Sodium Conductance Changes Research Articles

HP58 : Effects of sodium channel blockers on “nerve exciatability threshold tracking“

Selective voltage gated sodium channel blockers are of growing interest as treatment for pain. For drug development of such compounds, it would be critical to have a biomarker that can be used for proof-of-mechanism. We aimed to evaluate whether drug-induced changes in sodium conductance can be detected using nerve excitability threshold tracking in 18 healthy subjects. In a randomized, double-blind, three-way crossover study, effects of single doses of mexiletine and lacosamide were compared to placebo. On each study visit, motor- and sensory nerve excitability measurements of the median nerve were performed (pre-dose; 2- and 5-hours post-dose). Stimulation was guided by QTRAC-S. Treatment effects were calculated using an ANCOVA, with baseline as covariate. Mexiletine and lacosamide had significant effects on a multitude of motor- and sensory nerve excitability parameters. In motor nerves, TEd 40-60ms was significantly decreased when compared to placebo, with an estimated difference -1.37% (95%CI:-2.20,-0.55;p=0.002) after mexiletine and -1.27% (95%CI:-2.0968,-0.4430; p=0.004) after lacosamide. Moreover, mexiletine and lacosamide significantly increased superexcitability in motor nerves, with an estimated difference of 1.74% (95%CI:0.61,2.87; p=0.004) and 1.47% (95%CI:0.34, 2.60;p=0.013), respectively. The strength-duration time constant decreased after lacosamide in both motor nerves -0.03ms (95%CI:-0.06,-0.01;p=0.005) and sensory nerves -0.08ms (95%CI:-0.12,-0.05;p<0.001). Mexiletine and lacosamide significantly decrease excitability of motor and sensory nerves, in line with the mechanism of action. This study shows that threshold tracking can be an effective biomarker in pharmacological studies. The method would therefore be a valuable tool in drug development, to help identifying target engagement in healthy subjects.

Reliable and efficient processing of sensory information at body temperature by rodent cortical neurons

Some pathological conditions, such as infections and febrile seizures, may cause temperature fluctuations in the brain. We ask here how these temperature fluctuations, particularly hypothermia and hyperthermia, affect the cortical information coding performance and response reliability of warm-blooded animals. We studied this issue by a combination of in vitro whole-cell patch clamp recordings from cortical pyramidal neurons and computational simulations of Hodgkin–Huxley cortical neuronal model at different temperatures. A significantly higher reliability of the neuronal response to repeated input signals was observed at physiological temperature ( $$\sim 35\,^{\circ }\hbox {C}$$ ) than that at a much lower temperature ( $$\sim 24\,^{\circ }\hbox {C}$$ ) or upon hyperthermia ( $$\sim 41\,^{\circ }\hbox {C}$$ ). In addition, the firing rate of excitatory neurons (i.e., pyramidal neurons) was increased gradually, while it decreased gradually for inhibitory neurons (e.g., PV interneurons) as the temperature increased from 25 to $$40\,^{\circ }\hbox {C}$$ . The opposite changes in the response activity level between pyramidal neurons and interneurons suggested a shift in the excitatory/inhibitory (E/I) balance in the local network circuit as a function of changing temperature. An analysis of the information coding efficiency suggested that the pyramidal neurons displayed the maximal response reliability with the highest coding efficiency and information transmission rate at body temperature, suggesting that the E/I balance observed at this temperature might be optimal for enhancing information coding in cortical neurons. In addition, we applied tetrodotoxin and 4-aminopyridine to partially block $$\hbox {Na}^{+}$$ and $$\hbox {K}^{+}$$ channels, respectively, and observed that a change in sodium or potassium conductance could also alter the neuronal response reliability of pyramidal neurons.

Neuron Morphology Influences Axon Initial Segment Plasticity.

