Active and passive electrical properties of single bullfrog atrial cells

Cytoplasm resistivity of mammalian atrial myocardium determined by dielectrophoresis and impedance methods

Many cardiac arrhythmias are caused by slowed conduction of action potentials, which in turn can be due to an abnormal increase of intracellular myocardial resistance. Intracellular resistivity is a linear sum of that offered by gap junctions between contiguous cells and the cytoplasm of the myocytes themselves. However, the relative contribution of the two components is unclear, especially in atrial myocardium, as there are no precise measurements of cytoplasmic resistivity, R(c). In this study, R(c) was measured in atrial tissue using several methods: a dielectrophoresis technique with isolated cells and impedance measurements with both isolated cells and multicellular preparations. All methods yielded similar values for R(c), with a mean of 138 ± 5 Ω·cm at 23°C, and a Q(10) value of 1.20. This value is about half that of total intracellular resistivity and thus will be a significant determinant of the actual value of action potential conduction velocity. The dielectrophoresis experiments demonstrated the importance of including divalent cations (Ca(2+) and Mg(2+)) in the suspension medium, as their omission reduced cell integrity by lowering membrane resistivity and increasing cytoplasm resistivity. Accurate measurement of R(c) is essential to develop quantitative computational models that determine the key factors contributing to the development of cardiac arrhythmias.

Inhibition of L-type Ca2+ current by C-type natriuretic peptide in bullfrog atrial myocytes: an NPR-C-mediated effect

Single atrial myocytes were isolated from the bullfrog heart and studied under current and voltage clamp conditions to determine the electrophysiological effects of the C-type natriuretic peptide (CNP). CNP (10(-8) M) significantly shortened the action potential and reduced its peak amplitude after the application of isoproteronol (10(-7) M). In voltage clamp studies, CNP inhibited isoproteronol-stimulated L-type Ca2+ current (ICa) without any significant effect on the inward rectifier K+ current. The effects of cANF (10(-8) M), a selective agonist of the natriuretic peptide C receptor (NPR-C), were very similar to those of CNP. Moreover, HS-142-1, an antagonist of the guanylyl cyclase-linked NPR-A and NPR-B receptors did not alter the inhibitory effect of CNP on ICa. Inclusion of cAMP in the recording pipette to stimulate ICa at a point downstream from adenylyl cyclase increased ICa, but this effect was not inhibited by cANF. These results provide the first demonstration that CNP can inhibit ICa after binding to NPR-C, and suggest that this inhibition involves a decrease in adenylyl cyclase activity, which leads to reduced intracellular levels of cAMP.

The sustained inward current and inward rectifier K+ current in pacemaker cells dissociated from rat sinoatrial node

1. Myocytes were dissociated from the sinoatrial (SA) node of rat heart using a new enzymatic dissociation technique. Only a small number of isolated SA node myocytes showed regular rhythmic contractions and spontaneous action potentials, and these were used in the present study. 2. The spontaneous action potential was resistant to TTX, and the action potential parameters were similar to those of rabbit and guinea-pig pacemaker cells. Major time- and voltage-dependent currents were the delayed rectifier K+ current IKr, the L-type Ca2+ current ICa,L and the sodium current INa. The hyperpolarization-activated cation current (If) was recorded from approximately 50 % of the cells with hyperpolarization beyond -90 mV. 3. The instantaneous current jump at the onset of a hyperpolarizing pulse showed inward rectification and was largely blocked by Ba2+. This Ba2+-sensitive current corresponded well to the inward rectifier K+ current (IK1), although it was much smaller in amplitude than in the ventricle. 4. A sustained inward current was activated on depolarization from -80 mV to the voltage range of slow diastolic depolarization. The current was blocked by nicardipine, enlarged by isoprenaline and was insensitive to removal of external Ca2+. These characteristics were similar to the sustained inward current, Ist, previously described in the rabbit and guinea-pig SA node cells. 5. The role of Ist was considered by constructing empirical equations, which were applied to the experimental record of the action potential. It is demonstrated that the voltage-dependent activation of Ist constitutes a positive feedback loop with the depolarization of the membrane.

