A Multi-Scale Approach to Model K+ Permeation Through the KcsA Channel

K+ channels allow a very efficient passage of K+ ions through the membrane while excluding Na+ ions, and these properties are essential for life. The 3D structure of the KcsA K+ channel, solved more than 20 years ago, allows to address many relevant aspects of K+ permeation and selectivity mechanisms at the molecular level. Recent crystallographic data and molecular dynamics (MD) studies suggest that no water is normally present inside the selectivity filter (SF), which can instead accommodate four adjacent K+ ions. Using a multi-scale approach, whereby information taken from a low-level simulation approach is used to feed a high-level model, we studied the mechanism of K+ permeation through KcsA channels. More specifically, we used MD to find stable ion configurations under physiological conditions. They were characterized by two adjacent K+ ions occupying the more central positions of the SF (sites S2 and S3), while the other two K+ ions could be found at the external and internal entrances to the SF. Sites S1 and S4 were instead not occupied by K+. A continuum Bikerman–Poisson–Boltzmann model that takes into account the volume of the ions and their dehydration when entering the SF fully confirmed the MD results, showing peaks of K+ occupancy at S2, S3, and the external and internal entrances, with S1 and S4 sites being virtually never occupied by K+. Inspired by the newly found ion configuration in the SF at equilibrium, we developed a simple kinetic permeation model which, fed with kinetic rate constants assessed from molecular meta-dynamics, reproduced the main permeation properties of the KcsA channel found experimentally, including sublinear current-voltage and saturating conductance-concentration relationships. This good agreement with the experimental data also implies that the ion configuration in the SF we identified at equilibrium would also be a key configuration during permeation.

KCa3.1 channels are involved in the infiltrative behavior of glioblastoma in vivo

Glioblastoma multiforme (GBM) is a diffuse brain tumor characterized by high infiltration in the brain parenchyma rendering the tumor difficult to eradicate by neurosurgery. Efforts to identify molecular targets involved in the invasive behavior of GBM suggested ion channel inhibition as a promising therapeutic approach. To determine if the Ca(2+)-dependent K(+) channel KCa3.1 could represent a key element for GBM brain infiltration, human GL-15 cells were xenografted into the brain of SCID mice that were then treated with the specific KCa3.1 blocker TRAM-34 (1-((2-chlorophenyl) (diphenyl)methyl)-1H-pyrazole). After 5 weeks of treatment, immunofluorescence analyses of cerebral slices revealed reduced tumor infiltration and astrogliosis surrounding the tumor, compared with untreated mice. Significant reduction of tumor infiltration was also observed in the brain of mice transplanted with KCa3.1-silenced GL-15 cells, indicating a direct effect of TRAM-34 on GBM-expressed KCa3.1 channels. As KCa3.1 channels are also expressed on microglia, we investigated the effects of TRAM-34 on microglia activation in GL-15 transplanted mice and found a reduction of CD68 staining in treated mice. Similar results were observed in vitro where TRAM-34 reduced both phagocytosis and chemotactic activity of primary microglia exposed to GBM-conditioned medium. Taken together, these results indicate that KCa3.1 activity has an important role in GBM invasiveness in vivo and that its inhibition directly affects glioma cell migration and reduces astrocytosis and microglia activation in response to tumor-released factors. KCa3.1 channel inhibition therefore constitutes a potential novel therapeutic approach to reduce GBM spreading into the surrounding tissue.

Trigeminal ganglion neuron subtype-specific alterations of Ca(V)2.1 calcium current and excitability in a Cacna1a mouse model of migraine

Familial hemiplegic migraine type-1 (FHM1), a monogenic subtype of migraine with aura, is caused by gain-of-function mutations in Ca(V)2.1 (P/Q-type) calcium channels. The consequences of FHM1 mutations on the trigeminovascular pathway that generates migraine headache remain largely unexplored. Here we studied the calcium currents and excitability properties of two subpopulations of small-diameter trigeminal ganglion (TG) neurons from adult wild-type (WT) and R192Q FHM1 knockin (KI) mice: capsaicin-sensitive neurons without T-type calcium currents (CS) and capsaicin-insensitive neurons characterized by the expression of T-type calcium currents (CI-T). Small TG neurons retrogradely labelled from the dura are mostly CS neurons, while CI-T neurons were not present in the labelled population. CS and CI-T neurons express Ca(V)2.1 channels with different activation properties, and the Ca(V)2.1 channels are differently affected by the FHM1 mutation in the two TG neuron subtypes. In CI-T neurons from FHM1 KI mice there was a larger P/Q-type current density following mild depolarizations, a larger action potential (AP)-evoked calcium current and a longer AP duration when compared to CI-T neurons from WT mice. In striking contrast, the P/Q-type current density, voltage dependence and kinetics were not altered by the FHM1 mutation in CS neurons. The excitability properties of mutant CS neurons were also unaltered. Congruently, the FHM1 mutation did not alter depolarization-evoked CGRP release from the dura mater, while CGRP release from the trigeminal ganglion was larger in KI compared to WT mice. Our findings suggest that the facilitation of peripheral mechanisms of CGRP action, such as dural vasodilatation and nociceptor sensitization at the meninges, does not contribute to the generation of headache in FHM1.

