of ions is also quantified.

Resealing of the proximal and distal cut ends of transected axons: electrophysiological and ultrastructural analysis

The fates of the proximal and distal segments of transected axons differ. Whereas the proximal segment usually recovers from injury and regenerates, the distal segment degenerates. In the present report we studied the kinetics of the recovery processes of both proximal and distal axonal segments following axotomy and its temporal relations to the alterations in the cytoarchitecture of the injured neuron. The experiments were performed on primary cultured metacerebral neurons (MCn) isolated from Aplysia. We transected axons while monitoring the changes in transmembrane potential and input resistance (Rn) by inserting intracellular microelectrodes into the soma and axon. Correlation between the electrophysiological status of the injured axon and its ultrastructure was provided by rapid fixation of the neuron at selected times postaxotomy. Axotomy leads to membrane depolarization from a mean of -55.7 S.D. 12.8 mV to -12.7 S.D. 3.3 mV and decreased Rn from tens of M omega to 1-3 M omega. The transected axons remained depolarized for a period of 10-260 s for as long as the axoplasm was in direct contact with the bathing solution. Rapid repolarization and partial recovery of Rn was associated with the formation of a membrane seal over the cut ends by the constriction and subsequent fusion of the axolema. Prior to the formation of a membraneous barrier, electron-dense deposits aggregate at the tip of the cut axon and appear to form an axoplasmic "plug." Electrophysiological analysis revealed that this "plug" does not provide resistance for current flow and that the axoplasmic resistance is homogenously distributed. The kinetics of injury and recovery processes as well as the ultrastructural changes of the proximal and distal segments are identical suggesting that the different fates of the segments cannot be attributed to differences in the immediate response of the segments to axotomy.

Cytoplasmic hydrogen ion diffusion coefficient

The apparent cytoplasmic proton diffusion coefficient was measured using pH electrodes and samples of cytoplasm extracted from the giant neuron of a marine invertebrate. By suddenly changing the pH at one surface of the sample and recording the relaxation of pH within the sample, an apparent diffusion coefficient of 1.4 +/- 0.5 x 10(-6) cm2/s (N = 7) was measured in the acidic or neutral range of pH (6.0-7.2). This value is approximately 5x lower than the diffusion coefficient of the mobile pH buffers (approximately 8 x 10(-6) cm2/s) and approximately 68x lower than the diffusion coefficient of the hydronium ion (93 x 10(-6) cm2/s). A mobile pH buffer (approximately 15% of the buffering power) and an immobile buffer (approximately 85% of the buffering power) could quantitatively account for the results at acidic or neutral pH. At alkaline pH (8.2-8.6), the apparent proton diffusion coefficient increased to 4.1 +/- 0.8 x 10(-6) cm2/s (N = 7). This larger diffusion coefficient at alkaline pH could be explained quantitatively by the enhanced buffering power of the mobile amino acids. Under the conditions of these experiments, it is unlikely that hydroxide movement influences the apparent hydrogen ion diffusion coefficient.

Properties of calcium binding by Myxicola axoplasmic protein

The 45Ca2+ binding properties of axoplasmic protein from the Myxicola giant axon have been investigated using a centrifugal/concentration-dialysis technique. Scatchard plot analysis of these binding data suggest that Ca2+ is attached to a site with an equilibrium dissociation constant of 7.7 +/- 0.5 microM and a capacity of 4.4 +/- 0.2 mumol/g axoplasmic protein (n = 11). Addition of other cations--Cd2+, Mn2+, Al3+, Cu2+, Ba2+, and Zn2(+)--at concentrations up to 10 microM did not displace 0.2 microM 45Ca2+ from its binding site, probably because of buffering of these cations by amino acid residues within the protein solutions. The protein could be stored at 4 degrees C for up to 16 days with no appreciable change in the number of calcium sites. Ca2+ binding equilibrium took place in less than 30 min of incubation. Increasing the incubation temperature from 4 degrees C to 37 degree C reduced the number of Ca2+ sites. The binding capacity was reduced by one-half when the protein was dialyzed with 4 M urea or high ionic strength KCl (2 M). Calcium binding was examined as a function of pH. When the protein was dialyzed overnight at different pH values and all the binding was done at pH 7.0, the apparent number of Ca2+ sites decreased as the pH of the dialysis medium was increased. When the protein was dialyzed overnight at pH 7.0 and the binding was done at different pH values, the apparent binding capacity increased as pH increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ binding by Myxicola neurofilament proteins

