scholarly journals Ionic currents in single isolated bullfrog atrial cells.

1983 ◽  
Vol 81 (2) ◽  
pp. 153-194 ◽  
Author(s):  
J R Hume ◽  
W Giles
Keyword(s):  
Voltage Clamp ◽  
Single Cells ◽  
Ionic Current ◽  
Technical Difficulty ◽  
Ionic Currents ◽  
Cardiac Cells ◽  
Small Cells ◽  
Inward Current ◽  
Electrophysiological Properties ◽  
Atrial Cells

Enzymatic dispersion has been used to yield single cells from segments of bullfrog atrium. Previous data (Hume and Giles, 1981) have shown that these individual cells are quiescent and have normal resting potentials and action potentials. The minimum DC space constant is approximately 920 microns. The major goals of the present study were: (a) to develop and refine techniques for making quantitative measurements of the transmembrane ionic currents, and (b) to identify the individual components of ionic current which generate different phases of the action potential. Initial voltage-clamp experiments made using a conventional two-microelectrode technique revealed a small tetrodotoxin (TTX)-insensitive inward current. The small size of this current (2.5-3.0 X 10(-10)A) and the technical difficulty of the two-microelectrode experiments prompted the development of a one-microelectrode voltage-clamp technique which requires impalements using a low-resistance (0.5-2 M omega) micropipette. Voltage-clamp experiments using this new technique in isolated single atrial cells reveal five distinct ionic currents: (a) a conventional transient Na+ current, (b) a TTX-resistant transient inward current, carried mainly by Ca++, (c) a component of persistent inward current, (d) a slowly developing outward K+ current, and (e) an inwardly rectifying time-independent background current. The single suction micropipette technique appears well-suited for use in the quantitative study of ionic currents in these cardiac cells, and in other small cells having similar electrophysiological properties.

scholarly journals "Creep currents" in single frog atrial cells may be generated by electrogenic Na/Ca exchange.

1986 ◽  
Vol 87 (6) ◽  
pp. 857-884 ◽  
Author(s):  
J R Hume ◽  
A Uehara
Keyword(s):  
Voltage Clamp ◽  
Time Course ◽  
Single Cells ◽  
Mechanical Activity ◽  
Common Mechanism ◽  
Mechanical Relaxation ◽  
Equilibrium Conditions ◽  
Atrial Cells ◽  
Current Voltage ◽  
Wide Range

The objective of these experiments was to test the hypothesis that the "creep currents" induced by Na loading of single frog atrial cells (Hume, J. R., and A. Uehara. 1986. Journal of General Physiology. 87:833) may be generated by an electrogenic Na/Ca exchanger. Creep currents induced by Na loading were examined over a wide range of membrane potentials. During depolarizing voltage-clamp pulses, outward creep currents were observed, followed by inward creep currents upon the return to the holding potential. During hyperpolarizing voltage-clamp pulses, creep currents of the opposite polarity were observed: inward creep currents were observed during the pulses, followed by outward creep currents upon the return to the holding potential. The current-voltage relations for inward and outward creep currents in response to depolarizing or hyperpolarizing voltage displacements away from the holding potential all intersect the voltage axis at a common potential, which indicates that inward and outward creep currents may have a common reversal potential under equilibrium conditions and may therefore be generated by a common mechanism. Measurements of inward creep currents confirm that voltage displacements away from the holding potential rapidly alter equilibrium conditions. Current-voltage relationships of inward creep currents after depolarizing voltage-clamp pulses are extremely labile and depend critically upon the amplitude and duration of outward creep currents elicited during preceding voltage-clamp pulses. An optical monitor of mechanical activity in single cells revealed (a) a similar voltage dependence for the outward creep currents induced by Na loading and tonic contraction, and (b) a close correlation between the time course of the decay of the inward creep current and the time course of mechanical relaxation. A mathematical model of electrogenic Na/Ca exchange (Mullins, L.J. 1979. Federation Proceedings. 35:2583; Noble, D. 1986. Cardiac Muscle. 171-200) can adequately account for many of the properties of creep currents. It is concluded that creep currents in single frog atrial cells may be attributed to the operation of an electrogenic Na/Ca exchange mechanism.

