L-type calcium current in rod- and spindle-shaped myocytes isolated from rabbit atrioventricular node

1994 ◽  
Vol 267 (5) ◽  
pp. H1670-H1680
Author(s):  
J. C. Hancox ◽  
A. J. Levi
Keyword(s):  
Calcium Current ◽  
Time Course ◽  
Present Report ◽  
Atrioventricular Node ◽  
Time To Peak ◽  
Voltage Clamps ◽  
Voltage Dependent ◽  
Slope Factor ◽  
Spontaneous Action ◽  
Spontaneous Action Potentials

The atrioventricular node (AVN) is vital to normal cardiac function. The present report describes the properties of L-type calcium current (ICa) in rod- and spindle-shaped myocytes isolated from the rabbit AVN. With depolarizing voltage clamps from a holding potential of -40 mV, a rapidly activating ICa was observed, which peaked at +10 mV in most cells and exhibited a “bell-shaped” current-voltage relation. ICa was abolished by nifedipine (2–20 microM) and cadmium (100–200 microM) and was greatly reduced by manganese (1 mM). At +10 mV, time to peak ICa was 3.3 +/- 0.15 (SE) ms (n = 12) and ICa current density was 9.3 +/- 1.2 pA/pF (n = 9). Steady-state activation and inactivation curves for ICa showed half-maximal activation at -3.6 mV [slope factor (k) = 6.6 mV] and half-maximal inactivation at -25.8 mV (k = 6.5 mV). The time course of decay of ICa during a depolarizing pulse was voltage dependent and biexponential. The time course of recovery of ICa from inactivation was also biexponential (with two time constants tau 1 = 194.7 and tau 2 = 907.4 ms). Under current clamp, spontaneous action potentials from AVN cells were blocked by nifedipine as well as by cadmium, suggesting that L-type ICa was largely responsible for the action potential upstroke.

Voltage-dependent K+ current in capillary endothelial cells isolated from guinea pig heart

1999 ◽  
Vol 277 (1) ◽  
pp. H119-H127 ◽  
Author(s):  
Michael Dittrich ◽  
Jürgen Daut
Keyword(s):  
Guinea Pig ◽  
Time Course ◽  
Single Channel ◽  
Patch Clamp Technique ◽  
Guinea Pig Heart ◽  
Potential Oscillations ◽  
Time To Peak ◽  
Voltage Dependent ◽  
Capillary Endothelial Cells ◽  
Single Channel Currents

Capillary fragments were isolated from guinea pig hearts, and their electrical properties were studied using the perforated-patch and cell-attached mode of the patch-clamp technique. A voltage-dependent K+ current was discovered that was activated at potentials positive to −20 mV and showed a sigmoid rising phase. For depolarizing voltage steps from −128 to +52 mV, the time to peak was 71 ± 5 ms (mean ± SE) and the amplitude of the current was 3.7 ± 0.5 pA/pF in the presence of 5 mM external K+. The time course of inactivation was exponential with a time constant of 7.2 ± 0.5 s at +52 mV. The current was blocked by tetraethylammonium (inhibitory constant ∼3 mM) but was not affected by charybdotoxin (1 μM) or apamin (1 μM). In the cell-attached mode, depolarization-activated single-channel currents were found that inactivated completely within 30 s; the single-channel conductance was 12.3 ± 2.4 pS. The depolarization-activated K+current described here may play a role in membrane potential oscillations of the endothelium.

