Ion Current Rectification,Limiting and Overlimiting Conductances in Nanopores

Previous reports on Poisson-Nernst-Planck (PNP) simulations of solid-state nanopores have focused on steady state behaviour under simplified boundary conditions. These are Neumann boundary conditions for the voltage at the pore walls, and in some cases also Donnan equilibrium boundary conditions for concentrations and voltages at both entrances of the nanopore. In this paper, we report time-dependent and steady state PNP simulations under less restrictive boundary conditions, including Neumann boundary conditions applied throughout the membrane relatively far away from the nanopore. We simulated ion currents through cylindrical and conical nanopores with several surface charge configurations, studying the spatial and temporal dependence of the currents contributed by each ion species. This revealed that, due to slow co-diffusion of oppositely charged ions, steady state is generally not reached in simulations or in practice. Furthermore, it is shown that ion concentration polarization is responsible for the observed limiting conductances and ion current rectification in nanopores with asymmetric surface charges or shapes. Hence, after more than a decade of collective research attempting to understand the nature of ion current rectification in solid-state nanopores, a relatively intuitive model is retrieved. Moreover, we measured and simulated current-voltage characteristics of rectifying silicon nitride nanopores presenting overlimiting conductances. The similarity between measurement and simulation shows that overlimiting conductances can result from the increased conductance of the electric double-layer at the membrane surface at the depletion side due to voltage-induced polarization charges. The MATLAB source code of the simulation software is available via the website http://micr.vub.ac.be.     

Depolarization affects neuromuscular junction of streptozotocin-diabetic mice.

The effect of increasing extracellular calcium concentration on spontaneous transmitter release was studied at both soleus (slow) and fast extensor digitorum longus (EDL) nerve terminals of control and streptozotocin-induced diabetic (STZ-D) young C57 BL mice (7 months old) depolarized by high (20 mM) extracellular potassium [K]o. Diabetes was induced by i.p. injection with a single dose of streptozotocin (200 mg/kg) at the age 5 months and the electrophysiological studies were carried out after 8 more weeks. By using intracellular recording, miniature endplate potentials (MEPPs) were first recorded in a normal [K]o Krebs solution. Subsequently, MEPPs were recorded in high [K]o Krebs solution with 4 different Ca concentrations: Ca-free/ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetra acetic acid (EGTA), 0.5, 1.5 and 2 mM Ca. MEPP frequency was lower at STZ-D than control nerve terminals in EDL but not soleus. However, MEPP frequency was progressively higher at both EDL and soleus of STZ-D than control with increasing Ca concentration in Krebs that contained 20 mM [K]o. In STZ-D slow soleus muscle, depolarization produced 0.7, 4.3, 41.6 and 62.7 vs 1.4, 2.8, 20.7 and 31.6 Hz for control in the 4 different Ca concentrations. In STZ-D fast EDL muscle, depolarization produced 0.5, 4.9, 48.2 and 66.8 vs 1.2, 2.5, 27 and 35.4 Hz for control in the 4 different Ca concentrations. Bimodal and unimodal MEPP amplitude were present at both slow and fast nerve terminals. However, depolarization increased the percentage of bimodal MEPP amplitude in STZ-D compared to control (p<0.01) mice in EDL but not soleus. The results revealed that these changes in muscle firing pattern may provide a protective effect against diabetes-induced neuropathy at the neuromuscular junction.     

Regulation of peroxo-bridge formation between cobalt(II)-carnosine complexes by histidine: Possible involvement in polycythemia