Titrimetric, 45Ca dialysis, and autoradiographic methods were used to examine how axoplasmic proteins from the giant neuron of the marine annelid Myxicola infundibulum bind calcium. Following the autoradiographic method of Maruyama et al., the 150-160 kD neurofilament subunits were identified as prominent intracellular Ca-binding peptides. Using equilibrium dialysis, extracts of axoplasmic proteins (greater than 50% neurofilament subunits) were examined in 300 mM KCl at different concentrations of free Ca and Mg, and at different pH. Axoplasmic proteins showed a high affinity Ca binding site (K1/2 3-6 microM, capacity 3-7 mumole g-1 protein) at pH 6.8 or pH 7.5. Changing the Mg concentration from 0 to 5 mM had no effect on the Ca binding. Elevating the dialysis pH from 7.0 to 9.0 reduced the apparent number of binding sites for Ca. Using microelectrodes to record the free Ca, microtitrations of axoplasmic proteins were completed by adding small amounts of CaCl2 to 100 microliters volumes of protein solutions. In a medium containing ionic constituents closely resembling those of the Myxicola axon, a Ca binding capacity of 5.0 mumole g-1 protein and a K1/2 of approximately 1 microM were measured.

A mechanosensitive ion channel in the yeast plasma membrane

Mechanosensitive ion channels use mechanical energy to gate the dissipation of electrochemical gradients across cell membranes. This function is fundamental to physiological processes such as hearing and touch. In electrophysiological studies of ion channels in the plasma membrane of the yeast Saccharomyces cerevisiae, channels were observed that were activated by, and adapted to, stretching of the membrane. Adaptation of channel activity to mechanical stimuli was voltage-dependent. Because these mechanosensitive channels pass both cations and anions, they may play a role in turgor regulation in this walled organism.

Long-term analysis of organelle translocation in isolated axoplasm of Myxicola infundibulum

Moving intra-axonal organelles demonstrate frequent variations in speed when viewed over several seconds. To evaluate these and other motion variations, a long-term analysis of organelle motion in isolated axoplasm of Myxicola infundibulum was carried out using differential interference contrast optics and analog and digital image enhancement techniques. Motion characteristics of individual organelles were analyzed for periods of up to 58 minutes. Three principle observations on organelle motion were made: 1) Classes of organelles of the same size demonstrated a 5- to 25-fold variation of speed, with the slowest speeds occurring most frequently; 2) organelle speeds over individual translocations (motion without stopping) are inversely proportional to their size, but the speeds calculated for the long-term analysis of organelle motion (total distance travelled/total observation time, including pauses) did not reflect this observation; and 3) organelles displayed variable trip lengths, durations, mean speeds, and pause durations, and the relationships between these variations showed no repetitive patterns. In contrast to reported observations of uniform velocities of organelles moving on isolated microtubule preparations, these observations suggest that a variety of factors must play a role in organelle translocation in Myxicola axoplasm.

Free diffusion coefficient of ionic calcium in cytoplasm

The free diffusion coefficient of ionic Ca was measured in isolated samples of Myxicola axoplasm by following the migration of 45Ca. When precautions were taken to minimize the sequestration and chelation of 45Ca (i.e., using inhibitors, energy deprivation, and saturation of Ca chelation sites), a diffusion coefficient of 5.3 x 10(-6) cm2 s-1 was measured. The diffusion coefficient was not appreciably changed by lowering free calcium from 100 microM to approximately 10 microM or by increasing the diffusion time from ten to twenty minutes. In untreated cytoplasm taken directly from the giant axon of Myxicola, the migration of Ca was more complex and could not be described by a single diffusion coefficient. This result is interpreted to suggest that bulk movement of Ca-buffers may occur in untreated Myxicola axoplasm, a system that contains few microtubules.