G-Protein-Modulated Ca2+ Current With Slowed Activation Does Not Alter the Kinetics of Action Potential-Evoked Ca2+ Current

2000 ◽  
Vol 84 (5) ◽  
pp. 2417-2425 ◽  
Author(s):  
Debra E. Artim ◽  
Stephen D. Meriney
Keyword(s):  
Action Potential ◽  
Calcium Channel ◽  
G Protein ◽  
Calcium Current ◽  
Time Course ◽  
Channel Activation ◽  
Tail Current ◽  
Voltage Dependent ◽  
Dependent Inhibition ◽  
Kinetics Of

We have studied voltage-dependent inhibition of N-type calcium currents to investigate the effects of G-protein modulation-induced alterations in channel gating on action potential-evoked calcium current. In isolated chick ciliary ganglion neurons, GTPγS produced voltage-dependent inhibition that exhibited slowed activation kinetics and was partially relieved by a conditioning prepulse. Using step depolarizations to evoke calcium current, we measured tail current amplitudes on abrupt repolarization to estimate the time course of calcium channel activation from 1 to 30 ms. GTPγS prolonged significantly channel activation, consistent with the presence of kinetic slowing in the modulated whole cell current evoked by 100-ms steps. Since kinetic slowing is caused by an altered voltage dependence of channel activation (such that channels require stronger or longer duration depolarization to open), we asked if GTPγS-induced modulation would alter the time course of calcium channel activation during an action potential. Using an action potential waveform as a voltage command to evoke calcium current, we abruptly repolarized to −80 mV at various time points during the repolarization phase of the action potential. The resulting tail current was used to estimate the relative number of calcium channels that were open. Using action potential waveforms of either 2.2- or 6-ms duration at half-amplitude, there were no differences in the time course of calcium channel activation, or in the percent activation at any time point tested during the repolarization, when control and modulated currents were compared. It is also possible that modulated channels might open briefly and that these reluctant openings would effect the time course of action potential-evoked calcium current. However, when control and modulated currents were scaled to the same peak amplitude and superimposed, there was no difference in the kinetics of the two currents. Thus voltage-dependent inhibition did not alter the kinetics of action potential-evoked current. These results suggest that G-protein-modulated channels do not contribute significantly to calcium current evoked by a single action potential.

Inhibition of the voltage-dependent calcium current by extracellular ATP in hamster ventricular cardiomyocytes

1997 ◽  
Vol 273 (1) ◽  
pp. H250-H256 ◽  
Author(s):  
F. Von zur Muhlen ◽  
B. D. Gonska ◽  
H. Kreuzer
Keyword(s):  
Calcium Current ◽  
Time Course ◽  
Ventricular Myocytes ◽  
Purinergic Receptor ◽  
Extracellular Atp ◽  
Patch Clamp Technique ◽  
Ventricular Cardiomyocytes ◽  
Current Voltage ◽  
Voltage Dependent ◽  
High Level

The modulation of the high-voltage-activated calcium current (ICa) by external ATP was examined in single ventricular cardiomyocytes of the hamster using the whole-cell configuration of the patch-clamp technique. Extracellular application of ATP (0.1-100 microM) was found to inhibit ICa reversibly. The inhibition followed a slow time course (half time approximately 25 s) and was accompanied by very small changes of the holding current and no shift in the current-voltage relationship. With 100 microM ATP, peak ICa was reduced by approximately 30%. This response was not blocked by the P1 inhibitor 8-cyclopentyl-1,3-dipropylxanthine. The nonhydrolyzable ATP analogs adenosine 5'-O-(3-thiotriphosphate) and AMP-adenosine 5'-[beta,gamma-imido]triphosphate also reduced ICa. The ATP analog alpha,beta-methylene-ATP was about equipotent with ATP at 50 microM. Internal guanosine 5'-O-(3-thiotriphosphate) (200 microM) rendered the ATP-mediated inhibition of ICa poorly reversible, whereas internal guanosine 5'-O-(2-thiodiphosphate) (200-500 microM) had no effect. Holding the intracellular adenosine 3',5'-cyclic monophosphate concentration at a constant high level did not alter the ATP response. We conclude that external ATP inhibits ICa via a P2 purinergic receptor in hamster ventricular myocytes. Our results suggest the involvement of a G protein not coupled to adenylate cyclase. The inhibition of ICa by extracellular ATP might have pathophysiological relevance under conditions of myocardial injury.