The mechanisms by which histidine stabilizes the cobalt(II)-carnosine complex from oxidation to cobalt(III) in aqueous solution are investigated with ¹H-nmr, laser Raman, and Fourier transform-infrared spectroscopy. Histidine has at least three effects on the cobalt(II)-carnosine complex. First, over the concentration range of at least 5 to 250 mM, histidine stabilizes the cobalt(II)-carnosine complex from oxidation by excluding solvent molecules from the equatorial coordination positions. Second, at the upper end of this concentration range, histidine reduces the strained nonplanarity of the equatorial coordination positions around the cobalt(II) ion that results from tridentate chelation by carnosine. Bidentate ligation by histidine causes the carnosine to bind as a bidentate ligand also. Third, bidentate ligation of two carnosine molecules to the equatorial coordination positions of Co(II) ion places the β-alanyl residues inthe vicinity of the two axial coordination positions and thereby inhibits the binding of molecular oxygen. Substitution of a molecule of histidine for one of these two carnosine molecules makes an axial coordination position available for binding oxygen. The first two effects are expected to stabilize the cobalt(II) ion from rapid oxidation, whereas the third effect is expected to give long-term stability of the peroxo-bridged complex. Since bidentate ligation of histidine is favored over monodentate ligation only when the concentration of Co(II) ion is not limiting and is inhibited by high concentrations of carnosine in the same solution, the results presented provide a possible explanation for the observation that the stability of the Co(II) complexes toward oxidation and their ability to bind molecular oxygen depend on both the relative and absolute concentrations of Co(II) ion, carnosine, and histidine in solution. Furthermore, these results provide additional support to the suggestion that the high activity of carnosinase in kidney is involved in part in regulation of the oxygen sensor in this organ.     

On the mechanism of rectification of the isoproterenol-activated chloride current in guinea-pig ventricular myocytes

The whole cell configuration of the patch clamp technique was used to investigate the mechanism underlying rectification of the isoproterenol- activated chloride (Cl-) current in isolated guinea pig ventricular myocytes. When extracellular Cl- was replaced with either bromide (Br- ), glutamate (Glut), iodide (I-), isethionate (Iseth), or nitrate (NO3- ), the magnitude of the shift in reversal potential of the macroscopic current suggested the following selectivity sequence: NO3- > Br- > or = Cl- > or = I- > Iseth > or = Glut. This information was used to investigate the role of permeant ions in rectification of this current. Consistent with previous observations, when the concentration of intracellular Cl- (Cli-) was less than the concentration of extracellular Cl- (Clo-) (40 mM Cli-/150 mM Clo-) the current exhibited outward rectification, but when Cli- was increased to equal that outside (150 Cli-/150 Clo-), the current no longer rectified. Rectification in the presence of asymmetrical concentrations of permeant ions on either side of the membrane is predicted by constant field theory, as described by the Goldman-Hodgkin-Katz current equation. However, when the Cl- gradient was reversed (150 Cli-/40 Clo- ) the current did not rectify in the opposite direction, and in the presence of lower symmetrical concentrations of Cl- inside and out (40 Cli-/40 Clo-), outward rectification did not disappear. Reducing Cli- by equimolar replacement with glutamate caused a concentration dependent increase in the degree of rectification. However, when Cli- was replaced with more permeant anions (NO3- and Br-), rectification was not observed. These results can be explained by a single binding site model based on Eyring rate theory, indicating that rectification is a function of the concentration and the permeability of the anions in the intracellular solution.     

Positive charges at the intracellular mouth of the pore regulate anion conduction in the CFTR chloride channel

Many different ion channel pores are thought to have charged amino acid residues clustered around their entrances. The so-called surface charges contributed by these residues can play important roles in attracting oppositely charged ions from the bulk solution on one side of the membrane, increasing effective local counterion concentration and favoring rapid ion movement through the channel. Here we use site-directed mutagenesis to identify arginine residues contributing important surface charges in the intracellular mouth of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel pore. While wild-type CFTR was associated with a linear current-voltage relationship with symmetrical solutions, strong outward rectification was observed after mutagenesis of two arginine residues (R303 and R352) located near the intracellular ends of the fifth and sixth transmembrane regions. Current rectification was dependent on the charge present at these positions, consistent with an electrostatic effect. Furthermore, mutagenesis-induced rectification was more pronounced at lower Cl(-) concentrations, suggesting that these mutants had a reduced ability to concentrate Cl(-) ions near the inner pore mouth. R303 and R352 mutants exhibited reduced single channel conductance, especially at negative membrane potentials, that was dependent on the charge of the amino acid residue present at these positions. However, the very low conductance of both R303E and R352E-CFTR could be greatly increased by elevating intracellular Cl(-) concentration. Modification of an introduced cysteine residue at position 303 by charged methanethiosulfonate reagents reproduced charge-dependent effects on current rectification. Mutagenesis of arginine residues in the second and tenth transmembrane regions also altered channel permeation properties, however these effects were not consistent with changes in channel surface charges. These results suggest that positively charged arginine residues act to concentrate Cl(-) ions at the inner mouth of the CFTR pore, and that this contributes to maximization of the rate of Cl(-) ion permeation through the pore.     

