Effect of intracellular and extracellular acidosis on sodium current in ventricular myocytes

1995 ◽  
Vol 268 (4) ◽  
pp. H1749-H1756 ◽  
Author(s):  
C. L. Watson ◽  
M. R. Gold
Keyword(s):  
Steady State ◽  
Time Course ◽  
Ventricular Myocytes ◽  
Sodium Current ◽  
Essential Element ◽  
Neonatal Rat ◽  
Cardiac Conduction ◽  
Inactivation Kinetics ◽  
Extracellular Acidosis ◽  
Steady State Inactivation

Conduction slowing is an essential element in the generation of ischemic ventricular arrhythmias and is determined in part by the inward Na+ current (INa). Because intracellular acidosis is an early consequence of ischemia, we hypothesized that lowering intracellular pH (pHi) would reduce or kinetically modulate INa and thus affect cardiac conduction. To test this hypothesis, the whole cell patch-clamp method was used to measure INa in neonatal rat ventricular myocytes exposed to varying extracellular pH (pHo 6.4–7.4), while perfusing the cells with acidic solutions (pHi 6.2–7.2). With simultaneous acidification of pHo and pHi there was a progressive increase in time to peak current, a 31% decrease in peak INa (298 +/- 18 to 206 +/- 16 pA/pF), and a complex slowing of inactivation kinetics. At the most extreme levels of acidification, there was a 5-mV hyperpolarizing shift in steady-state inactivation and a 6-mV depolarizing shift in activation. Independent changes of pHo and pHi indicate that the reduction of peak INa is a function of pHo. However, steady-state inactivation is modulated by pHi. The time course of activation and inactivation appears to depend on both pHo and pHi. We conclude that both intracellular and extracellular acidosis are significant but distinct modulators of INa amplitude and kinetics in cardiac myocytes.

scholarly journals Modulation of late sodium current by Ca2+, calmodulin, and CaMKII in normal and failing dog cardiomyocytes: similarities and differences

2008 ◽  
Vol 294 (4) ◽  
pp. H1597-H1608 ◽  
Author(s):  
Victor A. Maltsev ◽  
Vitaliy Reznikov ◽  
Nidas A. Undrovinas ◽  
Hani N. Sabbah ◽  
Albertas Undrovinas
Keyword(s):  
Heart Failure ◽  
Ventricular Myocytes ◽  
Sodium Current ◽  
Feedback Mechanism ◽  
Electrical Performance ◽  
Inactivation Kinetics ◽  
Whole Cell Patch Clamp ◽  
Steady State Inactivation ◽  
Similarities And Differences ◽  
Positive Feedback Mechanism

Augmented and slowed late Na+ current ( INaL) is implicated in action potential duration variability, early afterdepolarizations, and abnormal Ca2+ handling in human and canine failing myocardium. Our objective was to study INaL modulation by cytosolic Ca2+ concentration ([Ca2+]i) in normal and failing ventricular myocytes. Chronic heart failure was produced in 10 dogs by multiple sequential coronary artery microembolizations; 6 normal dogs served as a control. INaL fine structure was measured by whole cell patch clamp in ventricular myocytes and approximated by a sum of fast and slow exponentials produced by burst and late scattered modes of Na+ channel gating, respectively. INaL greatly enhanced as [Ca2+]i increased from “Ca2+ free” to 1 μM: its maximum density increased, decay of both exponentials slowed, and the steady-state inactivation (SSI) curve shifted toward more positive potentials. Testing the inhibition of CaMKII and CaM revealed similarities and differences of INaL modulation in failing vs. normal myocytes. Similarities include the following: 1) CaMKII slows INaL decay and decreases the amplitude of fast exponentials, and 2) Ca2+ shifts SSI rightward. Differences include the following: 1) slowing of INaL by CaMKII is greater, 2) CaM shifts SSI leftward, and 3) Ca2+ increases the amplitude of slow exponentials. We conclude that Ca2+/CaM/CaMKII signaling increases INaL and Na+ influx in both normal and failing myocytes by slowing inactivation kinetics and shifting SSI. This Na+ influx provides a novel Ca2+ positive feedback mechanism (via Na+/Ca2+ exchanger), enhancing contractions at higher beating rates but worsening cardiomyocyte contractile and electrical performance in conditions of poor Ca2+ handling in heart failure.