Spontaneous electrical rhythmicity and the role of the sarcoplasmic reticulum in the excitability of guinea pig gallbladder smooth muscle cells

2006 ◽  
Vol 290 (4) ◽  
pp. G655-G664 ◽  
Author(s):  
Onesmo B. Balemba ◽  
Matthew J. Salter ◽  
Thomas J. Heppner ◽  
Adrian D. Bonev ◽  
Mark T. Nelson ◽  
...  
Keyword(s):  
Smooth Muscle ◽  
Sarcoplasmic Reticulum ◽  
Action Potentials ◽  
Cyclopiazonic Acid ◽  
Cellular Mechanisms ◽  
Voltage Dependent ◽  
Spontaneous Action ◽  
Potential Activity ◽  
Spontaneous Action Potentials

Spontaneous action potentials and Ca2+ transients were investigated in intact gallbladder preparations to determine how electrical events propagate and the cellular mechanisms that modulate these events. Rhythmic phasic contractions were preceded by Ca2+ flashes that were either focal (limited to one or a few bundles), multifocal (occurring asynchronously in several bundles), or global (simultaneous flashes throughout the field). Ca2+ flashes and action potentials were abolished by inhibiting sarcoplasmic reticulum (SR) Ca2+ release via inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] channels with 2-aminoethoxydiphenyl borate and xestospongin C or by inhibiting voltage-dependent Ca2+ channels (VDCCs) with nifedipine or diltiazem or nisoldipine. Inhibiting ryanodine channels with ryanodine caused multiple spikes superimposed upon plateaus of action potentials and extended quiescent periods. Depletion of SR Ca2+ stores with thapsigargin or cyclopiazonic acid increased the frequency and duration of Ca2+ flashes and action potentials. Acetylcholine, carbachol, or cholecystokinin increased synchronized and increased the frequency of Ca2+ flashes and action potentials. The phospholipase C (PLC) inhibitor U-73122 did not affect Ca2+ flash or action potential activity but inhibited the excitatory effects of acetylcholine on these events. These results indicate that Ca2+ flashes correspond to action potentials and that rhythmic excitation in the gallbladder is multifocal among gallbladder smooth muscle bundles and can be synchronized by excitatory agonists. These events do not depend on PLC activation, but agonist stimulation involves activation of PLC. Generation of these events depends on Ca2+ entry via VDCCs and on Ca2+ mobilization from the SR via Ins(1,4,5)P3 channels.

LVA and HVA Ca2+ Currents in Ventricular Muscle Cells of the Lymnaea Heart

1999 ◽  
Vol 82 (5) ◽  
pp. 2428-2440 ◽  
Author(s):  
M. S. Yeoman ◽  
B. L. Brezden ◽  
P. R. Benjamin
Keyword(s):  
Low Voltage ◽  
Muscle Cells ◽  
Voltage Sensitivity ◽  
Excitation Contraction Coupling ◽  
Contraction Coupling ◽  
Ventricular Cells ◽  
Voltage Dependent ◽  
Heart Muscle Cells ◽  
Spontaneous Action ◽  
Spontaneous Action Potentials