Accumulation and radial transport of ions from potassium salts by cucumber roots

Accumulation of K(+) is insensitive to the anion supplied with it at a solution concentration below 1 mm. Rates of K(+) transport to the xylem from the same solutions are, however, dependent upon the anion present and decrease in the order NO(3) (-) > Cl(-) > SO(4) (2-). Parallel effects on rates of exudation and anion transport result from kind and concentration of anion supplied and time of exposure to the solution. When high K salt concentrations are used, only linear relationships are found between solution concentrations and transport rates. However, ion concentration in the exudate increases more than external solution concentration, while exudation rate is unaffected. It is suggested that some of the ions transported are from compartments within the cells. At high solution concentrations KNO(3) results in more exudation and in higher ion concentration in the exudate than is found with KCl.     

Analysis of Rat Hindlimb Muscle Proteins by Two-Dimensional Gels Following Spinal Cord Injury

Proteins from extensor digitorum longus (EDL), plantaris (PLN), and soleus (SOL) muscles of adult, female rats were examined by high resolution two-dimensional gel electrophoresis up to 4 weeks following spinal cord transection. The electrophoretograms were analyzed by computer imaging and densitometry. Reproducible and significant changes in the relative concentrations of several proteins in each muscle type were detected. Whereas changes involving the largest number of proteins were observed in SOL, changes in EDL and PLN were also detected. In SOL, approximately 50% of the altered proteins increased in concentration and the remaining decreased: Actin and myosin light chains LCF-1 and LCF-2 were among those proteins whose concentrations increased, whereas myosin light chains LCS-1 and LCS-2 were among those proteins whose concentrations decreased. The present findings regarding the reversal in myosin light chain composition provide biochemical support for previously published data on changes in contractile properties of muscles following spinalization. In EDL, the relative concentration of only one protein was elevated in a time-dependent manner. The concentrations of two protein species in PLN were increased following cord transection. These findings provide new biochemical markers on the effects of spinal cord on gene expression in specific hindlimb skeletal muscles.     

Biophysical properties of Opuntia ficus-indica mucilage

The purified mucilage from Opuntia ficus-indica is a high MW polysaccharide which behaves as a polyelectrolyte. Viscosity of its solution is dependent on the Ca²⁺ ion concentration and on pH, being greatest at alkaline pH. The sedimentation coefficient was dependent on concentration. The molecule had an estimated axial ratio of 256 in water, and this was reduced at low pH and in the presence of high concentrations of Ca²⁺. The molecule was studied with light scattering and CD techniques and its UV spectrum was recorded. All these parameters were influenced by pH and by ion concentration. The gelation properties also changed with pH and with Ca²⁺ giving dense gels in its presence and loose ones in its absence. The results are interpreted in terms of changes in conformation of the molecule, changes in Ca²⁺ binding and degree of ionization of the molecule. An attempt is made to relate the molecular properties to the physiological function of the mucilage in the calcium and water economy of the plant.     