Histamine activates a background, arachidonic acid-sensitive K channel in embryonic chick dorsal root ganglion neurons

Histamine has been proposed to be an important modulator of developing neurons, but its mechanism of action remains unclear. In embryonic chick dorsal root ganglion neurons we found that histamine activates, through the pyrilamine-sensitive H1 receptor, a K-selective, background channel. The K channel activated by histamine was also activated by arachidonic acid in a dose-dependent way, with a KD of 4 microM and a slope of 2.5, had a unitary conductance of about 150 pS (symmetrical 140 KCl) and a moderate voltage dependence. The channel was insensitive to the classical K channel blockers tetraethylammonium, charybdotoxin, 4-aminopyridine, but inhibited by millimolar Ba2+. Channel activity could also be increased by lowering the intracellular pH from 7.2 to 5.5, or by applying negative pressure pulses through the patch pipette. Experiments aimed at delineating the metabotropic pathway leading to K channel activation by histamine indicated the involvement of a pertussis toxin-insensitive G protein, and a quinacrine-sensitive cytosolic phospholipase A2. The histamine-induced K channel activation was observed only with elevated internal Ca2+ (achieved using 0.5 microM ionomycin or elevated external KCl). An increase in the histamine-induced phosphoinositide hydrolysis was also observed upon internal Ca2+ elevation, showing the presence of a Ca2+ dependent step upstream to inositol 1,4,5-triphosphate production. In view of the functional importance of K conductances during cell differentiation, we propose that histamine activation of this K channel may have a significant role during normal development of embryonic chick neurons.

A fast transient outward monovalent current in rat saphenous myocytes passing through Ca2+ channels

Transient outward currents in rat saphenous arterial myocytes were studied using the perforated configuration of the patch-clamp method. When myocytes were bathed in a Na-gluconate solution containing TEA to block large-conductance Ca2+-activated K+ (BK) currents, depolarizing pulses positive to +20 mV from a holding potential of -100 mV induced fast transient outward currents. The activation and inactivation time constants of the current were voltage dependent, and at +40 mV were 3.6 +/- 0.8 ms and 23.9 +/- 6.4 ms (n = 4), respectively. The steady-state inactivation of the transient outward current was steeply voltage dependent (z = 1.7), with 50% of the current inactivated at -55 mV. The current was insensitive to the A-type K+ channel blocker 4-AP (1-5 mM), and was modulated by external Ca, decreasing to approximately 0.85 of control values upon raising Ca2+ from 1 to 10 mM, and increasing approximately 3-fold upon lowering it to 0.1 mM. Transient outward currents were also recorded following replacement of internal K+ with either Na+ or Cs+, raising the possibility that the current was carried by monovalent ions passing through voltage-gated Ca2+ channels. This hypothesis was supported by the finding that the transient outward current had the same inactivation rate as the inward Ba2+ current, and that both currents were effectively blocked by the L-type Ca2+ channel blocker, nifedipine and enhanced by the agonist BAYK8644.