In most vertebrate neurons, action potentials are initiated in the axon initial segment (AIS), a specialized region of the axon containing a high density of voltage-gated sodium and potassium channels. It has recently been proposed that neurons use plasticity of AIS length and/or location to regulate their intrinsic excitability. Here we quantify the impact of neuron morphology on AIS plasticity using computational models of simplified and realistic somatodendritic morphologies. In small neurons (e.g., dentate granule neurons), excitability was highest when the AIS was of intermediate length and located adjacent to the soma. Conversely, neurons having larger dendritic trees (e.g., pyramidal neurons) were most excitable when the AIS was longer and/or located away from the soma. For any given somatodendritic morphology, increasing dendritic membrane capacitance and/or conductance favored a longer and more distally located AIS. Overall, changes to AIS length, with corresponding changes in total sodium conductance, were far more effective in regulating neuron excitability than were changes in AIS location, while dendritic capacitance had a larger impact on AIS performance than did dendritic conductance. The somatodendritic influence on AIS performance reflects modest soma-to-AIS voltage attenuation combined with neuron size-dependent changes in AIS input resistance, effective membrane time constant, and isolation from somatodendritic capacitance. We conclude that the impact of AIS plasticity on neuron excitability will depend largely on somatodendritic morphology, and that, in some neurons, a shorter or more distally located AIS may promote, rather than limit, action potential generation.

Basolateral P2X4-like receptors regulate the extracellular ATP-stimulated epithelial Na+ channel activity in renal epithelia

Extracellular ATP initiates potent effects on sodium transport across renal epithelia through membrane-associated purinergic receptors. Dependent on the location of these receptors, ATP either inhibits or stimulates sodium reabsorption. Using A6 cells, transepithelial electrical resistance measurements, and scanning ion conductance microscopy, we have identified the purinergic receptors involved in the stimulatory action on the epithelial cell basolateral plasma membrane. Addition of the potent P2X(4) receptor agonist 2-methylthio-ATP (2MeSATP) to the basolateral side of the A6 monolayer stimulated amiloride-sensitive sodium conductance and produced similar cell morphological changes to those found with ATPgammaS, aldosterone, or hypotonic stress. The agonist potency order determined by sodium conductance changes of the monolayer was: 2MeSATP >or= ATPgammaS > CTP, a similar agonist potency profile to that of cloned P2X(4) receptors but with higher sensitivity for beta, gamma-methylene-ATP and alpha,beta-methylene-ATP. We further demonstrated that the ATP effect on sodium transport was potentiated by ivermectin, not blocked by suramin and PPADS, enhanced by Zn(2+) but not by Cu(2+), and significantly reduced but not totally inhibited by brilliant blue G. These results led us to conclude that basolateral P2X(4)-like receptors were involved. We suggest that there is a reciprocal purinergic system acting both at a basolateral and apical location for control of Na(+) transport. This requires a mechanism within the cell that leads to either basolateral or apical ATP release to regulate renal tubular function.

Hypertonic saline inhibits luminal sodium channels in respiratory epithelium

Physical exercise with increased ventilation leads to a considerable rise in water loss from the airways. The mechanisms underlying the regulation of transepithelial fluid transport necessary to compensate for these losses are unknown but may include changes in luminal ion channel conductance. The present study was designed to examine the effects of an increase in luminal chloride and sodium concentrations which may locally occur during hyperventilation on luminal ion conductance in the respiratory epithelium of healthy controls and patients diagnosed with cystic fibrosis (CF). Changes in luminal chloride and sodium conductance were inferred by recording nasal potential difference in eight healthy subjects and 10 patients with CF, using superfusing solutions based on isotonic saline (150 mM) on one occasion and solutions based on hypertonic saline (300 mM) on the other. Switching from isotonic to hypertonic saline superfusion decreased potential difference in controls and CF patients significantly. Amiloride induced a decrease of potential difference which was larger with isotonic than with hypertonic saline (controls 9.5 +/- 6.1 vs. 3.7 +/- 4.6 mV; CF 17.2 +/- 7.2 vs. 9.8 +/- 7.6 mV). Chloride conductance stimulated with solutions low in chloride and containing isoproterenol was not significantly changed by hypertonic saline solutions compared with isotonic solutions in both groups. The findings indicate a significant inhibition of luminal sodium conductance by high luminal sodium concentrations. This mechanism may be involved in the regulation of fluid transport across the respiratory epithelium during exercise and in the improvement of mucociliary clearance and lung functions with inhalation of hypertonic saline in CF.