A general approach to modeling conduction and concentration dynamics in excitable cells of concentric cylindrical geometry

This paper discusses mathematical approaches for modeling the propagation of the action potential and ion concentration dynamics in a general class of excitable cells and cell assemblies of concentric cylindrical geometry. Examples include myelinated and unmyelinated axons, single strands of interconnected cardiac cells and outer hair cells. A key feature in some of the cells is the presence of a small working volume such as the periaxonal space between the myelin sheath and the axon in the myelinated axon and the extracisternal space between the plasma membrane and the subsurface cisterna of the outer hair cell. Proper treatment of these cell types requires a modeling approach which can readily address these anatomical properties and the non-uniform biophysical properties of the concentric membranes and the ionic composition of the volumes between the membranes. An electrodiffusion approach is first developed in which the Nernst-Planck equation is used to characterize axial ion fluxes. It is then demonstrated that this "full" model can be stepwise reduced, eventually becoming equivalent to the standard cable equation formulation. This is done in a manner that permits direct comparisons between the full and simplified models by running simulations using a single parameter set. An intermediate approach where the contributions of the axial currents to ion concentration changes and the effect of varying ion concentrations on solution conductivities are ignored is derived and is found adequate in many cases. Two application examples are given: a "cardiac strand" model, for which the intermediate formulation is shown sufficient and a model of the myelinated axon, for which the full electrodiffusion formulation is clearly necessary. The latter finding is due to spatial inhomogeneities in the anatomy and distribution of ion channels and transporters in the myelinated axon and the restricted periaxonal space between the myelin sheath and the axon.

Transmembrane ICa contributes to rate-dependent changes of action potentials in human ventricular myocytes

The mechanism of action potential abbreviation caused by increasing rate in human ventricular myocytes is unknown. The present study was designed to determine the potential role of Ca2+ current (ICa) in the rate-dependent changes in action potential duration (APD) in human ventricular cells. Myocytes isolated from the right ventricle of explanted human hearts were studied at 36 degreesC with whole cell voltage and current-clamp techniques. APD at 90% repolarization decreased by 36 +/- 4% when frequency increased from 0.5 to 2 Hz. Equimolar substitution of Mg2+ for Ca2+ significantly decreased rate-dependent changes in APD (to 6 +/- 3%, P < 0.01). Peak ICa was decreased by 34 +/- 3% from 0.5 to 2 Hz (P < 0.01), and ICa had recovery time constants of 65 +/- 12 and 683 +/- 39 ms at -80 mV. Action potential clamp demonstrated a decreasing contribution of ICa during the action potential as rate increased. The rate-dependent slow component of the delayed rectifier K+ current (IKs) was not observed in four cells with an increase in frequency from 0.5 to 3.3 Hz, perhaps because the IKs is so small that the increase at a high rate could not be seen. These results suggest that reduction of Ca2+ influx during the action potential accounts for most of the rate-dependent abbreviation of human ventricular APD.

L-type Ca2+ current and excitation-contraction coupling in single atrial myocytes from rainbow trout