Aging-associated changes in whole cell K+ and L-type Ca2+ currents in rat ventricular myocytes

2000 ◽  
Vol 279 (3) ◽  
pp. H889-H900 ◽  
Author(s):  
Shi J. Liu ◽  
Richard P. Wyeth ◽  
Russell B. Melchert ◽  
Richard H. Kennedy
Keyword(s):  
Steady State ◽  
Young Adult ◽  
Action Potential ◽  
Action Potential Duration ◽  
Ventricular Myocytes ◽  
Inactivation Kinetics ◽  
Whole Cell ◽  
Rat Ventricular Myocytes ◽  
Whole Cell Patch Clamp ◽  
Steady State Inactivation

The effect of aging on cardiac membrane currents remains unclear. This study examined the inward rectifier K+ current ( I K1), the transient outward K+current ( I to), and the L-type Ca2+ channel current ( I Ca,L) in ventricular myocytes isolated from young adult (6 mo) and aged (>27 mo) Fischer 344 rats using whole cell patch-clamp techniques. Along with an increase in the cell size and membrane capacitance, aged myocytes had the same magnitude of peak I K1 with a greater slope conductance but displayed smaller steady-state I K1. Aged myocytes also had a greater I to with an increased rate of activation, but the I to inactivation kinetics, steady-state inactivation, and responsiveness to l-phenylephrine, an α1-adrenergic agonist, were unaltered. The magnitude of peak I Ca,L in aged myocytes was decreased and accompanied by a slower inactivation, but the I Ca,L steady-state inactivation was unaltered. Action potential duration in aged myocytes was prolonged only at 90% of full repolarization (APD90) when compared with the action potential duration of young adult myocytes. Aged myocytes from Long-Evans rats showed similar changes in I toand I Ca,L but an increased I K1. These results demonstrate aging-associated changes in action potential, in morphology, and in I K1, I to, and I Ca,L of rat ventricular myocytes that possibly contribute to the decreased cardiac function of aged hearts.

Pi inhibits the SR Ca2+ pump and stimulates pump-mediated Ca2+ leak in rabbit cardiac myocytes

2000 ◽  
Vol 279 (2) ◽  
pp. H577-H585 ◽  
Author(s):  
G. L. Smith ◽  
A. M. Duncan ◽  
P. Neary ◽  
L. Bruce ◽  
F. L. Burton
Keyword(s):  
Steady State ◽  
Inorganic Phosphate ◽  
Time Course ◽  
Ventricular Myocytes ◽  
Cyclopiazonic Acid ◽  
Ruthenium Red ◽  
Uptake Capacity ◽  
Exponential Decline ◽  
Pump Rate ◽  
Uptake Process

Measurements of sarcoplasmic reticulum (SR) Ca2+ uptake were made from aliquots of dissociated permeabilized ventricular myocytes using fura 2. Equilibration with 10 mM oxalate ensured a reproducible exponential decline of [Ca2+] from 600 nM to a steady state of 100–200 nM after addition of Ca2+. In the presence of 5 μM ruthenium red, which blocks the ryanodine receptor, the time course of the decline of [Ca2+] can be modeled by a Ca2+-dependent uptake process and a fixed Ca2+leak. Partial inhibition of the Ca2+ pump with 1 μM cyclopiazonic acid or 50 nM thapsigargin reduced the time constant for Ca2+ uptake but did not affect the SR Ca2+leak. Addition of 10 mM inorganic phosphate (Pi) decreased the rate of Ca2+ accumulation by the SR and increased the Ca2+ leak rate. This effect was reversed on addition of 10 mM phosphocreatine. 10 mM Pi had no effect on Ca2+ leak from the SR after complete inhibition of the Ca2+ pump. In conclusion, Pi decreases the Ca2+ uptake capacity of cardiac SR via a decrease in pump rate and an increase in Ca2+ pump-dependent Ca2+ leak.