Sir Alan Hodgkin (1914-1998).

E very now and then giants come along, raise the bar, and lift us all; Alan Hodgkin was one of them. He never worked for a Ph.D.; didn't have a research supervisor; and built, borrowed, or begged equipment to ask and answer his own questions. Yet despite—or because of—what might be viewed today as these initial “handicaps,” as a second-year undergraduate, Hodgkin began an active research career that spanned more than 50 years. His studies resulted in landmark discoveries that rewrote textbooks, established the keystones of modern physiology, and set new standards for experimental neuroscience. He is most recognized for his work on nerves. As a 20-year-old undergraduate at Trinity College, Cambridge, Hodgkin established that the action potential is propagated electrically by currents spreading passively in a local circuit. Later, with Andrew Huxley, he made the first intracellular recordings from squid axon and discovered unexpectedly that the action potential overshot zero by many tens of millivolts. This demonstrated that the action potential was generated by selective changes in the electrical properties of the surface membrane and did not involve proteins in the axoplasm as many thought. Their excitement was cut short 3 weeks later when Hitler's army marched into Poland and England was plunged into war. Hodgkin was assigned to research on radar and spent the next 5 years working as a physicist, an unusual assignment for a biologist who was self-taught in mathematics and physics, but he was an excellent choice. Hodgkin was a quick study, creative, with penetrating intelligence and common honesty. “Alan, what was the most important thing you learned at school?” “To read widely and work on my own.” “Yes, but what did you like the best about school?” “The holidays.” [A. L. Hodgkin, Chance and Design , 1992]. Boyhood holidays were spent exploring the outdoors, which was the original source of Hodgkin's enthusiasm for natural history. It was through a keen interest in bird watching that he first recognized the essential relationship between observation (research) and learning. The war ended, and Hodgkin and Huxley returned to their work on the giant axon. Using a feedback circuit to clamp membrane voltage at fixed levels they dissected the ionic basis of the action potential, and discovered voltage-gated sodium and potassium conductance changes, showing how their properties could account for the excitation and propagation of the nerve impulse. The Nobel Prize in Physiology or Medicine recognized their work in 1963. Hodgkin became president of the Royal Society in 1970 and switched to research on vision, the focus of his experimental work for the rest of his career. He began working with M. G. F. Fuortes at Woods Hole on the photoreceptors of the horseshoe crab, Limulus . They investigated the long delay that preceded the electrical response evoked by a light flash and attributed it to the time taken for a signal to pass through a cascade of chemical reactions. They recognized that stages of chemical amplification may underlie the generation of the photoresponse and that feedback mechanisms were likely to be responsible for the reciprocal changes in response sensitivity and time resolution that occur when the receptor is light- or dark-adapted. Rarely working with more than one person at a time, Hodgkin undertook research on retinal cones and rods that lasted 17 years and through elegant experimentation and imaginative quantitative analysis established many tenets of retinal photoreceptor physiology. As with his work on the electrical properties of nerve, Hodgkin's research on vision redefined the landscape, expanded the vocabulary, and focused attention on the next set of crucial questions. Alan Hodgkin was a tall man, with a quiet disposition, a good sense of humor, and lively eyes that could express a full range of emotions. He was modest despite his achievements. He did not put his name on any work that he did not fully participate in. He had no interest in having a large research group and felt that one collaborator at a time was best, two were awkward, and more than that impossible. He was fun to work with. Experiments were rarely planned ahead of time, as there was the unexpressed sense that this would somehow ruin the chase by quenching the feeling of exploration and discovery that was, after all, the point of the process—the chance in Chance and Design .

Mastoparan activates apical chloride and potassium conductances, decreases cell volume, and increases permeability of cultured epithelial cell monolayers.