Measurements of amino acid transport in internally dialyzed giant axons

It is shown that the axoplasmic composition of acidic and neutral amino acids can be controlled effectively by the method of internal dialysis. Direct assay for specific binding and measurement of diffusion coefficients in axoplasm show that there is no significant binding or compartmentalization of amino acids. The dependence of amino acid efflux on substrate concentration can be measured under well-defined, true steady-state conditions. The taurine efflux-concentration relation in the Myxicola giant axon conforms to a second-order Hill equation. This fact is consistent with either a cooperative process or a mechanism in which membrane translocation is not the rate-controlling step. The effluxes of taurine and glycine from squid axon are an order of magnitude smaller than in Myxicola. The efflux-concentration relations are essentially linear up to 200 mM substrate concentration. This result may be produced by specific transporters which have very high asymmetry, or by simple diffusive leak in the absence of specific transporters.

Calcium and proton buffering and diffusion in isolated cytoplasm from Myxicola axons

Ion-selective electrodes recorded the pH (7.49 +/- 0.05, n = 8) and pCa (6.72 +/- 0.03, n = 40) in samples (approximately 1 microliter) of isolated Myxicola axoplasm mounted within 760-micron diameter plastic tubes. We determined the interactions between Ca2+ and H+ on axoplasmic buffers by microinjecting CaCl2 or HCl into the axoplasmic samples at a distance 75-125 micron from the tips of the electrodes (distance = r). When axoplasmic pH was lowered 0.97 +/- 0.095 from its resting value (measured at r = 125 micron) by injecting 4 nmol HCl, pCa dropped 0.30 +/- 0.05 (n = 6). When expressed in units of concentration, these data show that a HCl injection of approximately 4 mmol/l axoplasm increased H+ and Ca2+ activity by approximately 0.3 microM. Lowering axoplasmic pCa 2.20 +/- 0.43 (r = 75 micron) (n = 3) by injecting 40 pmol CaCl2 had only a small effect on pH. In other experiments, two Ca2+ electrodes measured the Ca2+ activity 125 and 375 micron from the site of CaCl2 injection. Evidence of Ca2+ buffering was obtained when the Ca2+ activity at these two locations was below that expected for simple Ca2+ diffusion away from the injection site. Centrifuged axoplasm (100,000 g) taken from the bottom of the centrifuge tube had a somewhat greater Ca2+ buffering capacity than that taken from the top of the tube. Electron microscopic studies of the centrifuged axoplasm showed a greater concentration of mitochondria and other axoplasmic vesicles in the bottom of the centrifuge tube. Ruthenium red (20-40 micrograms/ml) greatly reduced Ca2+ buffering. The mitochondrial inhibitors CN (2 mM) and oligomycin (a mixture of oligomycin A, B, and C, 5 micrograms/ml) also reduced Ca2+ buffering but were not as effective as ruthenium red. Axoplasm in which ATP and mitochondrial substrates were removed by dialysis was unable to lower free Ca2+ when the concentration of this ion was elevated to approximately 10 microM. In the presence of oligomycin to block mitochondrial ATPase, and with Mg2+ -ATP as the only source of energy, axoplasm lowered Ca2+ activity slowly; with succinate as the only metabolic substrate, axoplasm rapidly lowered the Ca2+ activity from approximately 10 microM to below 1 microM.

Cell composition as assessed from osmolality and concentrations of sodium, potassium and chloride: total contributions of other substances to osmolality and charge balance

From published data for various tissues on intracellular Na, K and Cl were calculated the net anionic charge on all other substances present and also the total contribution of these to osmolality, assuming osmotic equilibrium with extracellular fluid. These parameters were compared for muscle and nerve of animals differing widely in osmolality and also for other mammalian cells. Cell volume regulation in some euryhaline species was considered; it is only partly due to ninhydrin-positive materials. In sheep mammary glands K seems to be sequestered with lactose and anion. Changes in mammalian muscle due to adrenalectomy, hypophysectomy, K deficiency, myotonia, acid-base imbalance and treatment with deoxycorticosterone or insulin were discussed.

The composition of animal cells: solutes contributing to osmotic pressure and charge balance

The cytoplasmic solutes of vertebrates and invertebrates, other than Na, K and Cl, are surveyed in relation to their influence on ionic regulation through osmolality and charge balance. The most abundant include MgATP, phosphagens, amino acids, various other nitrogen and phosphorus compounds and sometimes anaerobic end products and antifreeze agents. Differences in muscle osmolality, e.g. between marine and non-marine animals, affect mainly nitrogenous solutes of no net charge, such as certain amino acids, taurine, betaine, trimethylamine oxide and urea. The high osmolality of axoplasm in marine invertebrates is due more to anions such as aspartate, glutamate and isethionate.