Effects of cytochalasin B, N-formyl peptide and plasma on polarisation, zeiosis (blebbing) and degranulation of polymorphonuclear leukocytes in suspension

The effects were studied of cytochalasin B and N-formyl peptide (FMLP) in various concentrations on the morphology and degranulation (release of the granule contents lysozyme and beta-glucuronidase) of polymorphonuclear leukocytes (PMN) suspended in either Hanks' solution or 100% fresh heparinised plasma. PMN in low concentrations of FMLP in Hanks' solution or in plasma alone showed "long" polarisation and did not degranulate. Cytochalasin B caused the PMN in low concentrations of FMLP or in plasma to become spherical, but no degranulation of the cells occurred. High concentrations of FMLP in Hanks' solution induced "short" polarisation of PMN with slight degranulation of the cells. Cytochalasin B together with high concentrations of FMLP in Hanks' solution induced zeiosis ("blebbing") and marked degranulation of the cells. However, cytochalasin B and high concentrations of FMLP in plasma caused PMN to exhibit "short" polarised morphology and markedly degranulate. These results suggest that degranulation of PMN can be associated with either the "short" polarised shape of the cells or zeiosis, but not the "long" polarised form. Furthermore, the results indicate that plasma, although capable of causing "long" polarisation of the cells, inhibits zeiosis without affecting the degranulation of the cells induced by cytochalasin B.     

Predicted profiles of ion concentrations in olfactory cilia in the steady state

The role of ciliary geometry for transduction events was explored by numerical simulation. The changes in intraciliary ion concentrations, suspected to occur during transduction, could thus be estimated. The case of a single excised cilium, having a uniform distribution of membrane channels, voltage clamped to -80 mV, was especially investigated. The axial profile of membrane voltage was that of a leaky cable. The Ca(2+) concentration profile tended to show a maximum in proximal segments, due to a preponderance of Ca(2+) inflow over Ca(2+) export at those locations. The local increase in Ca(2+) concentration activated Cl(-) channels. The resulting current caused a local drop in Cl(-) concentration, especially at the tip of the cilium and in distal segments, accompanied by a drop in ciliary K(+) concentration. In consequence, the membrane Cl(-) current was low in distal segments but stronger in proximal segments, where resupply was sufficient. The model predicts that the Cl(-) depletion will codetermine the time course of the receptor potential or current and the ciliary stimulus-response curve. In conclusion, when modeling with transduction elements presently known to participate, the ciliary geometry has large effects on ion distributions and transduction currents because ciliary ion transport is limited by axial electrodiffusion.     

Concentrations of signal transduction proteins exercise and insulin responses in rat extensor digitorum longus and soleus muscles

Differences in the concentrations of signal transduction proteins often alter cellular function and phenotype, as is evident from numerous, heterozygous knockout mouse models for signal transduction proteins. Here, we measured signal transduction proteins involved in the adaptation to exercise and insulin signalling in fast rat extensor digitorum longus (EDL; 3% type I fibres) and the slow soleus muscles (84% type I fibres). The EDL and soleus were excised from four rats, the proteins extracted and subjected to Western blots for various signal transduction proteins. Our results show major differences in signal transduction protein concentrations between EDL and soleus. The EDL to soleus concentration ratios were: Calcineurin: 1.43 +/- 0.10; ERK1: 0.38 +/- 0.18; ERK2: 0.61 +/- 0.16; p38alpha, beta: 1.36 +/- 0.15; p38gamma/ERK6: 0.95 +/- 0.11; PKB/AKT: 1.44 +/- 0.08; p70S6k: 6.86 +/- 3.58; GSK3beta: 0.69 +/- 0.03; myostatin: 1.95 +/- 0.43; NF-kappaB: 0.32 +/- 0.10 (values >1 indicate higher expression in the EDL, and values < 1 indicate higher expression in the soleus). With the exception of p38gamma/ERK6, the concentration of each signal transduction protein was uniformly higher in one muscle than in the other in all four animals. These experiments show that signal transduction protein concentrations vary between fast and slow muscles, presumably reflecting a concentration difference on a fibre level. Proteins that promote particular functions such as growth or slow phenotype are not necessarily higher in muscles with that particular trait (e.g. higher in larger fibres or slow muscle). Interindividual differences in fibre composition might explain variable responses to training and insulin.     