Mastoparan is a tetradecapeptide. Mastoparan added to the apical surface of monolayers of Madin-Darby canine kidney (MDCK) epithelial cells, cultured on micropore filters, activated ion transport and increased the permeability of the paracellular pathway across the monolayers. In monolayers of similar MDCK cells in which the basolateral membrane was permeabilized with Staphylococcus aureus alpha toxin (Staph. alpha toxin), the effects of mastoparan on apical membrane ion conductances were dependent on the presence of guanosine triphosphate (GTP). Mastoparan and GTP increased apical membrane chloride conductance more than potassium conductance, with very little change in sodium conductance. In intact monolayers, addition of barium to the apical bath prevented mastoparan activation of ion transport and the increase in paracellular permeability. Increasing bath potassium to 130 mM also reduced ion transport and prevented the increase in paracellular permeability. We hypothesized that these observations could be linked by mastoparan activation of apical chloride and potassium conductances, with consequent decreases in cell volume and resultant increases in paracellular permeability. Addition of 270 mM mannitol to isosmotic media to decrease cell volume decreased MDCK monolayer transepithelial resistance. Addition of mastoparan to monolayers of MDCK cells grown on micropore filters decreased cell volume to the same extent as addition of 270 mM mannitol to isosmotic media. Addition of the potassium channel inhibitor, barium, prevented the decrease in cell volume in response to mastoparan. Mastoparan activates apical membrane chloride and potassium conductances in MDCK cells. The loss of these ions from the cells decreases cell volume, and the decrease in cell volume increases the permeability of the paracellular pathway.

Identification of frog photoreceptor plasma and disk membrane proteins by radioiodination.

Several functions have been identified for the plasma membrane of the rod outer segment, including control of light-dependent changes in sodium conductance and a sodium-calcium exchange mechanism. However, little is known about its constituent proteins. Intact rod outer segments substantially free of contaminants were prepared in the dark and purified on a density gradient of Percoll. Surface proteins were then labeled by lactoperoxidase-catalyzed radioiodination, and intact rod outer segments were reisolated. Membrane proteins were identified by polyacrylamide gel electrophoresis and autoradiography. The surface proteins labeled included rhodopsin, the major membrane protein, and 12 other proteins. Several control experiments indicated that the labeled proteins are integral membrane proteins and that label is limited to the plasma membrane. To compare the protein composition of plasma membrane with that of the internal disk membrane, purified rod outer segments were lysed by hypotonic disruption or freeze-thawing, and plasma plus disk membranes were radioiodinated. In these membrane preparations, rhodopsin was the major iodinated constituent, with 12 other proteins also labeled. Autoradiographic evidence indicated some differences in protein composition between disk and plasma membranes. A quantitative comparison of the two samples showed that labeling of two proteins, 24 kilodaltons (kDa) and 13 kDa, was enriched in the plasma membrane, while labeling of a 220-kDa protein was enriched in the disk membrane. These plasma membrane proteins may be associated with important functions such as the light-sensitive conductance and the sodium-calcium exchanger.

The Physical Mechanism of Sodium Conductance Change in Nerve: An Electron Transfer Model

The Physical Mechanism of Sodium Conductance Change in Nerve: An Electron Transfer Model

The Physical Mechanism of Sodium Conductance Change in Nerve: An Electron Transfer Model

Assuming that the nerve membrane contains electron transfer complexes and energy traps, many Na channel-related properties can be explained, including the Na activation-inactivation process, refractory state, recovery of excitability, effects of Ca ions and local anesthetics, heat production and absorption, anode break excitation, and the significance of positive afterpotential in repetitive firing. This model also suggests that the nerve impulse could be elicited by laser beams.

Single sodium channels from rat brain incorporated into planar lipid bilayer membranes.