We have examined the contribution of L-type Ca2+ current (ICa) to the activation of contraction in trout atrial myocytes under basal and phosphorylating conditions. The average myocyte length was 197 +/- 14 micrometer, width was 5.5 +/- 0.2 micrometer, and cell capacitance was 36.2 +/- 2.2 pF. With 25 microM EGTA in the patch pipette and a stimulation frequency of 0.125 Hz, ICa was 2.6 +/- 0.4 pA/pF and it carried a total charge of 0.10 +/- 0.01 pC/pF, giving rise to a contraction of 15.2 +/- 2.8% of the resting cell length. With a cell volume of 2.4 +/- 0.3 pl, the charge carried by ICa corresponded to 14.7 +/- 2.2 micromol Ca2+/l nonmitochondrial cell volume (microM). This can account for only 30-40% of the Ca2+ binding to the myofilaments during a contraction. Increasing the stimulation frequency from 0.25 to 2 Hz decreased ICa amplitude and charge by 66 +/- 5 and 80 +/- 3%, respectively. Elevating the pipette EGTA concentration from 25 microM to 5 mM increased ICa amplitude and charge by approximately 290%. Both isoproterenol and cAMP increased ICa by approximately 230%. The total charge carried by the isoproterenol- or cAMP-stimulated current was increased by 170%. We conclude that the use of high-EGTA concentration may overestimate the total Ca2+ carried by ICa under physiological conditions. Furthermore, the results suggest that, in contrast to previous reports from other lower vertebrates, Ca2+ flux through L-type Ca2+ channels alone is not sufficient to fully activate contraction in trout atrial myocytes at room temperature.

Longitudinal distribution of Na+ and Ca2+ channels and beta-adrenoceptors on the sarcolemmal membrane of frog cardiomyocytes

1. The distribution of L-type Ca2+ and tetrodotoxin-sensitive Na+ channels and of beta-adrenergic receptors was examined in frog ventricular myocytes using the whole-cell patch-clamp technique and a double capillary for extracellular microperfusion. 2. Rod-shaped cells (250-300 microns long) were sealed at both ends to two patch-clamp pipettes and positioned transversally at different positions between the mouths of two microcapillaries separated by a thin wall. A combination of nifedipine (1 microM) and tetrodotoxin (0.3 microM) (blocking solution) was added to one capillary in order to inhibit macroscopic Ca2+ and Na+ currents (Ica and INa, respectively) in the part of the cell exposed to this capillary. 3. Moving the cell in 10-20 microns steps from the control capillary to the capillary containing the blocking solution induced step decreases in Ica and INa amplitudes. Complete block of both currents occurred when the entire cell was exposed to the blocking solution. 4. Each step decrease in current was due to the loss of activity of the functional Ca2+ and Na+ channels present in the slice of sarcolemmal membrane newly exposed to the blocking solution. These step current changes allowed longitudinal mapping of current density for Ca2+ and Na+ channels on the sarcolemmal membrane. 5. Addition of a submaximal concentration of isoprenaline (10 nM) to the control capillary induced a local increase in Ica which enabled examination of the distribution of functional beta-adrenergic receptors as well. 6. Our results demonstrate that Ca2+ and Na+ channels and beta-adrenergic receptors are equally and essentially uniformly distributed on the sarcolemmal of frog ventricular myocytes.

Active and passive forces of isolated myofibrils from cardiac and fast skeletal muscle of the frog

1. Force measurements in isolated myofibrils (15 degrees C; sarcomere length, 2.10 microns) were used in this study to determine whether sarcomeric proteins are responsible for the large differences in the amounts of active and passive tension of cardiac versus skeletal muscle. Single myofibrils and bundles of two to four myofibrils were prepared from glycerinated tibialis anterior and sartorius muscles of the frog. Skinned frog atrial myocytes were used as a model for cardiac myofibrils. 2. Electron microscope analysis of the preparations showed that: (i) frog atrial myocytes contained a small and variable number of individual myofibrils (from 1 to 7); (ii) the mean cross-sectional area and mean number of myosin filaments of individual cardiac myofibrils did not differ significantly from those of single skeletal myofibrils; and (iii) the total myofibril cross-sectional area of atrial myocytes was on average comparable to that of bundles of two to four skeletal myofibrils. 3. In maximally activated skeletal preparations, values of active force ranged from 0.45 +/- 0.03 microN for the single myofibrils (mean +/- S.E.M.; n = 16) to 1.44 +/- 0.24 microN for the bundles of two to four myofibrils (n = 9). Maximum active force values of forty-five cardiac myocytes averaged 1.47 +/- 0.10 microN and exhibited a non-continuous distribution with peaks at intervals of about 0.5 microN. The results suggest that variation in active force among cardiac preparations mainly reflects variability in the number of myofibrils inside the myocytes and that individual cardiac myofibrils develop the same average amount of force as single skeletal myofibrils. 4. The mean sarcomere length-resting force relation of atrial myocytes could be superimposed on that of bundles of two to four skeletal myofibrils. This suggests that, for any given amount of strain, individual cardiac and skeletal sarcomeres bear essentially the same passive force. 5. The length-passive tension data of all preparations could be fitted by an exponential equation. Equation parameters obtained for both types of myofibrils were in reasonable agreement with those reported for larger preparations of frog skeletal muscle but were very different from those estimated for multicellular frog atrial preparations. It is concluded that myofibrils are the major determinant of resting tension in skeletal muscle; structures other than the myofibrils are responsible for the high passive stiffness of frog cardiac muscle.