Effects of caffeine at different temperatures on contractile properties of slow-twitch and fast-twitch rat muscles

The slow-twitch soleus muscle (SOL) exhibits decreased twitch tension (cold depression) in response to a decreased temperature, whereas the fast-twitch extensor digitorum longus (EDL) muscle shows enhanced twitch tension (cold potentiation). On the other hand, the slow-twitch SOL muscle is more sensitive to twitch potentiation and contractures evoked by caffeine than the fast-twitch EDL muscle. In order to reveal the effects of these counteracting conditions (temperature and caffeine), we have studied the combined effects of temperature changes on the potentiation effects of caffeine in modulating muscle contractions and contractures in both muscles. Isolated muscles, bathed in a Tyrode solution containing 0.1-60 mM caffeine, were stimulated directly and isometric single twitches, fused tetanic contractions and contractures were recorded at 35 degrees C and 20 degrees C. Our results showed that twitches and tetani of both SOL and EDL were potentiated and prolonged in the presence of 0.3-10 mM caffeine. Despite the cold depression, the extent of potentiation of the twitch tension by caffeine in the SOL muscle at 20 degrees C was by 10-15 % higher than that at 35 degrees C, while no significant difference was noted in the EDL muscle between both temperatures. Since the increase of twitch tension was significantly higher than potentiation of tetani in both muscles, the twitch-tetanus ratio was enhanced. Higher concentrations of caffeine induced contractures in both muscles; the contracture threshold was, however, lower in the SOL than in the EDL muscle at both temperatures. Furthermore, the maximal tension was achieved at lower caffeine concentrations in the SOL muscle at both 35 degrees C and 20 degrees C compared to the EDL muscle. These effects of caffeine were rapidly and completely reversed in both muscles when the test solution was replaced by the Tyrode solution. The results have indicated that the potentiation effect of caffeine is both time- and temperature-dependent process that is more pronounced in the slow-twitch SOL than in the fast-twitch EDL muscles.     

ACTION OF POTASSIUM AND NARCOTICS ON RECTIFICATION IN NERVE AND MUSCLE

Electrical rectification was demonstrated in whole sartorius muscle and sciatic nerve of Rana pipiens and also in the single giant nerve fiber of the northern squid, Ommastrephes illecibrossus. It is probably a property of the plasma membrane. Rectification decreases reversibly under the influence of increased concentrations of the potassium ion and with chloroform, veratrine sulfate and isoamyl carbamate. No effect was found with lack of calcium, excess calcium, or barium chloride. Decrease in rectification is invariably accompanied by simultaneous decrease in resting potential. A proposed explanation of the mechanism of rectification is discussed. Rectification in a living membrane, viz. a change in resistance with change in direction of current flow, may possibly be explained in terms of a change in the concentration of potassium ions in the membrane.     

External potassium and action potential propagation in rat fast and slow twitch muscles.

The role of extracellular K+ concentration in the propagation velocity of action potential was tested in isolated rat skeletal muscles. Different K+ concentrations were produced by KCl additions to extracellular solution. Action potentials were measured extracellularly by means of two annular platinum electrodes. Fibre bundles of m. soleus (SOL), m. extensor digitorum longus (EDL), red (SMR) and white (SMW) part of m. sternomastoideus were maximum stimulated. The conduction velocity (c.v.) was calculated from the distance between the electrodes and the time delay of the potentials measured at 22 degrees C. In Tyrode solution containing 5 mmol/l K+, the c.v. was close to 1 m.s-1. Bundles of the fast muscle type seemed to have a somewhat higher c.v. The differences observed in these studies were not significant. At higher temperatures, the c.v. increased (Q10 of approx. 2) and a dissociation between SMR and SMW muscles appeared. An elevation of K+ concentration to 10 mmol/l induced a drop of the c.v. by approx. 25% and 15% in EDL and SOL muscles, respectively. After return to normal solution, the recovery was not complete within 30 min. In K+ free solution the c.v. of EDL and SM muscles rose by a factor of 1.5, but less in SOL muscles. The weaker response of SOL to K+ modification was related to the higher resistance of this muscle to fatigue. This suggestion was supported by experiments on fatigued fibre bundles. Immediately after a tetanic stimulation producing fatigue, the c.v. of EDL and SOL muscles dropped similarly as in 10 mmol/l K+; again, the drop was less for SOL muscles. Adrenaline (0.5-10.0 mumol/l) enhanced both the c.v. and the twitch amplitude. The results support the suggestion that extracellular K+ accumulation during activity is an essential factor of muscle fatigue.     