Amino acid transport in Myxicola giant axon: stability of the amino acid pool, taurine efflux, and trans effect of sodium

1. The giant axon of Myxicola infundibulum was assessed for its suitability as a model preparation for study of amino acid transport mechanisms.2. The amino acid composition of axoplasm was measured and compared with those of coelomic fluid, muscle and axon sheath. The axoplasmic composition is unique. Axoplasm/coelomic fluid concentration ratios are all much larger than 1. The axoplasmic amino acid concentrations are (mmol/kg plasm): cysteic acid (104), aspartic acid (75), glutamic acid (10), taurine (64), serine (5), glycine (191) and alanine (5). Other amino acids or primary amines, if present, must have concentrations of less than 1 mm.3. The size of the sheath amino acid pool is 12% or less of the axoplasmic pool.4. The amino acid pool of axons soaked in sea water for up to 24 h is stable. Removal of Na from sea water causes a large increase of net efflux and net production of amino acids.5. Net amino acid production can not be detected in sheath. Metabolic production occurs in axoplasm with little accumulation. Time scales for production and net efflux are therefore similar.6. The Myxicola axon has a vigorous amino acid metabolism and transport systems capable of relatively large fluxes. Homeostasis is strongly linked to Na and may involve Na-coupled co-transport. Conservation of transmembrane amino acid gradients could be promoted in part by trans inhibition of efflux by external Na.7. Taurine is a useful model substrate because it is not catabolized in Myxicola and its net efflux is sensitive to Na. [(3)H]taurine efflux was measured from injected axons. Fluxes and internally recorded action potentials are stable for up to 6 h.8. Axon sheaths take up [(3)H]taurine from 10 mm-taurine sea water with an apparent half-time of 5 h. [(3)H]taurine washout from the apparent extracellular space has a half-time of 5 min. Washout from sheath cells has a half-time of 2-3 h. Sheath is not an important parallel compartment for taurine fluxes in the axon.9. Taurine efflux has a Q(10) of 1.8.10. Taurine efflux is insensitive to external taurine concentrations up to 10 mm.11. Taurine efflux is sensitive to external Na, but only if internal Na is high.12. Taurine is transported by a low-affinity Na-dependent system in Myxicola axon. Results could be explained by a carrier which is more mobile in the empty state than in the substrate-loaded state. Trans inhibition of taurine efflux by external Na is an important property of the system, and contributes to conservation of axoplasmic taurine.

Properties of a calcium-activated protease in squid axoplasm which selectively degrades neurofilament proteins

Axoplasm extruded from the giant axon of the squid contains Ca2+-activated proteases. The protease in the 100,000 x g of supernatant of axoplasm is very specific and degrades only the 200,000 MW, neurofilament protein (NF200), whereas the protease(s) in the pellet has a much wider range of substrate specificity. The activation of the supernatant protease is restricted to the Ca2+ ion, and no other divalent cation will substitute. The protease requires Ca2+ at a higher concentration than 0.5 mM for activation, and has a pH optimum of about 7.5. Degradation of the NF200 appears to proceed through a 100,000 MW and possibly a 47,000--50,000-MW intermediate form before degradation to TCA-soluble peptides. Activity of the protease is inhibited by divalent cation chelators, Cu2+ and Fe2+, sulphydryl inhibitors, and leupeptin. This specific Ca2+-activated protease in squid axoplasm has identical properties to Ca2+-activated proteases found in various non-neural tissues. Despite its narrow protein substrate specificity, Ca2+-activated protease purified from human platelets effectively degrades squid NF200, suggesting a possible structural relationship between platelet and muscle actin-binding proteins and neurofilament proteins.