A voltage- and time-dependent conductance for sodium ions is responsible for the generation of impulses in most nerve and muscle cells. Changes in the sodium conductance are produced by the opening and closing of many discrete transmembrane channels. We present here the first report of electrical recordings from voltage-dependent sodium channels incorporated into planar lipid bilayers. In bilayers with many channels, batrachotoxin (BTX) induced a steady-state sodium current that was blocked by saxitoxin (STX) at nanomolar concentrations. All channels appeared in the bilayer with their STX blocking sites facing the side of vesicle addition, allowing us to define that as the extracellular side. Current fluctuations due to the opening and closing of single BTX-activated sodium channels were voltage-dependent (unit conductance, 30 pS in 0.5 M NaCl): the channels closed at large hyperpolarizing potentials. Slower fluctuations of the same amplitude, due to the blocking and unblocking of individual channels, were seen after addition of STX. Block of the sodium channels by STX was voltage-dependent, with hyperpolarizing potentials favouring block. The voltage-dependent gating, ionic selectivity and neurotoxin sensitivity suggest that these are the channels that normally underlie the sodium conductance change during the nerve impulse.

Molecular mechanism of sodium conductance changes in nerve: The role of electron transfer and energy migration

Molecular mechanism of sodium conductance changes in nerve: The role of electron transfer and energy migration

Sodium currents in the giant axon of the crab Carcinus maenas.

Measurements were made of the kinetics and steady-state properties of the sodium conductance changes in the giant axon of the crab Carcinus maenas. The conductance measurements were made in the presence of small concentrations of tetrodotoxin and as much electrical compensation as possible in order to minimize errors caused by the series resistance. After an initial delay of 10-150 microsec, the conductance increase during depolarizing voltage clamp pulses followed the Hodgkin-Huxley kinetics. Values of the time constant for the activation of the sodium conductance lay on a bell-shaped curve with a maximum under 180 microsec at -40 mV (at 18 degrees C). Values of the time constant for the inactivation of the sodium conductance were also fitted using a bell-shaped curve with a maximum under 7 msec at -70 mV. The effects of membrane potential on the fraction of Na channels available for activation studied using double pulse protocols suggest that hyperpolarizing potentials more negative than -100 mV lock a fraction of the Na channels in a closed conformation.

Recent advances in mode of action of insecticides.

The field of insecticide mode of action has become so large that a review of this length cannot hope to be comprehensive. I have therefore attempted to clarify the current status of only a few unresolved or controversial problems and to indicate certain possibilities for future research. I have omitted any discussion of the acetylcholinesterase-inhibitory organophos­ phate or carbamate insecticides. Many other important topics and novel concepts, including juvenile hormone (JR) mimics (44, 176), anti-JRs (21, 27, 156, 157, 175), cholinergic receptor agonists (98, 172), choline acetyl­ transferase inhibitors (164), I-phenylcarbamoyl-2-pyrazolines (73), gluta­ mate analogs (76, 122), and antibodies as insecticides (142) are likewise not discussed. Recent accounts of various aspects of the mode of action of pyrethroids and DDT analogs can be cited, including effects on sodium and potassium conductance changes (135-137), structure-activity relationships (55,83,84, 134, 138), and inhibition of calcium-independent ATPases (33, 45, 50). I have focused my discussion of this group of insecticides on recent attempts to locate their primary sites of action in vivo, and to relate their mode of neurotoxic action to calcium function. Other paths traced in the present review are the aminergic theory of formamidine action, the mechanism of synaptic facilitation by cyc10dienes and y-BHC, and the question of chitin synthetase inhibition by benzoylphenyl urea insecticides. It is assumed that the reader is familiar with the most basic principles of neurophysiology.