The TTX metabolite 4,9-anhydro-TTX is a highly specific blocker of the Nav1.6 voltage-dependent sodium channel

2007 ◽  
Vol 293 (2) ◽  
pp. C783-C789 ◽  
Author(s):  
Christian Rosker ◽  
Birgit Lohberger ◽  
Doris Hofer ◽  
Bibiane Steinecker ◽  
Stefan Quasthoff ◽  
...  
Keyword(s):  
Steady State ◽  
Sodium Channels ◽  
Therapeutic Intervention ◽  
Time Course ◽  
Specific Interaction ◽  
Xenopus Laevis Oocytes ◽  
Recovery From Inactivation ◽  
Voltage Dependent ◽  
Steady State Inactivation ◽  
Invaluable Tool

The blocking efficacy of 4,9-anhydro-TTX (4,9-ah-TTX) and TTX on several isoforms of voltage-dependent sodium channels, expressed in Xenopus laevis oocytes, was tested (Nav1.2, Nav1.3, Nav1.4, Nav1.5, Nav1.6, Nav1.7, and Nav1.8). Generally, TTX was 40–231 times more effective, when compared with 4,9-ah-TTX, on a given isoform. An exception was Nav1.6, where 4,9-ah-TTX in nanomole per liter concentrations sufficed to result in substantial block, indicating that 4,9-ah-TTX acts specifically at this peculiar isoform. The IC50 values for TTX/4,9-ah-TTX were as follows (in nmol/l): 7.8 ± 1.3/1,260 ± 121 (Nav1.2), 2.8 ± 2.3/341 ± 36 (Nav1.3), 4.5 ± 1.0/988 ± 62 (Nav1.4), 1,970 ± 565/78,500 ± 11,600 (Nav1.5), 3.8 ± 1.5/7.8 ± 2.3 (Nav1.6), 5.5 ± 1.4/1,270 ± 251 (Nav1.7), and 1,330 ± 459/>30,000 (Nav1.8). Analysis of approximal half-maximal doses of both compounds revealed minor effects on voltage-dependent activation only, whereas steady-state inactivation was shifted to more negative potentials by both TTX and 4,9-ah-TTX in the case of the Nav1.6 subunit, but not in the case of other TTX-sensitive ones. TTX shifted steady-state inactivation also to more negative potentials in case of the TTX-insensitive Nav1.5 subunit, where it also exerted profound effects on the time course of recovery from inactivation. Isoform-specific interaction of toxins with ion channels is frequently observed in the case of proteinaceous toxins. Although the sensitivity of Nav1.1 to 4,9-ah-TTX is not known, here we report evidence on a highly isoform-specific TTX analog that may well turn out to be an invaluable tool in research for the identification of Nav1.6-mediated function, but also for therapeutic intervention.

Persistent Sodium Current, Membrane Properties and Bursting Behavior of Pre-Bötzinger Complex Inspiratory Neurons In Vitro

2002 ◽  
Vol 88 (5) ◽  
pp. 2242-2250 ◽  
Author(s):  
Christopher A. Del Negro ◽  
Naohiro Koshiya ◽  
Robert J. Butera ◽  
Jeffrey C. Smith
Keyword(s):  
Sodium Current ◽  
Neonatal Rat ◽  
Membrane Properties ◽  
Negative Slope ◽  
Inactivation Kinetics ◽  
Pacemaker Neurons ◽  
Inspiratory Neurons ◽  
Voltage Ramp ◽  
Bötzinger Complex