Large-conductance calcium-activated potassium channels in neonatal rat intracardiac ganglion neurons

The properties of single Ca2+-activated K+ (BK) channels in neonatal rat intracardiac neurons were investigated using the patch-clamp recording technique. In symmetrical 140 mM K+, the single-channel slope conductance was linear in the voltage range -60/+60 mV, and was 207+/-19 pS. Na+ ions were not measurably permeant through the open channel. Channel activity increased with the cytoplasmic free Ca2+ concentration ([Ca2+]i) with a Hill plot giving a half-saturating [Ca2+] (K0.5) of 1.35 microM and slope of approximately equals 3. The BK channel was inhibited reversibly by external tetraethylammonium (TEA) ions, charybdotoxin, and quinine and was resistant to block by 4-aminopyridine and apamin. Ionomycin (1-10 microM) increased BK channel activity in the cell-attached recording configuration. The resting activity was consistent with a [Ca2+]i <100 nM and the increased channel activity evoked by ionomycin was consistent with a rise in [Ca2+]i to > or =0.3 microM. TEA (0.2-1 mM) increased the action potential duration approximately equals 1.5-fold and reduced the amplitude and duration of the afterhyperpolarization (AHP) by 26%. Charybdotoxin (100 nM) did not significantly alter the action potential duration or AHP amplitude but reduced the AHP duration by approximately equals 40%. Taken together, these data indicate that BK channel activation contributes to the action potential and AHP duration in rat intracardiac neurons.

Verapamil block of large-conductance Ca-activated K channels in rat aortic myocytes

The effects of verapamil on the large conductance Ca-activated K (BK) channel from rat aortic smooth muscle cells were examined at the single channel level. Micromolar concentrations of verapamil produced a reversible flickering block of the BK channel activity. Kinetic analysis showed that verapamil decreased markedly the time constants of the open states, without any significant change in the time constants of the closed states. The appearance of an additional closed state-specifically, a nonconducting, open-blocked state--was also observed, whose time constant would reflect the mean residence time of verapamil on the channel. These observations are indicative of a state-dependent, open-channel block mechanism. Dedicated kinetic (group) analysis confirmed the state-dependent block exerted by verapamil. D600 (gallopamil), the methoxy derivative of verapamil, was also tested and found to exert a similar type of block, but with a higher affinity than verapamil. The permanently charged and membrane impermeant verapamil analogue D890 was used to address other important features of verapamil block, such as the sidedness of action and the location of the binding site on the channel protein. D890 induced a flickering block of BK channels similar to that observed with verapamil only when applied to the internal side of the membrane, indicating that D890 binds to a site accessible from the cytoplasmic side. Finally, the voltage dependence of D890 block was assessed. The experimental data fitted with a Langmuir equation incorporating the Woodhull model for charged blockers confirms that the D890-binding site is accessed from the internal mouth of the BK channel, and locates it approximately 40% of the membrane voltage drop along the permeation pathway.

Characterization of the large-conductance Ca-activated K channel in myocytes of rat saphenous artery

We used the patch-clamp method to characterize the BK channel in freshly isolated myocytes from the saphenous branch of the rat femoral artery. Single-channel recordings revealed that the BK channel had a conductance of 187 pS in symmetrical 150 mM KCl, was blocked by external tetraethylammonium (TEA) with a KD(TEA) of approx. 300 microM at +40 mV, and by submicromolar charybdotoxin (CTX). The sensitivity of the BK channel to Ca was especially high (KD(ca) approx. 0.1 microM at +60 mV) compared to skeletal muscle and neuronal tissues. We also investigated the macroscopic K current, which under certain conditions is essentially sustained by BK channels. This conclusion is based on the findings that the macroscopic current activated upon depolarization follows a single exponential time course and is virtually fully blocked by 100 nM CTX and 5 mM external TEA. We made use of this occurrence to assess the voltage and Ca dependence of the macroscopic BK current. In intact myocytes, the BK channel showed a strong and voltage-dependent reduction of the outward current (62% at +40 mV), most likely due to block by intracellular Ba and polyamines. The results obtained from macroscopic and unitary current indicate that approx. 2.5% of the BK channels are active under physiological conditions, sustaining approx. 20 pA of outward current. Given the high input resistance of these cells, few BK channels are required to open in order to cause a significant membrane hyperpolarization, and thus function to limit the contraction resulting from acute increases in intravascular pressure, or in response to hypertensive pathologies.