Molecular Mechanism of Transduction in Visual Cells

Absorption of light by rhodopsin causes a change in receptor potential as hyperpolarization for vertebrate rod outer segment and depolarization for invertebrate rhabdomal microvilli. Since rhodopsin is located in disk membranes which are separated from the external plasma membrane of the outer segment where the sodium conductance change occurrs, the concept of intermediary transmitter is proposed in order to interprete communication between the separated membranes. This concept apply equally to cephalopod microvilli in which rhodopsin is located in the plasma membranes. I will intend to review current knowledge of the molecular transmitter systems of vertebrate rod outer segment and cephalopod microvilli.Isomerzation of retinal in rhodopsin by light induces conformational changes of opsin protein to produce intermediates. Metarhodopsin II, one of the intermediates in the photolysis of vertebrate rhodopsin, associates with Gprotein and information flows. Amplified information carrying mechanism take place in GTP/GDP exchange of G-protein. Phosphodiesterase is activated by G·GTP and hydorize cGMP. Activity of these enzymes is influenced by concentration of Ca++. Release of Ca++ from the disk membrane is influenced by these enzymes.Although a lot of evidences seem sufficient to place cGMP and Ca++ in the transduction mechanism, the role of these substances are subject of controversy. Studies using cephalopod photoreceptor might give an insight into underestanding of the transduction mechansim because rhodopsin and ion channel located in the same plasma membranes.

Ionic mechanisms of two types of on-center bipolar cells in the carp retina. II. The responses to annular illumination.

On-center bipolar cells in the dark-adapted carp retina were divided into four types (A, B, C, and D) on the basis of response wave forms, spectral response properties, and electrical membrane properties. Type A and B cells responded to a spot of light with a transient depolarization followed by a plateau, whereas the response of type C and D cells were approximately rectangular in shape. The center and surround responses of type A cells had maximum spectral response of approximately 525 nm in the lower mesopic range; the polarity of both responses was reversed at positive membrane potentials as the membrane was depolarized by extrinsic current. The center and surround responses of type D cells had a maximum spectral response of approximately 625 nm in the mesopic or photopic range; the polarity of both responses was reversed at membrane potentials that were more negative than those at the dark level. The results suggest that the center and surround responses mediated by rods are generated by changes in sodium conductance, but in opposite ways; whereas those mediated by red cones are generated by changes in potassium and/or chloride conductances. In type B and C cells, which probably receive inputs from both rods and/or green cones as well as red cones, the center responses were composed of the two ionic mechanisms described above. The surround responses of many type B and C cells were dominated by only one ionic mechanism with a negative reversal potential, but in some type B cells the surround responses were resulted from two ionic mechanisms similar to those of the center responses.

The dipole flip-flop theory for the excitation and conduction of excitable tissues--I. Dipole's behavior and gating currents.

To explain the sodium conductance change using Wei's dipole model (Wei, 1969), we may expect that during depolarization the dipole's population difference, ΔN, is first reduced and then returns more slowly to its resting value. This paper shows that the experimental results of gating currents support this idea. Such time course of ΔN, however, is not a usual relaxation process. To account for the unusual behavior of ΔN, we propose two additional assumptions: (1) there exists a special coupling system (probably the intramolecular vibrations) whose coupling strength with the dipoles is much stronger than with the thermal bath (intermolecular vibrations), and (2) there also exist “traps” for the dipole's excitation energy so that this energy is transformed into other energy forms at a rate increasing with the increase of depolarization. Experiments suggest that the traps are proteins located at the inner membrane surface.

The dipole flip-flop theory for the excitation and conduction of excitable tissues—I. Dipole's behavior and gating currents

To explain the sodium conductance change using Wei's dipole model (Wei, 1969), we may expect that during depolarization the dipole's population difference,ΔN, is first reduced and then returns more slowly to its resting value. This paper shows that the experimental results of gating currents support this idea. Such time course ofΔN, however, is not a usual relaxation process. To account for the unusual behavior ofΔN, we propose two additional assumptions: (1) there exists a special coupling system (probably the intramolecular vibrations) whose coupling strength with the dipoles is much stronger than with the thermal bath (intermolecular vibrations), and (2) there also exist “traps” for the dipole's excitation energy so that this energy is transformed into other energy forms at a rate increasing with the increase of depolarization. Experiments suggest that the traps are proteins located at the inner membrane surface.

Characteristics of sodium and calcium conductance changes produced by membrane depolarization in an Aplysia neurone.