We measured persistent Na+current and membrane properties of bursting-pacemaker and nonbursting inspiratory neurons of the neonatal rat pre-Bötzinger complex (pre-BötC) in brain stem slice preparations with a rhythmically active respiratory network in vitro. In whole-cell recordings, slow voltage ramps (≤100 mV/s) inactivated the fast, spike-generating Na+ current and yielded N-shaped current-voltage relationships with nonmonotonic, negative-slope regions between −60 and −35 mV when the voltage-sensitive component was isolated. The underlying current was a TTX-sensitive persistent Na+ current ( I NaP) since the inward current was present at slow voltage ramp speeds (3.3–100 mV/s) and the current was blocked by 1 μM TTX. We measured the biophysical properties of I NaP after subtracting the voltage-insensitive “leak” current ( I Leak) in the presence of Cd2+ and in some cases tetraethylammonium (TEA). Peak I NaP ranged from −50 to −200 pA at a membrane potential of −30 mV. Decreasing the speed of the voltage ramp caused time-dependent I NaPinactivation, but this current was present at ramp speeds as low as 3.3 mV/s. I NaP activated at −60 mV and obtained half-maximal activation near −40 mV. The subthreshold voltage dependence and slow inactivation kinetics of I NaP, which closely resemble those of I NaP mathematically modeled as a burst-generation mechanism in pacemaker neurons of the pre-BötC, suggest that I NaP predominantly influences bursting dynamics of pre-BötC inspiratory pacemaker neurons in vitro. We also found that the ratio of persistent Na+conductance to leak conductance ( g NaP/ g Leak) can distinguish the phenotypic subpopulations of bursting pacemaker and nonbursting inspiratory neurons: pacemaker neurons showed g NaP/ g Leak> g NaP/ g Leakin nonpacemaker cells ( P < 0.0002). We conclude that I NaP is ubiquitously expressed by pre-BötC inspiratory neurons and that bursting pacemaker behavior within the heterogeneous population of inspiratory neurons is achieved with specific ratios of these two conductances, g NaP and g Leak.

scholarly journals Actin dynamics is rapidly regulated by the PTEN and PIP2 signaling pathways leading to myocyte hypertrophy

2014 ◽  
Vol 307 (11) ◽  
pp. H1618-H1625 ◽  
Author(s):  
Jieli Li ◽  
Elaine J. Tanhehco ◽  
Brenda Russell
Keyword(s):  
Time Course ◽  
Ventricular Myocytes ◽  
Initial Step ◽  
Heart Tissue ◽  
Neonatal Rat ◽  
Actin Dynamics ◽  
Mouse Heart ◽  
Dependent Manner ◽  
Ang Ii ◽  
Myocyte Hypertrophy

Mature cardiac myocytes are terminally differentiated, and the heart has limited capacity to replace lost myocytes. Thus adaptation of myocyte size plays an important role in the determination of cardiac function. The hypothesis tested is that regulation of the dynamic exchange of actin leads to cardiac hypertrophy. ANG II was used as a hypertrophic stimulant in mouse heart and neonatal rat ventricular myocytes (NRVMs) in culture for assessment of a mechanism for regulation of actin dynamics by phosphatidylinositol 4,5-bisphosphate (PIP2). Actin dynamics in NRVMs rapidly increased in a PIP2-dependent manner, measured by imaging and fluorescence recovery after photobleaching (FRAP). A significant increase in PIP2 levels was found by immunoblotting in both adult mouse heart tissue and cultured NRVMs. Inhibition of phosphatase and tensin homolog (PTEN) in NRVMs markedly blunted ANG II-induced increases in actin dynamics, the PIP2 level, and cell size. Furthermore, PTEN activity was dramatically upregulated in ANG II-treated NRVMs but downregulated when PTEN inhibitors were used. The time course of the rise in the PIP2 level was inversely related to the fall in the PIP3 level, which was significant by 30 min in ANG II-treated NRVMs. However, significant translocation of PTEN to the plasma membrane occurred by 10 min, suggesting a crucial initial step for PTEN for the cellular responses to ANG II. In conclusion, PTEN and PIP2 signaling may play an important role in myocyte hypertrophy by the regulation of actin filament dynamics, which is induced by ANG II stimulation.