Bimodal kinetics of a chloride channel from human fibroblasts

Excised patches were used to study the kinetics of a Cl channel newly identified in cultured human fibroblasts (L132). The conductance of ca. 70 pS in 150 mm symmetrical Cl, and the marked outward rectification ascribe this channel to the ICOR family. Long single-channel recordings (>30 min) revealed that the channel spontaneously switches from a kinetic mode characterized by high voltage dependence (with activity increasing with depolarization; mode 1), into a second mode (mode 2) insensitive to voltage, and characterized by a high activity in the voltage range +/-120 mV. On patch excision the channel always appeared in mode 1, which was maintained for a variable time (5-20 min). In most instances the channels then switched into mode 2, and never were seen to switch back, in spite of the eight patches that cumulatively dwelled in this mode 2.33-fold as compared to mode 1. Stability plots of long recordings showed that the channel was kinetically stable in both modes, allowing standard analysis of steady-state kinetics to be performed. Open and closed time distributions of mode 1 and mode 2 revealed that the apparent number of kinetic states of the channel was the same in the two modes. The transition from mode 1 into mode 2 was not instantaneous, but required a variable time in the range 5-60 sec. During the transition the channel mean open time was intermediate between mode 1 and mode 2. The intermediate duration in the stability plot however is not to be interpreted as if the channel, during the transition, rapidly switches between mode 1 and mode 2, but represents a distinct kinetic feature of the transitional channel.

Mechanisms of verapamil inhibition of action potential firing in rat intracardiac ganglion neurons

The effects of verapamil and related phenylalkylamines on neuronal excitability were investigated in isolated neurons of rat intracardiac ganglia using whole-cell perforated patch-clamp recording. Verapamil (>/=10 microM) inhibits tonic firing observed in response to depolarizing current pulses at 22 degrees C. The inhibition of discharge activity is not due to block of voltage-dependent Ca2+ channels because firing is not affected by 100 microM Cd2+. The K+ channel inhibitors charybdotoxin (100 nM), 4-aminopyridine (0.5 mM), apamin (30-100 nM), and tetraethylammonium ions (1 mM) also have no effect on firing behavior at 22 degrees C. Verapamil does not antagonize the acetylcholine-induced inhibition of the muscarine-sensitive K+ current (M-current) in rat intracardiac neurons. Verapamil inhibits the delayed outwardly rectifying K+ current with an IC50 value of 11 microM, which is approximately 7-fold more potent than its inhibition of high voltage-activated Ca2+ channel currents. These data suggest that verapamil inhibits tonic firing in rat intracardiac neurons primarily via inhibition of delayed outwardly rectifying K+ current. Verapamil inhibition of action potential firing in intracardiac neurons may contribute, in part, to verapamil-induced tachycardia.

Mechanism of verapamil block of a neuronal delayed rectifier K channel: active form of the blocker and location of its binding domain

1. The mechanism of verapamil block of the delayed rectifier K currents (I K(DR)) in chick dorsal root ganglion (DRG) neurons was investigated using the whole-cell patch clamp configuration. In particular we focused on the location of the blocking site, and the active form (neutral or charged) of verapamil using the permanently charged verapamil analogue D890. 2. Block by D890 displayed similar characteristics to that of verapamil, indicating the same state-dependent nature of block. In contrast with verapamil, D890 was effective only when applied internally, and its block was voltage dependent (136 mV/e-fold change of the on rate). Given that verapamil block is insensitive to voltage (Trequattrini et al., 1998), these observations indicate that verapamil reaches its binding site in the uncharged form, and accesses the binding domain from the cytoplasm. 3. In external K and saturating verapamil we recorded tail currents that did not decay monotonically but showed an initial increase (hook). As these currents can only be observed if verapamil unblock is significantly voltage dependent, it has been suggested (DeCoursey, 1995) that neutral drug is protonated upon binding. We tested this hypothesis by assessing the voltage dependence of the unblock rate from the hooked tail currents for verapamil and D890. 4. The voltage dependence of the off rate of D890, but not of verapamil, was well described by adopting the classical Woodhull (1973) model for ionic blockage of Na channels. The voltage dependence of verapamil off rate was consistent with a kinetic scheme where the bound drug can be protonated with rapid equilibrium, and both charged and neutral verapamil can unbind from the site, but with distinct kinetics and voltage dependencies.