Internally perfused Myxicola giant axons showing long-term survival

An improved method for internally perfusing the Myxicola giant axon based on removing the axoplasm by dispersing it in KCl-KF salt solutions is described. Proteolytic enzymes are not introduced. With this improved method perfused preparations show long-term stability of their electrical properties and the ability to generate action potentials for many hours. Mean initial values for resting membrane potential, action potential amplitude, and peak inward current were -68 mV, 118 mV, and 3.62 mA/cm2, respectively. Mean resting membrane resistance was 75% of that in intact axons. In one series of voltage clamp experiments, perfused preparations remained excitable for a mean period of 5 1/2 h, but this period could exceed 10 h. 4 min are needed for exchange of internal solutions. At least 50 mM KF is required both in the axoplasm liquefying solution and in the standard perfusate to obtain stable preparations.

Sodium and potassium fluxes across the dialyzed giant axon of Myxicola

Resting and stimulated fluxes of sodium and potassium across the giant axon of the marine annelid, Myxicola infundibulum, have been characterized using the technique of internal dialysis. In most respects the ion movements were found to be similar to those in squid axons. Sodium efflux and potassium influx were found to be active, cardiac glycoside-sensitive fluxes, with a variable coupling ratio. However, when [ATP]i was lowered to less than 20 microM by treatment with cyanide and continuous dialysis, or to less than 2 microM by dialysis with glucose following injection of hexokinase, Na efflux and K influx were unaltered. The maintained fluxes were not accounted for by an increased passive permeability of the axolemma, although 30-60% of the Na efflux appeared to be due to Na-Na exchange. An altered form of Na pump operation at low [ATP]i is a more likely explanation than an alternate energy source, or an ATP source proximate to the axolemma. The transient response of 22Na efflux to a change in [22Na]i was found to be much slower than in squid, tau = 360 sec. The efflux delay could only be accounted for by an extra-axonal diffusion barrier, which is probably the basement membrane surrounding the ventral nerve cord.

The flow properties of axoplasm in a defined chemical environment: influence of anions and calcium

The flow properties of axoplasm have been studied in a defined chemical environment. Axoplasm extruded from squid giant axons was introduced into porous cellulose acetate tubes of diameter roughly equal to that of the original axon. Passage of axoplasm along the tube rapidly coated the tube walls with a layer of protein. By measuring the rate of low back and forth along the tube, the rheological properties of the axoplasm plug were investigated at a range of pressures and in a variety of media. Axoplasm behaves as a classical Bingham body the motion of which can be characterized by a yield stress (theta) and a plastic viscosity (eta p). In a potassium methanesulphonate medium containing 65 nM free Ca2+, theta averaged 109 +/- 46 dyn/cm2 and eta p1 146 +/- 83 P. These values were little affected by ATP, COLCHICINE, CYTOCHOLASIN B or by replacing K by Na but were sensitive to the anion composition of the medium. The effectiveness of different anions at reducing theta and eta p1 was in the order SCN greater than I greater then Br greater than Cl greater than methanesulphonate. Theta and eta p1 were also drastically reduced by increasing the ionized Ca. This effect required millimolar amounts of Ca, was unaffected by the presence of ATP and was irreversible. It could be blocked by the protease inhibitor TLCK. E.p.r. measurements showed that within the matrix of the axoplasm gel there is a watery space that is largely unaffected by anions or calcium.

Cytoplasmic gel and water relations of axon

A previous method of measuring the swelling pressure (delta IIg) of the cytoplasmic gel of the giant axon of Loligo vulgaris was refined. The estimates of delta IIg made with the improved method were consistent with those made with the earlier method. In these methods the activity of the solvent in the gel is measured by increasing the activity of the solvent in the internal phase of the gel by application of hydrostatic pressure to the gel directly. Comparable values for the activity of the solvent in the gel were obtained also by an alternate method, in which the deswelling of the gel is measured upon decreasing the activity of the solvent in the external phase by addition of a nonpenetrating high mol wt polymer (i.e., Ficoll). Additional support was obtained for the earlier suggestion that delta IIg contributes to the swelling and shrinkage pattern of the whole axon. In part, the new evidence involved two consecutive direct measurements of intraxonal pressure. The first measurement was that of a mixed pressure composed of delta IIg and delta IIm (delta IIm being the effective osmotic pressure due to the intra-extraxonal gradient in the activity of mobile solutes). The subsequent measurement was that of delta IIg alone. The latter measurement was made feasible by destroying the axolemma, thereby eliminating the contribution of delta IIm. An estimate of delta IIm was obtained by subtracting delta IIg from the total pressure measured initially. The delta IIm determined by the above method was two orders of magnitude smaller than the theoretical osmotic pressure. This is consistent with the delta IIm determined previously, where osmotic intra-extraxonal filtration coefficients were compared to the hydrostatic. The mixed pressure experiments lend credence to the idea that the substantial contribution of delta IIg to the water relations of the whole axon is due to delta IIg being of the same order of magnitude as delta IIm. The degree of free swelling of axoplasmic gels was studied as a function of pH, salt concentration, and hydration radius of the anion of the salt used. The swelling increased with an increase in the reciprocal of the hydration radius, a decrease in salt concentration, and at pH below or above similar to 4.5. The nature of the constraints to the free swelling of axoplasm in axons immersed in seawater was studied. With the seawater employed, these constraints appeared to be due more to the retractive forces of the sheath than to delta IIm.