Changes of action potential and L-type calcium channel current of Sprague–Dawley rat ventricular myocytes by different amlodipine isomers

2008 ◽  
Vol 86 (9) ◽  
pp. 620-625 ◽  
Author(s):  
Ru-xing Wang ◽  
Wen-ping Jiang
Keyword(s):  
Steady State ◽  
Action Potential ◽  
Calcium Channel ◽  
Ventricular Myocytes ◽  
Channel Blockers ◽  
Sprague Dawley ◽  
Patch Clamp Technique ◽  
Rat Ventricular Myocytes ◽  
Channel Current ◽  
Steady State Inactivation

To investigate the effects of S- and R-amlodipine (Aml) on action potential (AP) and L-type calcium channel current (ICa-L), the whole-cell patch-clamp technique was used on rat ventricular myocytes to record AP, ICa-L, peak currents, steady-state activation currents, steady-state inactivation currents, and recovery currents from inactivation with S-Aml and R-Aml at various concentrations. Increasing concentrations of S-Aml gradually shortened AP durations (APDs). At concentrations of 0.1, 0.5, 1, 5, and 10 μmol/L, S-Aml blocked 1.5% ± 0.2%, 25.4% ± 5.3%, 65.2% ± 7.3%, 78.4% ± 8.1%, and 94.2% ± 5.0% of ICa-L, respectively (p < 0.05), and the half-inhibited concentration was 0.62 ± 0.12 µmol/L. Current–voltage curves were shifted upward; steady-state activation and inactivation curves were shifted to the left. At these concentrations of S-Aml, the half-activation voltages were –16.01 ± 1.65, –17.61 ± 1.60, –20.17 ± 1.46, –21.87 ± 1.69, and –24.09 ± 1.87 mV, respectively, and the slope factors were increased (p < 0.05). The half-inactivation voltages were –27.16 ± 4.48, –28.69 ± 4.52, –31.19 ± 4.17, –32.63 ± 4.34, and –35.16 ± 4.46 mV, respectively, and the slope factors were increased (p < 0.05). The recovery times from inactivation of S-Aml were prolonged (p < 0.05). In contrast, R-Aml had no effect on AP and ICa-L (p > 0.05) at the concentrations tested. Thus, only S-Aml has calcium channel blockade activity, whereas R-Aml has none of the pharmacologic actions associated with calcium channel blockers.

scholarly journals The calcium-independent transient outward potassium current in isolated ferret right ventricular myocytes. II. Closed state reverse use-dependent block by 4-aminopyridine.

1993 ◽  
Vol 101 (4) ◽  
pp. 603-626 ◽  
Author(s):  
D L Campbell ◽  
Y Qu ◽  
R L Rasmusson ◽  
H C Strauss
Keyword(s):  
Steady State ◽  
Right Ventricular ◽  
Ventricular Myocytes ◽  
Patch Clamp Technique ◽  
Time Constants ◽  
Closed State ◽  
Whole Cell Patch Clamp ◽  
Dependent Behavior ◽  
Steady State Inactivation ◽  
K Current

Block of the calcium-independent transient outward K+ current, I(to), by 4-aminopyridine (4-AP) was studied in ferret right ventricular myocytes using the whole cell patch clamp technique. 4-AP reduces I(to) through a closed state blocking mechanism displaying "reverse use-dependent" behavior that was inferred from: (a) development of tonic block at hyperpolarized potentials; (b) inhibition of development of tonic block at depolarized potentials; (c) appearance of "crossover phenomena" in which the peak current is delayed in the presence of 4-AP at depolarized potentials; (d) relief of block at depolarized potentials which is concentration dependent and parallels steady-state inactivation for low 4-AP concentrations (V1/2 approximately -10 mV in 0.1 mM 4-AP) and steady-state activation at higher concentrations (V1/2 = +7 mV in 1 mM 4-AP, +15 mV in 10 mM 4-AP); and (e) reassociation of 4-AP at hyperpolarized potentials. No evidence for interaction of 4-AP with either the open or inactivated state of the I(to) channel was obtained from measurements of kinetics of recovery and deactivation in the presence of 0.5-1.0 mM 4-AP. At hyperpolarized potentials (-30 to -90 mV) 10 mM 4-AP associates slowly (time constants ranging from approximately 800 to 1,300 ms) with the closed states of the channel (apparent Kd approximately 0.2 mM). From -90 to -20 mV the affinity of the I(to) channel for 4-AP appears to be voltage insensitive; however, at depolarized potentials (+20 to +100 mV) 4-AP dissociates with time constants ranging from approximately 350 to 150 ms. Consequently, the properties of 4-AP binding to the I(to) channel undergo a transition in the range of potentials over which channel activation and inactivation occurs (-30 to +20 mV). We propose a closed state model of I(to) channel gating and 4-AP binding kinetics, in which 4-AP binds to three closed states. In this model 4-AP has a progressively lower affinity as the channel approaches the open state, but has no intrinsic voltage dependence of binding.