Verapamil block of the delayed rectifier K current in chick embryo dorsal root ganglion neurons

We have used the patch-clamp method in the whole-cell configuration to investigate the mechanism of block of the delayed rectifier K current (IDRK) by verapamil in embryonic chick dorsal root ganglion (DRG) neurons. Verapamil induced a dose-dependent decay of the current, without altering its activation kinetics. This observation, together with the good description of IDRK time course at various blocker concentrations with the computer simulation of a three-state chain model (closed left and right arrow open left and right arrow open-blocked), indicates that verapamil acts as a state-dependent, open-channel blocker. To account for the double-exponential time course of recovery from block, this minimal kinetics scheme was expanded to include a closed-blocked state resulting from channel closure (at hyperpolarized voltages) with verapamil still bound to it. The apparent block and unblock rate constants assessed from verapamil-induced current decay in the presence of external Na were 0.95 +/- 0.05 ms-1mM-1 and 0.0037 +/- 0.0016 ms-1, respectively. When external Na was replaced by K, only the unblock rate constant changed, to 0.02 +/- 0.009 ms-1. Under these ionic conditions it was also observed that the recovery from block was modified from the double-exponential time course in the presence of external Na (tau1 = 160 ms; tau2 = 1600 ms), to a faster single-exponential recovery (tau = 100 ms). We tested the voltage dependence of block by applying stimulation protocols aimed at eliminating bias easily introduced by the shift of the gating equilibrium and by the coupling of channel activation and block. Under these experimental conditions the resulting block rate constant was not measurably voltage dependent.

Characterization of a neuronal delayed rectifier K current permeant to Cs and blocked by verapamil

We have used the patch-clamp method in the whole-cell configuration to characterize the delayed rectifier K current (IDRK) in embryonic chick dorsal root ganglion (DRG) neurons. The IDRK is activated by depolarizing pulses positive to -40 mV, and its V1/2 is near -20 mV. The slope factor of 10.4 mV for an e-fold change in conductance indicates an equivalent gating charge of 2.4e. Inactivation during sustained depolarizing pulses displays two distinct time constants of 200-300 msec and 6-9 sec, respectively. Outward current through the delayed rectifier K (DRK) channels could also be carried by internal Cs, which however exerts mild block when in mixtures with K, as evidenced by the anomalous mole fraction effect. The relative permeability of Cs vs. K, PCs/PK, as calculated from reversal potential measurements, is 0.25. Rb likewise permeates the DRK channel (PRb/PK = 0.67). The IDRK was effectively suppressed by external application of the Ca channel blocker Verapamil, with apparent dissociation constant of ca. 4 microM. The time course of Verapamil block, its good description by equations derived from open-channel block kinetic scheme, and the frequency-dependent effect of the blocker indicate that Verapamil can bind to the channel only when it is in the open state.

Tityus bahiensis toxin IV-5b selectively affects Na channel inactivation in chick dorsal root ganglion neurons

A novel toxin was isolated from the venom of the Brazilian scorpion Tityus (T.) bahiensis. The N-terminal amino acid sequence of this toxin was shown to be 80% identical to the corresponding segment of T. serrulatus toxin IV-5. The new toxin was thus named toxin IV-5b. Toxin IV-5b was found to markedly slow inactivation of Na channel in dorsal root ganglion neurons from chick embryo. By contrast, Na channel activation was only negligibly delayed, and deactivation completely unaffected. Similarly unaffected by the toxin were K and Ca currents. The slowing effect of the toxin starts to appear at concentrations of c. 80 nM, and shows a KD of 143 nM. With a toxin concentration of 2.4 microM, the Na channel inactivation time constant was increased c. 3-fold with respect to the control. The slowing of inactivation was voltage dependent, and increased with depolarization.

A multi-ion permeation mechanism in neuronal background chloride channels

Unitary current/voltage relationships of background Cl channels of rat hippocampal neurons were determined for varied gradients and absolute concentrations of NaCl. The channels revealed permeabilities for both Cl and Na ions. A hyperlinear increase of unitary conductance, observed for a symmetrical increase of salt concentration from 300 and 600 mM, indicated a multi-ion permeation mechanism. A variety of kinetic models of permeation were tested against the experimental current/voltage relationships. Models involving a pore occupied by mixed complexes of up to five ions were necessary to reproduce all measurements. A minimal model included four equilibrium states and four rate-limiting transitions, such that the empty pore accepts first an anion and then can acquire one or two cation/anion pairs. Three transport cycles are formed: a slow anion cycle (between the empty and single-anion states), a slow cation cycle (between the one- and three-ion states), and a fast anion cycle (between the three- and five-ion states). Thus, permeant anions are required for cation permeation, and several bound anions and cations promote a high rate of anion permeation. The optimized free-energy and electrical charge parameters yielded a self-consistent molecular interpretation, which can account for the particular order in which the pore accepts ions from the solutions. Although the model describes the mixed anion/cation permeability of the channel observed at elevated concentrations, it predicts a high selectivity for Cl anion at physiological ionic conditions.