The single-electrode voltage-clamp technique was used to characterize voltage-gated Ca2+ currents in dissociated Lymnaea heart ventricular cells. In the presence of 30 mM tetraethylammonium (TEA), two distinct Ca2+ currents could be identified. The first current activated between −70 and −60 mV. It was fully available for activation at potentials more negative than −80 mV. The current was fast to activate and inactivate. The inactivation of the current was voltage dependent. The current was larger when it was carried by Ca2+ compared with Ba2+, although changing the permeant ion had no observable effect on the kinetics of the evoked currents. The current was blocked by Co2+ and La3+ (1 mM) but was particularly sensitive to Ni2+ ions (≈50% block with 100 μM Ni2+) and insensitive to low doses of the dihydropyridine Ca2+channel antagonist, nifedipine. All these properties classify this current as a member of the low-voltage–activated (LVA) T-type family of Ca2+ currents. The activation threshold of the current (−70 mV) suggests that it has a role in pacemaking and action potential generation. Muscle contractions were first seen at −50 mV, indicating that this current might supply some of the Ca2+necessary for excitation-contraction coupling. The second, a high-voltage–activated (HVA) current, activated at potentials between −40 and −30 mV and was fully available for activation at potentials more negative than −60 mV. This current was also fast to activate and with Ca2+ as the permeant ion, inactivated completely during the 200-ms voltage step. Substitution of Ba2+ for Ca2+ increased the amplitude of the current and significantly slowed the rate of inactivation. The inactivation of this current appeared to be current rather than voltage dependent. This current was blocked by Co2+ and La3+ ions (1 mM) but was sensitive to micromolar concentrations of nifedipine (≈50% block 10 μM nifedipine) that were ineffective at blocking the LVA current. These properties characterize this current as a L-type Ca2+ current. The voltage sensitivity of this current suggests that it is also important in generating the spontaneous action potentials, and in providing some of the Ca2+ necessary for excitation-contraction coupling. These data provide the first detailed description of the voltage-dependent Ca2+ currents present in the heart muscle cells of an invertebrate and indicate that pacemaking in the molluscan heart has some similarities with that of the mammalian heart.

Kinetic properties of a transient outward current in rat neocortical neurons

1992 ◽  
Vol 68 (4) ◽  
pp. 1133-1142 ◽  
Author(s):  
M. Andreasen ◽  
J. J. Hablitz
Keyword(s):  
Time Course ◽  
Outward Current ◽  
Transient Outward Current ◽  
Late Component ◽  
Exponential Time ◽  
Neocortical Neurons ◽  
Voltage Dependent ◽  
Steady State Inactivation ◽  
Slope Factor ◽  
Single Exponential

1. Whole-cell patch-clamp techniques were used to record outward currents in embryonic rat neocortical neurons maintained in culture. In the presence of tetrodotoxin and cadmium, depolarization evoked an outward current with a complex waveform. This outward current consisted of an initial fast transient component and a late, slowly inactivating component. 2. The two outward current components could be separated pharmacologically with the use of tetraethylammonium (TEA) and 4-aminopyridine (4-AP). TEA (20 mM) applied extracellularly completely blocked the late component, unmasking a fast transient outward current (TOC). 4-AP (5 mM) applied extracellularly blocked the early component while reducing the late component by 27.8 +/- 9.7% (mean +/- SE). 3. The TOC activated after a short delay and rose rapidly to a peak. The time to peak was voltage dependent and decreased with depolarization. In the presence of 200 microM extracellular cadmium, activation threshold was around -25 mV, and current amplitude increased with depolarization. The voltage-conductance relationship was well fitted by the use of the Boltzmann equation with a Vm of +19 mV for half activation and a slope factor of +6 mV. 4. On sustained depolarization the TOC rapidly inactivated and decayed to baseline within 500-600 ms. The decay phase followed a single exponential time course with a time constant of 55-65 ms. The decay time was most rapid at potentials from +5 to +20 mV and increased slightly with further depolarization. 5. Steady-state inactivation of the TOC, in the presence of cadmium, was complete near -10 mV and was totally relieved at potentials more negative than -75 mV. With the use of the Boltzmann equation, a Vm of -34 mV for half inactivation and a slope factor of -8.6 mV were found. 6. Recovery of the TOC from steady-state inactivation followed a single exponential time course and was voltage dependent. When the membrane potential was held at -84 mV during the conditioning pulse, the time constant of recovery was 17 ms, increasing to 45.2 and 58.1 ms at holding potentials of -64 and -44 mV, respectively. Holding at potentials more negative than -84 mV produced no further change in the recovery time course. 7. The presence of 200 microM external cadmium altered the TOC activation and inactivation curves. Removal of cadmium produced a -16-mV shift in the Vm for half activation and a -25-mV shift in the inactivation curve. This sensitivity to cadmium is higher than that reported in other systems.(ABSTRACT TRUNCATED AT 400 WORDS)

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