1. The time course and voltage dependence of Na and Ca conductance changes produced by depolarization of the soma of the neurone R15 in the abdominal ganglion of Aplysia juliana were examined at temperatures of 10--14 degrees C. 2. During a maintained depolarization, Na currents turned on then decayed (inactivated). Inactivation was exponential with time constant tauh. Activation (after correction for inactivation) was reasonably well described by the expression G'Na(t) = G'Na (infinity) (1 - exp [-t/taum])3 over a wide range of potentials. 3. taum and tauh were both voltage dependent. In the range -20 to +40 mV, taum varied from 5 to 0.5 msec and tauh from 25 to 8 msec (13.5 degrees C). Steady-state Na conductance (corrected for inactivation) was voltage dependent also, increasing sigmoidally with depolarization to a maximum of 25--30 muS at +10 to +20 mV. Half-maximal Na conductance occurred at a membrane potential of -8 mV and from -15 to -5 mV, a 5 mV change in membrane potential produced an e-fold change in steady-state Na conductance. 4. Steady-state inactivation of Na conductance (hNa(infinity)) was voltage dependent with half-inactivation occurring at a membrane potential of -32 mV. Recovery from Na inactivation followed an exponential time course with a voltage-dependent time constant. 5. During a maintained depolarization Ca currents activated then decayed (inactivated) more slowly than Na currents. The decay was exponential with time constant tauH. The decay of Ca current was not an artifact porduced by an outward current. The amplitude of calcium tail currents, produced by voltage steps back to epsilonK at different times during the decay of ICa, decayed also with a time constant close to tauH. 6. Ca conductance (after correction for inactivation) could be described approximately by the expression G'Ca(t) = G'Ca(infinity) (1 - exp [-t/tauM])p but it was necessary to vary p from 1 to 2 at different potentials. No value of p gave as good a fit to this model as that obtained for Na currents. 7. taum and tauH were voltage dependent. In the range of potentials from 0 to +60 mV, tauM varied from 9 to 5 msec and tauH from 300 to 50 msec (13.5 degrees C). Steady-state Ca conductance (corrected for inactivation) was voltage dependent also, increasing sigmoidally with depolarization to a maximum of 10--15 muS at +30 to +40 mV. Half-maximal Ca conductance occurred at a membrane potential of +12 mV, and from +10 to +20 mV a 6 mV change in membrane potential produced an e-fold change in Ca conductance. 8. Steady-state inactivation of Ca conductance (hCa(infinity)) varied with holding potential (VH). Half-inactivation occurred with depolarization to -20 mV. At potentials more negative than -40 mV, hCa(infinity) was less than at -40 mV, i.e. hyperpolarization produced Ca 'inactivation'. 9...

The relation of Vmax to INa, GNa, and h infinity in a model of the cardiac Purkinje fiber

The inward sodium current in cardiac muscle is difficult to study by voltage clamp methods, so various indirect experimental measures have been used to obtain insight into its characteristics. These methods depend on the relationship between maximal upstroke velocity of the action potential (Vmax) and the sodium current (INa), usually defined in terms of the Hodgkin-Huxley model. These relationships were explored using an adaptation of this model to cardiac Purkinje fibers. In general Vmax corresponded to INa, and it could be used to determine the relationship of membrane potential to GNa, and h infinity. The results, however, depended on the method of stimulation of the action potential, and an optimal stimulation method was determined. A commonly used experimental technique called "membrane responsiveness" was shown to distort seriously the properties of steady-state gating inactivation that is supposed to measure. Estimation of the changes in maximal sodium conductance, such as those produced by tetrodotoxin (TTX), would be accurately measured. Some experimental results have indicated a voltage-dependent effect of TTX. Characteristics of the measures of TTX effect under those conditions were illustrated. In summary, calculations with a model of the cardiac Purkinje fiber action potential provide insight into the accuracy of certain experimental methods using maximal upstroke velocity as a measure of INa, and cast doubt on other experimental methods, such as membrane responsiveness.