Anion-cation interactions in the pore of neuronal background chloride channels

Background Cl channels in neurons and skeletal muscle are significantly permeable for alkali cations when tested with asymmetrical concentrations of the same salt. Both anion and cation permeation were proposed to require binding of an alkali cation with the pore (Franciolini, F., and W. Nonner. 1987. Journal of General Physiology. 90:453-478). We tested this hypothesis by bilaterally substituting large alkali cations for Na and found no significant changes of unitary conductance at 300 mM symmetrical concentrations. In addition, all organic cations examined were permeant in a salt gradient test (1,000 mM internal@300 mM external), including triethanolamine, benzyltrimethylamine, and bis-tris-propane (BTP, which is divalent at the tested pH of 6.2). Inward currents were detected following substitution of internal NaCl by the Na salts of the divalent anions of phosphoric, fumaric, and malic acid. Zero-current potentials in gradients of the Na and BTP salts of varied anions (propionate, F, Br, nitrate) that have different permeabilities under bi-ionic conditions, were approximately constant, as if the permeation of either cation were coupled to the permeation of the anion. These results rule out our earlier hypothesis of anion permeation dependent on a bound alkali cation, but they are consistent with the idea that the tested anions and cations form mixed complexes while traversing the Cl channel.

Determination of dependence of spin-lattice relaxation rate in serum upon concentration of added iron by magnetic resonance imaging

Dependence of spin-lattice relaxation rate (1/T1) in serum upon concentration of added iron was studied in the concentration range 0.0179-0.179 mmol l-1 for each of ferrous and ferric iron. In conjunction with the serum study, 1/T1 in solutions of transferrin and a mixture of albumin and gamma globulin was also studied as a function of added iron concentration. At low concentrations 1/T1 in serum increases linearly with increasing amounts of iron for each ion, and then reaches saturation for ferrous iron, whereas it shows an inflection for ferric iron. To explain the partition of added iron between various serum components, the effect of iron on 1/T1 in serum was compared with those of transferrin and the mixture. This effect can be defined as relaxivity or the incremental increase in relaxation rate per millimolar of added iron. At low concentrations the relaxivities of iron in serum, about 0.91 mmol-1 l s-1 for ferric and 0.95 mmol-1 l s-1 for ferrous ion, approximate well to the relaxivity of iron in transferrin solutions, which was measured to be about 0.92 mmol-1 l s-1. Furthermore, at high concentrations the relaxivity of ferric iron in serum, 0.44 mmol-1 l s-1, becomes similar to that of the mixture which is about 0.39 mmol-1 l s-1. These findings imply that iron added to serum first satisfies the binding requirements of transferrin, and the binding of iron to the other serum proteins occurs at high concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Single-channel currents activated by low intracellular pH in cultured hippocampal neurons of rat

Patch clamp technique was applied to the plasma membrane of cultured hippocampal neurons of rat. Elementary currents of a cation-selective channel were elicited by low intracellular pH (pHi 3.5-4.5). Channel activity starts with 1-2 min delay from the application of low pHi, and persists upon restoration of physiological pH conditions. The channel has a conductance of approx. 110 pS in symmetrical 300 mM NaCl, and is strongly selective for cations over anions. The channel is active over the whole voltage range tested (from +75 mV to -75 mV). Mean open time is function of voltage, increasing with depolarization. Low pH applied extracellularly did not activate the channel.

Spin-lattice relaxation rates in Fe(III)-doped human serum measured by magnetic resonance imaging

The proton spin-lattice relaxation time T1 in iron-doped serum was measured with a magnetic resonance imager operating at 0.5 T. The T1 in aqueous solutions of iron and iron-doped solutions of albumin and gamma globulin was also measured in order to analyse the paramagnetic contribution in iron-doped serum. The enhancement in serum is not linearly dependent on concentration of iron added. It is known that Fe(III) added to serum is mainly complexed with transferrin, albumin, gamma globulin and water. With serum pH (7.9) Fe(III) in the protein solution results in enhancement. Fe(III) in water does not cause any enhancement. As a result, the 1/T1 enhancement in serum should be caused solely by iron-binding serum proteins.