Trichokonin VI, a new Ca2+ channel agonist in bullfrog cardiac myocytes

The effects of trichokonin VI (= gliodeliquescin A), a peptaibol isolated from the culture broth of Trichoderma koningii Oudemans, on L-type Ca2+ channel currents in single bullfrog atrial cells were investigated. Our results showed that trichokonin VI is a new potent agonist of L-type Ca2+ channels in cardiac membranes.

Force responses to rapid length changes in single intact cells from frog heart

1. Force transients in response to step perturbations in length were recorded in intact atrial cells from frog heart at various temperatures (6-15 degrees C). Length changes of various sizes and in either direction, complete in 0.5 ms, were applied to single myocytes near slack length (initial sarcomere length 2.1-2.2 microns) just before the peak of an isometric twitch. The frequency response of the force transducers used was 2-4 kHz in Ringer solution. 2. An early quick force recovery phase was clearly observed after the elastic force response to the length step and before the start of much slower recovery processes. The quick recovery phase became progressively faster with larger shortening steps and was almost as fast as that originally described in intact frog skeletal muscle fibres (rate constants above 1000 s-1 in large releases at 10 degrees C). 3. The force-extension relation determined at the end of the length change (T1 curve) indicates that an instantaneous shortening of 0.5-0.6% of the initial cell length (L0) brings the force to zero. The force--extension relation determined at the end of the quick recovery phase (T2 curve) showed that the early recovery leads to an almost complete restoration of the original force with small stretches and releases (up to 0.3% L0) and that it becomes negligible in shortening steps of about 1.4% L0. 4. The results suggest that the mechanical properties of attached cross-bridges and the rate of transitions between attached cross-bridge states are approximately the same in frog atrial cells and fast skeletal muscle fibres.(ABSTRACT TRUNCATED AT 250 WORDS)

Sustained depolarization-induced outward current in human atrial myocytes. Evidence for a novel delayed rectifier K+ current similar to Kv1.5 cloned channel currents