Alterations of voltage-activated sodium current by a novel conotoxin from the venom of Conus gloriamaris

1995 ◽  
Vol 73 (3) ◽  
pp. 1295-1301 ◽  
Author(s):  
A. Hasson ◽  
K. J. Shon ◽  
B. M. Olivera ◽  
M. E. Spira
Keyword(s):  
Steady State ◽  
Action Potential ◽  
Sodium Current ◽  
Land Snails ◽  
Calcium Currents ◽  
Inactivation Kinetics ◽  
The Novel ◽  
Inactivation Curve ◽  
Aplysia Neurons ◽  
Kinetics Of

1. The novel peptide toxin delta-conotoxin-GmVIA, recently purified by us from the mollusk-hunting snail Conus gloriamaris, induces convulsive-like contractions when injected into land snails but has no detectable effects in mammals. 2. At concentrations of 0.5-0.75 microM, the toxin induces action potential broadening and increased excitability of cultured Aplysia neurons. 3. Whole cell patch-clamp experiments on cultured Aplysia neurons revealed that the toxin does not alter potassium or calcium currents, but induces action potential broadening by slowing the inactivation kinetics of the sodium current. Under control conditions, the inactivation kinetics of the sodium current follows a single exponential with tau = 0.47 +/- 0.14 (SE) ms. After toxin application the sodium current inactivation is composed of two phases: an early phase with tau = 0.86 +/- 0.12 ms and a late phase of slowly inactivating sodium current with tau = 488 +/- 120 ms. In addition, the toxin shifts the voltage-dependent steady-state inactivation curve to more positive values and the steady-state activation curve to more negative values. These alterations are not associated with changes in the rise time or the peak value of the sodium current. 4. The novel delta-conotoxin-GmVIA, and the previously described "King Kong peptide," purified from another mollusk-hunting cone (Conus textile), share a similar cystein framework also found in the calcium channel blocking peptide omega-conotoxin but represent a new class of conotoxins with unusual specificity for molluscan sodium channels.

Stimulatory effect of ouabain on T- and L-type calcium currents in guinea pig cardiac myocytes

1990 ◽  
Vol 258 (5) ◽  
pp. H1620-H1623 ◽  
Author(s):  
B. Le Grand ◽  
E. Deroubaix ◽  
A. Coulombe ◽  
E. Coraboeuf
Keyword(s):  
Steady State ◽  
Guinea Pig ◽  
Ventricular Myocytes ◽  
Calcium Currents ◽  
High Threshold ◽  
Inactivation Curve ◽  
Cell Recording ◽  
Steady State Inactivation ◽  
Low Threshold ◽  
Free Media

The effect of 10(-6) M ouabain on macroscopic low-threshold T-type Ca2+ and high-threshold L-type Ca2+ currents was studied by whole cell recording in isolated guinea pig ventricular myocytes superfused with K-free, Na-free media, i.e., after suppression of Na-K-ATPase activity and Na influx through the Na-Ca exchanger. Under such conditions, the amplitudes of the two currents were significantly increased by ouabain. In particular, the current occurring in the -50 to -20 mV range (T-type Ca2+) was increased two- to threefold by ouabain and suppressed by 40 microM Ni2+. Ouabain shifted by approximately 10 mV toward negative potentials the steady-state inactivation curve of the T-type Ca2+ current but not that of the L-type Ca2+ current. It is concluded that ouabain enhances not only L-type Ca2+ current but also T-type Ca2+ current possibly through different mechanisms.

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