Evolution of ionic channels of biological membranes

This paper presents a view of the evolution and phylogenetic distribution of ionic channels of biological membranes. The view is based on the assumptions that ionic channels (1) appeared very early in the history of life, (2) have evolved from a common ancestor, and (3) have been subjected to evolutionary pressure to reach precision and high speed of signaling. We propose that Ca2+ was the intracellular messenger and modulator of the most primitive biological systems, which implies that the first channel to appear may have been a calcium channel. Then, very soon the entire group of potassium channels evolved from the calcium channel to improve the shape of signals and to restore initial conditions. Sodium channels probably appeared relatively late, diversifying from calcium channels in the early metazoan groups. Mainly because Na+ ions do not interfere with cellular metabolism (thus allowing the inward current--and, consequently, the speed of conduction--to be greatly increased), sodium channels probably proved advantageous in the generation of the action potential, and selection replaced calcium channels with sodium channels in this function. Finally, with the acquisition of multicellularity, channels responsible for synaptic transmission appeared. The case of the acetylcholine receptor channel is briefly discussed.

Calcium and voltage dependence of single Ca2+-activated K+ channels from cultured hippocampal neurons of rat

Calcium and voltage dependence of the Ca2+-activated K+ channel, K(Ca), was studied at the single-channel level in cultured hippocampal neurons from rat. The K(Ca) channel has approx. 220 pS conductance in symmetrical 150 mM K+, and is gated both by voltage and by Ca2+ ions. For a fixed Ca2+ concentration at the inner membrane surface, [Ca]i, channel open probability, Po, increases e-fold for 14 mV positive change in membrane potential. At a fixed membrane potential (0 mV), channel activity is first observed at [Ca]i = 10(-6) M, and increases with Ca2+ concentration approximating an absorption isotherm with power 1.4. The [Ca]i required to half activate (Po = 0.5) the channel is 4.10(-6) M. When compared to other preparations, the K(Ca) channel from hippocampal neurons reported here shows the lowest Ca2+ sensitivity and the highest voltage sensitivity. These findings are interpreted in evolutionary terms.

Single channel recording and gating function of ionic channels

This review discusses several aspects of the kinetic analysis of the gating function of membrane channels, as carried out on single channel data obtained by the patch clamp method. In particular, the following three aspects of channel behavior are reviewed in some detail: 1) estimate of the number of states or conformations the channel can enter; 2) the lifetime of each state as a function of transmembrane voltage (voltage-gated channels), or of ligand concentration (ligand-gated channels); 3) estimates of the rates at which the transitions between the various states occur, and assessment of reaction mechanisms. An introductory description of conformational transitions of channel proteins is also provided.

Single chloride channels in cultured rat neurones

Single-channel currents through chloride channels were recorded in cultured hippocampal neurones from rats using the patch-clamp method. The channel is active at voltages between -80 and +80 mV, and the time spent in the open state increases with depolarization (almost fourfold for 120 mV). The channel conductance is 62 pS in symmetrical 150 mM NaCl saline. In salt gradient conditions the channel was measurably permeable to Na+. Substitution of NO3- and Br- for Cl- gave higher single-channel currents, meaning a higher permeability of the channel toward the two anions than to Cl-. SO4(2-) ions were poorer charge carriers, yet contributed measurable inward current at negative voltages. Channel activity appeared independent of intracellular Ca2+ concentration. Taken together, these features would suggest for this channel a role in stabilizing resting membrane potential and in maintaining normal cell excitability.

Anion and cation permeability of a chloride channel in rat hippocampal neurons

The ionic permeability of a voltage-dependent Cl channel of rat hippocampal neurons was studied with the patch-clamp method. The unitary conductance of this channel was approximately 30 pS in symmetrical 150 mM NaCl saline. Reversal potentials interpreted in terms of the Goldman-Hodgkin-Katz voltage equation indicate a Cl:Na permeability ratio of approximately 5:1 for conditions where there is a salt gradient. Many anions are permeant; permeability generally follows a lyotropic sequence. Permeant cations include Li, Na, K, and Cs. The unitary conductance does not saturate for NaCl concentrations up to 1 M. No Na current is observed when the anion Cl is replaced by the impermeant anion SO4. Unitary conductance depends on the cation species present. The channel is reversibly blocked by extracellular Zn or 9-anthracene carboxylic acid. Physiological concentrations of Ca or Mg do not affect the Na:Cl permeability ratio. The permeability properties of the channel are consistent with a permeation mechanism that involves an activated complex of an anionic site, an extrinsic cation, and an extrinsic anion.