Diffusion of ions in myelinated nerve fibers

The diffusion of ions towards or away from the inner side of the nodal membrane in preparations, the cut ends of which are placed in various media, was investigated. The ion concentration changes were calculated by numerical solution of the unidimensional electrodiffusion equation under a variety of media compositions, axoplasmic diffusion coefficients, and internal anionic compositions. The potassium and cesium ion diffusion along the axon towards the node was determined experimentally by two different electrophysiological methods. On the basis of comparison between the experimental data and the computational predictions the axoplasmic potassium ion diffusion coefficient was determined to be almost equal to that in free aqueous solution, while that of cesium ion was close to one half of that in aqueous solution. Utilizing the values of diffusion parameters thus determined, we solved the electrodiffusion equation for a number of common experimental procedures. We found that in short fibers, cut 0.1-0.2 cm at each side of the node, the concentration approached values close to the new steady-state values within 5-30 min. In long fibers (over 1 cm long) steady-state concentrations were obtained only after a few hours. Under some conditions the internal concentrations transiently overshot the steady-state values. The diffusion potentials generated in the system were also evaluated. The ion concentration changes and generation of diffusion potential cannot be prevented by using side pools with cation content identical to that of the axoplasm.

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.

Uptake and binding of calcium by axoplasm isolated from giant axons of Loligo and Myxicola

1. Axoplasm isolated from giant axons of the squid Loligo and of the polychaete worm Myxicola continues to bind Ca and maintain an ionized Ca concentration close to 0.1 microgram which is similar to that seen in intact axons. 2. Injection of Ca into isolated axoplasm only produces a transient rise in ionized Ca showing that axoplasm can buffer a Ca challenge. 3. In order to characterize the Ca-binding systems isolated axoplasm was placed in small dialysis tubes and exposed to a variety of artificial axoplasms containing 45Ca. 4. In the presence of ATP, orthophosphate and succinate, Ca uptake appreciable and after 4 hr exposure of Loligo axoplasm to 0.1 microgram-Ca, approximately 100 mumole Ca/kg axoplasm was bound. Binding could be divided operationally into two distinct processes, one that requires ATP or succinate togeth with orthophosphate and is blocked by cyanide and oligomyocin, and one that is unaffected by these reagents. 5. Energy-dependent binding has a large capacity, but a rather low affinity for Ca, being half-maximal between 20 and 60 microgram-Ca. In Loligo, its properties closely parallel those of a crude mitochondrial preparation isolated from axoplasm; but there are some interesting differences in Myxicola. Energy-independent binding is half-maximal at ionized Ca concentrations between 80 and 160 nM but is readily saturated and has a capacity of 6-60 mumole/kg axoplasm. 6. Ca binding by Loligo is greatest in media containing roughly physiological concentrations of K and is reduced by isosmotic replacement of K by Na. This effect seems to be confined to the energy-dependent, presumed mitochondrial, component of binding. 7. Ca binding by Loligo axoplasm is reduced by both La and Mn ions.