Ionic currents of smooth muscle cells isolated from the ctenophore Mnemiopsis

1988 ◽  
Vol 233 (1271) ◽  
pp. 99-121 ◽  
Keyword(s):  
Smooth Muscle ◽  
Steady State ◽  
Smooth Muscle Cells ◽  
Voltage Clamp ◽  
Muscle Cells ◽  
Ionic Currents ◽  
Patch Clamping ◽  
Excitable Cells ◽  
Inward Current ◽  
Electrode Voltage

The ionic currents of smooth muscle cells isolated from the ctenophore Mnemiopsis were examined by using conventional two-electrode voltage clamp and whole-cell patch clamping methods. Several separable currents were identified. These include: (1) a transient and (2) a steady-state voltage-activated inward current; both are tetrodotoxin (TTX) and saxitoxin (STX) insensitive, partly reduced by decreasing external Ca 2+ or Na + or by addition of 5 mM Co 2+ , D-600 or verapamil and are totally blocked with 5 mM Cd 2+ ; (3) an early, transient, cation-dependent, outward K + current ( I Kca/Na ); (4) a transient, voltage-activated, outward K + current provisionally identified as I A ; (5) a delayed, steady-state, voltage-activated outward K + current ( I K ) and (6) a late, transient, outward K + current which is blocked by Cd 2+ and evident only during long voltage pulses. Despite their phylogenic origin, most of these currents are similar to currents identified in many vertebrate smooth and cardiac muscle preparations, and other excitable cells in higher animals.

scholarly journals Two Inward Currents in Frog Atrial Muscle

1971 ◽  
Vol 58 (5) ◽  
pp. 523-543 ◽  
Author(s):  
Merrill Tarr
Keyword(s):  
Action Potential ◽  
Voltage Clamp ◽  
Ionic Currents ◽  
Ringer Solution ◽  
Slow Inward Current ◽  
Slow Phase ◽  
Free Solution ◽  
Rapid Phase ◽  
Inward Current ◽  
Atrial Tissue

The double sucrose-gap voltage-clamp technique was applied to frog atrial tissue to investigate the ionic currents responsible for the action potential in this tissue. Membrane depolarization elicited two distinct components of inward current when the test node was exposed to normal Ringer solution: a fast inward current and a slow inward current. The fast inward current appeared to be carried by sodium ions, since it was rapidly abolished by exposure of the fiber to Na+-free solution or tetrodotoxin but persisted on exposure to Ca++-free solution. In contrast, in the majority of the preparations the slow inward current appeared to be primarily carried by calcium ions, since it was abolished on exposure of the fiber to Ca++-free solution but persisted on exposure to Na+-free solution. Action potential data supported the voltage-clamp findings. The normal action potential shows two distinct components in the upstroke phase: an initial rapid phase of depolarization followed by a slower phase of depolarization reaching the peak of the action potential. Abolition of the fast inward current resulted in abolition of the initial rapid phase of depolarization. Abolition of the slow inward current resulted in abolition of the slow phase of depolarization. These data support the hypothesis that two distinct and different ionic mechanisms contribute to the upstroke phase of the action potential in frog atrial tissue.

scholarly journals The pacemaker of snake heart is localized near the sinoatrial valve

2021 ◽  
Author(s):  
Denis V. Abramochkin ◽  
Vladislav S. Kuzmin ◽  
Vladimir Matchkov ◽  
Andrey A. Kamensky ◽  
Tobias Wang
Keyword(s):  
Optical Mapping ◽  
Ionic Currents ◽  
Inward Rectifier ◽  
Time Dependent ◽  
Sinus Venosus ◽  
Circular Region ◽  
Inward Current ◽  
Electrophysiological Properties ◽  
Exact Location ◽  
The Right

To provide the first description of the exact location of primary pacemaker of the squamate heart, we used sharp microelectrode impalements and optical mapping of isolated sinus venosus preparations from Burmese pythons. We located the dominant pacemaker site at the base of the right leaflet of the sinoatrial valve (SAV), but latent pacemakers were also identified in a circular region around the SAV. Acetylcholine (10−5M) or noradrenaline (10−6M) induced shifts of the leading pacemaker site to other points near the SAV. The ionic currents of most of the cardiomyocytes isolated enzymatically from the SAV region resembled those of typical working myocytes from the sinus venosus. However, seven cells lacked the background inward rectifier current (IK1) and had a time-dependent hyperpolarization-induced inward current identified as the “funny” current (If). Therefore, region proximal to SAV demonstrates pacemaking activity and contains cells that resemble the electrophysiological properties of mammalian pacemaker myocytes.