Changes in the synaptic activity in sciatic nerve-extensor digitorum longus (preparations induced by ethanol)

The aim of this research is the study of the modification of synaptic activity caused by ethanol in the rat sciatic nerve-extensor digitorum longus (EDL) muscle preparation. For such a purpose, intracellular recordings have been carried out, keeping the muscle immersed in normal Ringer solution and in Ringer solutions containing ethanol at different concentrations up to 0,8 M. Therefore, the resting potential of muscle cells and the frequency of m.e.p.p.s were measured. Qualitative observations of m.e.p.p.s shape were also carried out. Ethanol increases the frequency of m.e.p.p.s in the rat sciatic nerve - EDL muscle preparation. The logarithm of relative frequency (frequency in Ringer solution with ethanol/frequency in normal Ringer solution) is linear with respect to the concentration of ethanol, with a slope of 1.44. Furthermore, ethanol increases the amplitude and lengthens the time course of m.e.p.p.s. The muscle cells undergo a hyperpolarization of about 2-3% at the lowest concentrations of ethanol tested.     

Molecular-dynamic simulation of phospholipid bilayers: Ion distribution at the surface of neutral and charged bilayer in the liquid crystalline state

The electric field and ion distribution at the surface of neutral and charged lipid bilayers (BeCl₂ and dipalmitoyl phosphatidylcholine/dipalmitoyl phosphatidylserine (DPPC/DPPS) + KCl) were studied with molecular dynamic (MD) methods. It is shown that the contributions of lipid molecules, water and ions to the electric potential compensate each other in the region of the diffuse double layer and decrease the potential value close to zero. It is also demonstrated that the ion distribution at the charged surface is determined not only by the electrostatic ion-medium interaction. The total energy of this interaction was compared with the potential of mean ion force. It was shown that cations and anions have a different effect on the state of water molecules at the surface. The order parameter of water in the system DPPC + BeCl₂ and the Clion distribution have the extremum at the distance of 10 α atoms of the phospholipid glycerol. This position was chosen as the “electrical” interface of the electrical double layer (EDL) for all lipid systems studied. The potential of mean force of counter ions in EDL allows us to obtain the value of potential at the lipid surface suitable for experimental test of the MD data. This surface potential and surface charge density was found from MD simulation different electrolyte concentrations and DPPS content of 20, 40 and 60% in the mixture with DPPC and was shown to be in a good agreement with the Gouy-Chapman-Stern model upon fitting parameters close to their experimental values.     

Comparison of monovalent and divalent ion distributions around a DNA duplex with molecular dynamics simulation and a Poisson‐Boltzmann approach

The ion atmosphere created by monovalent (Na⁺) or divalent (Mg²⁺) cations surrounding a B‐form DNA duplex were examined using atomistic molecular dynamics (MD) simulations and the nonlinear Poisson‐Boltzmann (PB) equation. The ion distributions predicted by the two methods were compared using plots of radial and two‐dimensional cation concentrations and by calculating the total number of cations and net solution charge surrounding the DNA. Na⁺ ion distributions near the DNA were more diffuse in PB calculations than in corresponding MD simulations, with PB calculations predicting lower concentrations near DNA groove sites and phosphate groups and a higher concentration in the region between these locations. Other than this difference, the Na⁺ distributions generated by the two methods largely agreed, as both predicted similar locations of high Na⁺ concentration and nearly identical values of the number of cations and the net solution charge at all distances from the DNA. In contrast, there was greater disagreement between the two methods for Mg²⁺ cation concentration profiles, as both the locations and magnitudes of peaks in Mg²⁺ concentration were different. Despite experimental and simulation observations that Mg²⁺ typically maintains its first solvation shell when interacting with nucleic acids, modeling Mg²⁺ as an unsolvated ion during PB calculations improved the agreement of the Mg²⁺ ion atmosphere predicted by the two methods and allowed for values of the number of bound ions and net solution charge surrounding the DNA from PB calculations that approached the values observed in MD simulations. © 2014 Wiley Periodicals, Inc. Biopolymers 101: 834–848, 2014.     