Microtubules and actin in giant nerve fibers of the spiny lobster, Panulirus argus

Giant axons of the spiny lobster, Panulirus argus, are filled with microtubules that are decorated with fine, irregular filaments. Mitochondria and membrane-limited clear vesicles are the only other distinguishable elements in the axoplasm and are located around the periphery of the axon near the axolemma. Neither 100 A neurofilaments nor 70 A microfilaments are evident in fixed, intact axons or in negatively stained axoplasm. Actin-like microfilaments are a prominent constituent of the glial cells that closely ensheathe the axons, and gel electrophoresis studies suggest that most of the actin in the nerve fibers is located in the glia rather than in the axons. Studies of isolated axoplasm indicate that microtubules are the primary elements stabilizing the axoplasm. The microtubules in the isolated axoplasm are disrupted by Ca2+ in the medium in the presence of protease inhibitors.

Structural alterations of peripheral nerve induced by the calcium ionophore A23187

Desheathed segments of rat peripheral nerve were incubated at 37 degrees C in oxygenated Ringer's solution with and without the addition of calcium ionophore, A23187, 10 microgram/ml. Nerve fibers incubated in the presence of both ionophore and calcium revealed extensive granular disintegration of their axonal microtubules and neurofilaments after 30 and 60 min incubation intervals. These changes were not seen following control incubations in Ringer's solution without ionophore or in calcium-free Ringer's solution containing ionophore and EGTA, 1 mmole/1. Ionophore-induced alterations were also noted in Schwann cell cytoplasm. The granular degradative alteration of axoplasm caused by exposure of nerve fibers to ionophore and calcium were believed to be due to an ionophore-mediated influx of calcium into the axoplasm with resultant elevation of intra-axoplasmic calcium concentration. These axoplasmic changes were indistinguishable from the axoplasmic alteration occurring in the distal portions of transpected neurites during Wallerian degeneration. The findings support the view that abnormal calcium influxes are determinants in the degeneration of peripheral nerve.

Sodium efflux in Myxicola giant axons

Several properties of the Na pump in giant axons from the marine annelid Myxicola infundibulum have been determined in an attempt to characterize this preparation for membrane transport studies. Both NaO and KO activated the Na pump of normal microinjected Myxicola axons. In this preparation, the KO activation was less and the NaO activation much greater than that found in the squid giant axon. However, when the intracellular ATP:ADP ratio of the Myxicola axon was elevated by injection of an extraneous phosphagen system, the K sensitivity of Na efflux increased to the magnitude characteristic of squid axons and the activating effect of NaO disappeared. Several axons were injected with Na2SO4 in order to determine the effect of elevated Nai on the Na efflux. Increasing Nai enhanced a component of Na efflux which was insensitive to ouabain and dependent on [Ca] in Na-free (Li) seawater. After subtracting the CaO-dependent fraction, Na efflux was related linearly to [Na]i in all solutions except in K-free (Li) seawater, where it appeared to reach saturation at high [Na]i.

Axoplasm architecture and physical properties as seen in the Myxicola giant axon

A technique is described for extracting axoplasm from the giant axon of a marine worm, Myxicola infundibulum. The operation can be completed in 10 sec. 2. Axoplasm is pulled from the axon of a living worm as a long, clear cylinder, up to 35 cm long and 70 mg wet weight. The worm regenerates a new giant axon in about 4 months. 3. Myxicola axoplasm is a gel, 87% water, held together by protein neurofilaments. It contains small amounts of mitochondria and vesicles, but no detectable microtubules. 4. The internal structure of the gel is superficially similar to that of yarn. Closer inspection with light and electron microscopy, and X-ray diffraction, show it to be organized in a hierarchy of helical forms. Squid giant axons have a similar structure. 5. Initial estimates of the bulk physical properties of extracted Myxicola axoplasm give: breaking strength, 1400 g/cm2; specific gravity, 1-05 g/cm3; birefringence, 1-6 X 10(-4); index of refraction, 1-351; resistivity, 57 omega cm. These average values are shown to be compatible with the observed structure and composition. 6. Despite its mechanical strength, the axoplasm gel is so hydrated that Na+, K+ and homarine diffuse through it at rates approaching those in free solution. Fewer than about 5% of each of these ions are tightly bound to the gel. 7. It is argued that (a) the structure and physical properties of Myxicola axoplasm are representative of those in other axons, (b) the compound helix architecture results from twist of parallel, cross-linked fibrous proteins, and (c) this sturcture serves as a flexible internal skeleton for nerve cell processes.