Spontaneous firing and myelination of very small axons

In this article the question of what evolutionary factors guided acquisition of myelin in the nervous system is addressed. The conclusion that conduction velocity of action potentials along the axon has been the only motive force needs reformulation, as other factors may have played a central role as well. In particular, protection against firing of spontaneous action potentials which may result from the simultaneous opening of only few (less than 10) sodium channels at the nodes of small (less than 1 micron diameter) myelinated axons, may have greatly contributed to discouraging myelination of axons smaller than 1 micron.

Blocking of the squid axon K+ channel by noxiustoxin: a toxin from the venom of the scorpion Centruroides noxius

We have studied the selective effects of noxiustoxin (NTX), a fraction of the venom of the scorpion Centruroides noxius, on the K currents of perfused squid giant axons using the voltage-clamp technique. At concentrations below 1.5 microM, NTX blocked K currents in a voltage-independent manner, with little effect on their turning-on and turning-off kinetics. Above 1.5 microM, the block by NTX became voltage-dependent and could be partially removed by repetitive pulsing and strong depolarizations. Long repolarizations and more negative holding potentials favoured the slow restoration of channel block. Reduction of K currents by internally perfusing the fibers with solutions of low K+ concentration (200 mM), affected very little the removal of NTX-block during repetitive pulsing, suggesting that block removal depended on membrane potential and not on outward movements of K+ ions through open channels. In high extracellular K+ (300 mM) the blocking action of NTX was reduced and the instantaneous I-V characteristics showed a marked outward rectification. At 20 microM NTX, inward tail currents measured on step repolarizations to -70 mV were fully blocked, suggesting a direct interaction of the toxin with the open channel. The effects of the total venom Centruroides noxius Hoffmann was also studied. External application of 0.25 mg/ml of the venom caused a marked reduction of both Na and K currents, an effect similar to that of other scorpion venoms.

Patch clamp technique and biophysical study of membrane channels

The present work describes the patch clamp technique, which first allowed the recording of single channel currents in biological membranes. In particular, it describes procedures for preparation and applications of the four different patch clamp configurations. Briefly, the cell-attached configuration is widely used for investigating channel modulation by transmitters acting via second messengers. The cell-free configurations (inside-out and outside-out), complementary to one another with respect to the orientation of the membrane surface, are particularly indicated for the study of the biophysics (kinetics, conductivity, selectivity, mechanism of permeation and block) of ionic channels. Finally, the whole-cell configuration which, because of the remarkable feature that it allows voltage clamp of very small cells, has given access to a number of physiologically important preparations never studied before.

The ageing kidney: biochemical and morphological study after irradiation

The behaviour of some activities of the kidney was studied both in young-adult and in adult rats exposed to an 8-Gly dose of gamma-rays and killed at various intervals after irradiation (both in the morning and in the evening). Brush border and lysosomal enzymes did not show marked differences among control rats of the same age even if adult animals showed levels of maltase, alkaline phosphatase and LAP activities higher than the young-adult group. Moreover, irradiation did not induce typical modifications of the same enzyme activities in young-adult and adult rats. Adult animals showed a reduction in the brush border enzyme activities at 120 hours after irradiation while, at the same interval, lysosomal activities underwent an increase both in young and in adult animals.

14C leucine uptake in rat tissues at different times after irradiation

The uptake of 14C leucine administered at different intervals after irradiation, but always 4 and 8 h before the animals were killed, has been evaluated in tissues with different proliferative activity and protein synthesis. The results have demonstrated an increased uptake and a more rapid elimination of the tracer after irradiation. In the small intestine a lower amount of TCA insoluble fraction was observed when the morphologic injury was evident, while protein synthesis significantly increased during the initial phase of appearance of the injury and mainly during the recovery phase of epithelial cells. Kidney and plasma had levels higher than controls at all intervals.

Circadian activity of rat kidney enzymes

Alkaline phosphatase, LAP, beta-glucuronidase and cathepsin D activities and protein content of the kidney homogenate did not show any circadian rhythm in animals sacrificed at different hours of the day. The fluctuations of maltase appear modest and not dependent on a ligh/dark cycle.

Early effects in kidney enzyme activities after irradiation

Brush border enzymes of proximal tubules, lysosomal activities and protein content of rat kidney were analysed after whole-body irradiation using two different experimental schedules. Maltase, alkaline phosphatase and beta-glucuronidase activities increased moderately during the first days after irradiation, whereas LAP, cathespsin D activities and protein content were not modified. No evident morphologic alterations were observed.