Separation of Na-Ca exchange and transient inward currents in heart cells

1987 ◽  
Vol 253 (5) ◽  
pp. H1330-H1333
Author(s):  
Y. Shimoni ◽  
W. Giles
Keyword(s):  
Sodium Current ◽  
Single Cells ◽  
Cardiac Cells ◽  
Slow Inward Current ◽  
Heart Cells ◽  
Inward Current ◽  
Inward Currents ◽  
Isolated Cardiac Cells ◽  
Fast Sodium Current ◽  
External K

Enzymatically dispersed single cells from rabbit ventricle were voltage clamped using the suction pipette method to investigate whether in isolated cardiac cells a recently described slow inward current (IEX) due to the electrogenic Na+-dependent Ca2+ extrusion also underlies a transient inward current (ITI), which can trigger certain cardiac arrhythmias. The cells were held at -40 mV to inactivate the fast sodium current. After depolarizing pulses (to 0 or +10 mV for 50 to 200 ms), slow inward "tail" currents were consistently recorded. Previous results indicate that this tail current IEX is generated by the Na+-Ca2+ exchanger. After loading the cells with Ca2+ by blocking the Na+-K+ pump [either with strophanthidin (10(-5) M) treatment or by reducing external K+ to 1 mM or less], ITIS appeared. These were usually spontaneous but occasionally were time locked to the clamp pulses. It was possible to separate IEX and ITI by a variety of methods. These include the following. 1) Different stimulation protocols; repolarizing to more negative potentials augmented IEX and decreased or eliminated ITI. Increasing the rate of stimulation diminished IEX and increased ITI. 2) Pharmacological methods; adding BaCl2 (0.5-2.0 mM) or caffeine (5-10 mM) decreased IEX but abolished ITI. The findings suggest that different mechanisms regulate these two currents.

A mathematical model of a bullfrog cardiac pacemaker cell

1990 ◽  
Vol 259 (2) ◽  
pp. H352-H369 ◽  
Author(s):  
R. L. Rasmusson ◽  
J. W. Clark ◽  
W. R. Giles ◽  
E. F. Shibata ◽  
D. L. Campbell
Keyword(s):  
Action Potential ◽  
Voltage Clamp ◽  
Binding Proteins ◽  
Single Cells ◽  
Cardiac Pacemaker ◽  
Ionic Currents ◽  
Tissue Type ◽  
Delayed Rectifier ◽  
Single Action ◽  
Diastolic Depolarization

Previous models of cardiac cellular electrophysiology have been based largely on voltage-clamp measurements obtained from multicellular preparations and often combined data from different regions of the heart and a variety of species. We have developed a model of cardiac pacemaking based on a comprehensive set of voltage-clamp measurements obtained from single cells isolated from one specific tissue type, the bullfrog sinus venosus (SV). Consequently, sarcolemmal current densities and kinetics are not influenced by secondary phenomena associated with multicellular preparations, allowing us to realistically simulate processes thought to be important in pacemaking, including the Na(+)-K+ pump and Na(+)-Ca2+ exchanger. The membrane is surrounded extracellularly by a diffusion-limited space and intracellularly by a limited myoplasmic volume containing Ca2(+)-binding proteins (calmodulin, troponin). The model makes several predictions regarding mechanisms involved in pacing. 1) Primary pacemaking cannot be attributed to any single current but arises from both the lack of a background K+ current and a complex interaction between Ca2+, delayed-rectifier K+, and background leak currents. 2) Ca2+ current displays complex behavior and is important during repolarization. 3) Because of Ca2+ buffering by myoplasmic proteins, the Na(+)-Ca2+ exchanger current is small and has little influence on action potential repolarization but may modulate the maximum diastolic potential. 4) The Na(+)-K+ pump current does not play an active role in repolarization but is of sufficient size to modulate the rate of diastolic depolarization. 5) K+ accumulation and Ca2+ depletion may occur in the extracellular spaces but play no role in either the diastolic depolarization or repolarization of a single action potential. This model illustrates the importance of basing simulations on quantitative measurements of ionic currents in myocytes and of including both electrogenic transporter mechanisms and Ca2+ buffering by myoplasmic Ca2(+)-binding proteins.