Regulation of glucose uptake in rat slow and fast skeletal muscles

1. Regulation of glucose uptake was compared between extensor digitorum longus (EDL) and soleus (Sol) muscles in rats. 2. Insulin stimulated glucose uptake more in EDL than in Sol. 3. Under high concentrations of insulin, the glucose uptake was higher in EDL than Sol. 4. Inhibition of oxidative phosphorylation by anoxia or an uncoupler stimulated glucose uptake more in EDL than in Sol. 5. Anoxia abolished the effect of insulin on glucose uptake in both EDL and Sol. 6. The blocker to glucose transport system reduced glucose uptake more in Sol than in EDL.     

A kinetic study of phosphorus absorption by excised maize roots from flowing solutions with different phosphorus concentrations

The effect of two different mechanisms of phosphorus ion transport from the nutrient solution volume to the surface areas of excised maize roots was studied under concentrations ranging from 0.01 mM to 50.0 mM KH₂PO₄. A modified technique of study of kinetic ion absorption was used. In the control series, the roots were placed in absorption solution without flow (the dominant mechanism of ion transport to the roots being diffusion), while in the experimental series the absorption solution was flowing round the roots at a rate of 0.162 cm s^?1 (the dominant mechanism of ion transport to the roots being mass flow). The rate of phosphorus absorption by the roots from flowing solutions was highly significantly increased at all concentrations of absorption solution except for the 50.0 mM KH₂PO₄ concentration. The increase in phosphorus absorption in the case of 50.0 mM KH₂PO₄ concentration was non-significant due to the fact that the high concentration of phosphorus together with the diffusion of phosphorus ions ensured a sufficient supply of phosphorus to the roots, covering the requirement for their uptake. The results point to the need for an analysis of environmental factors to be carried out in studying ion absorption kinetics, and reveal the inadequacy of methods usually employed in such investigations, in particular with respect to the homogeneity of the nutrient solution in the whole of its volume and especially round the roots.     

Glucocorticoid-induced glycogen increases in extensor digitorum longus and soleus grafts in the rat

The purpose of the present study was to compare dexamethasone-induced glycogen increases in normal EDL and SOL muscles with that in free muscle grafts. Glycogen in mature EDL and SOL grafts in the rat equalled control concentrations irrespective of whether the graft was a nerve-intact (NI), nerve-crushed (NC), reimplanted, or cross-transplanted graft. The grafts also possessed the glycogen-regulatory mechanisms to respond to the glucocorticoid dexamethasone (DEX), which increases muscle glycogen. The increase in glycogen induced by DEX in the EDL and SOL grafts resembled that of the EDL and SOL muscles, respectively, whether the grafted muscle was originally an EDL or SOL. DEX induced an approximate twofold increase in glycogen concentration in control muscles and nerve-intact SOL grafts, and a smaller but significant increase in all other free grafts. Nerve crushing prior to grafting resulted in no significant change in muscle weight, glycogen concentration, or DEX-induced glycogen increase in these grafts. The data suggest that skeletal muscle grafts are qualitatively similar to normal muscles in terms of metabolic responsiveness to hormones. Leaving the nerve intact during grafting quantitatively enhances the graft's hormonal sensitivity but the technique of nerve crushing prior to grafting has no such effect.