A model of the electrophysiological properties of thalamocortical relay neurons

1992 ◽  
Vol 68 (4) ◽  
pp. 1384-1400 ◽  
Author(s):  
D. A. McCormick ◽  
J. R. Huguenard
Keyword(s):  
Model Neuron ◽  
Ionic Current ◽  
Ionic Currents ◽  
Dorsal Lateral Geniculate Nucleus ◽  
Resting Membrane Potential ◽  
Na Current ◽  
Electrophysiological Properties ◽  
K Current ◽  
Low Threshold ◽  
Relay Neurons

1. A model of the electrophysiological properties of single thalamocortical relay neurons in the rodent and cat dorsal lateral geniculate nucleus was constructed, based in part on the voltage dependence and kinetics of ionic currents detailed with voltage-clamp techniques. The model made the simplifying assumption of a single uniform compartment and incorporated a fast and transient Na+ current, INa; a persistent, depolarization-activated Na+ current, INap; a low-threshold Ca2+ current, I(T); a high-threshold Ca2+ current, IL; a Ca(2+)-activated K+ current, IC; a transient and depolarization-activated K+ current, IA; a slowly inactivating and depolarization-activated K+ current, IK2; a hyperpolarization-activated cation current, Ih; and K+ and Na+ leak currents IKleak and INaleak. 2. The effects of the various ionic currents on the electrophysiological properties of thalamocortical relay neurons were initially investigated through examining the effect of each current individually on passive membrane responses. The two leak currents, IKleak and INaleak, determined in large part the resting membrane potential and the apparent input resistance of the model neuron. Addition of IA resulted in a delay in the response of the model cell to a depolarizing current pulse, whereas addition of IK2, or IL combined with IC, resulted in a marked and prolonged decrease in the response to depolarization. Addition of Ih resulted in a depolarizing "sag" in response to hyperpolarization, whereas addition of IT resulted in a large rebound Ca2+ spike after hyperpolarization. Finally, addition of INap resulted in enhancement of depolarization. 3. The low-threshold Ca2+ spike of rodent neurons was successfully modeled with the active currents I(T), IL, IA, IC, and IK2. The low-threshold Ca2+ current I(T) generated the low-threshold Ca2+ spike. The transient K+ current IA slowed the rate of rise and reduced the peak amplitude of the low-threshold Ca2+ spike, whereas the slowly inactivating K+ current IK2 contributed greatly to the repolarization of the Ca2+ spike. Activation of IL during the peak of the Ca2+ spike led to activation of IC, which also contributed to the repolarization of the Ca2+ spike. Reduction of any one of the K+ currents resulted in an increase in the other two, thereby resulting in substantially smaller changes in the Ca2+ spike than would be expected on the basis of the amplitude of each ionic current alone.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage Sensitive Ca-Channels and the Transient Inward Current in Paramecium Tetraurelia

1979 ◽  
Vol 78 (1) ◽  
pp. 149-161 ◽  
Author(s):  
YOUKO SATOW ◽  
CHING KUNG
Keyword(s):  
Voltage Clamp ◽  
Resting Potential ◽  
Ca Channel ◽  
Paramecium Tetraurelia ◽  
Ca Channels ◽  
Action Current ◽  
Inward Current ◽  
Transient Inward Current ◽  
Inorganic Cation ◽  
Inward Currents

Transient inward currents across the membrane of P. tetraurelia are recorded upon step depolarizations with a voltage clamp in solutions where Ca2+ is the only added inorganic cation. It is shown that the current is normally carried by Ca2+ through the Ca-channels which activate and inactivate in time. The transient inward current is dependent on both the size of the depolarizing step and the holding level before the step. Maximum inward current (Imax) occurs when the membrane is first held at the resting level (- 30 mV), then stepped to 0 mV in a solution containing 0.91 mM-Ca2+. The Imax is smaller when the membrane is first held at depolarized level. This is due to the depolarization-sensitive inactivation of the Ca-channels. The Imax is also smaller when the membrane is first held at a hyperpolarized level. This may be explained by the activation of hyperpolarization-sensitive K-channels known to exist in the Paramecium membrane. I max increases with concentration of Ca2+ up to 0.9 mM. Further increase in the Ca2+ concentration does not affect Imax. This apparent saturation at 0.9 mM-Ca2+ may reflect a rate-limiting step of Ca2+ permeation. The increase in Ca2+ concentration shifts the V-Ipeak curve in the direction of less sensitivity. This result is best explained as the effect of bound Ca2+ on the surface potential of the Paramecium membrane. These results provide the first detailed description of the properties of the action current through the Ca-channel in Paramecium. They also define the conditions under which future voltage-clamp studies of wild-type and mutant membranes of P. tetraurelia should be performed, i.e. to maximize the resolution of the Ca-channel activity, the membrane should be held at or near the resting potential and there should be over 0.9 mM-Ca2+ in the test solutions. The behaviour of the Paramecium Ca-channel and small Imax in the presence of K+ are discussed.

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