Depolarization of human atrial myocytes activates a transient outward current that rapidly inactivates, leaving a sustained outward current after continued depolarization. To evaluate the ionic mechanism underlying this sustained current (Isus), we applied whole-cell voltage-clamp techniques to single myocytes isolated from right atrial specimens obtained from patients undergoing coronary bypass surgery. The magnitude of Isus was constant for up to 10 seconds at +30 mV and was unaffected by 40 mmol/L tetraethylammonium, 100 nmol/L dendrotoxin, 1 mmol/L Ba2+, 0.1 mumol/L atropine, or removal of Cl- in the superfusate. Isus could be distinguished from the 4-aminopyridine (4AP)-sensitive transient outward current (Ito1) by differences in voltage-dependent inactivation (1000-millisecond prepulse to -20 mV reduced Ito1 by 91.7 +/- 0.1% [mean +/- SEM], P < .001, versus 9.4 +/- 0.4% reduction of Isus) and 4AP sensitivity (IC50 for block of Ito1, 1.96 mmol/L; for Isus, 49 mumol/L). Isus activation had a voltage threshold near -30 mV, a half-activation voltage of -4.3 mV, and a slope factor of 8.0 mV. Isus was not inactivated by 1000-millisecond prepulses but was reduced by 16 +/- 8% (P < .05) at a holding potential of -20 mV relative to values at a holding potential of -80 mV. Isus activated very rapidly, with time constants (tau) at 25 degrees C ranging from 18.2 +/- 1.8 to 2.1 +/- 0.2 milliseconds at -10 to +50 mV, two orders of magnitude faster than previously described kinetics of the rapid component of the delayed rectifier K+ current. At 16 degrees C, Isus activation was greatly slowed (tau at +10 mV, 46.7 +/- 4.1 milliseconds; tau at 25 degrees C, 7.1 +/- 0.8 milliseconds; P < .01), and the envelope of tails test was satisfied. The reversal potential of Isus tail currents changed linearly with log [K+]o (slope, 55.3 +/- 2.9 mV per decade), and the fully activated current-voltage relation showed substantial outward rectification. Selective inhibition of Isus with 50 mumol/L 4AP increased human atrial action potential duration by 66 +/- 11% (P < .01). In conclusion, Isus in human atrial myocytes is due to a very rapidly activating delayed rectifier K+ current, which shows limited slow inactivation, is insensitive to tetraethylammonium, Ba2+, and dendrotoxin, and is highly sensitive to 4AP. These properties resemble the characteristics of channels encoded by the Kv1.5 group of cardiac cDNAs and may represent a physiologically significant manifestation of such channels in human atrium.

Simulations of passive properties and action potential conduction in an idealized bullfrog atrial trabeculum

This study investigates the properties of a distributed parameter model of an idealized trabeculum of cardiac muscle surrounded by a resistive-capacitive trabecular sheath. A mathematical approach is developed that permits the direct solution for the absolute potential in each medium [i.e., the intracellular (Vi), interstitial (Ve), and external (Vo) potentials), as opposed to obtaining solutions for the transmembrane potential V (where V identical to Vi-Ve). The mathematical description of the underlying individual cell is based upon quantitative whole-cell voltage-clamp measurements in bullfrog atrial myocytes. "Reduced" or "simplified" cell membrane models that lack the complete complement of transmembrane currents are compared with regard to their accuracy in representing the root, upstroke, and plateau regions of the propagated action potential in the complete model. The results show that a reduced cell membrane model must contain the sodium current INa, calcium current ICa, and background-rectifying K+ current IK1. A cell membrane model that contains a linear background K+ current IL instead of IK1 results in much poorer approximation to the upstroke, plateau, and conduction velocities of an action potential. The effects of varying the resistive-capacitive parameters of the trabecular sheath on both the passive properties (the time and space constants and the input resistance) and conduction parameters (time and space constants of the foot and conduction velocity of the action potential) of the trabeculum are also investigated. These simulations show that electrical activity within the trabeculum is much more sensitive to variations in the resistive component than in the capacitive component of the sheath. The trabecular sheath reduces the extracellular resistance seen by the cell by shunting current away from highly resistive interstitial medium into the volume conductor medium, which is of low resistance, and thereby increases conduction velocity. Finally, the addition of the cholinergic neurotransmitter acetylcholine to the extracellular medium reduces both the space constant of the trabeculum and the conduction velocity of propagated electrical activity.

Delayed rectifier outward current and repolarization in human atrial myocytes

Previous work has suggested that the primary time-dependent repolarizing current in human atrium is the transient outward current (Ito), but interventions known to alter the magnitude of the delayed rectifier current (IK) affect atrial electrophysiology and arrhythmias in humans. To explore the potential role of IK in human atrial tissue, we used the whole-cell configuration of the patch-clamp technique to record action potentials and ionic currents in isolated myocytes from human atrium. A delayed outward current was present in the majority of myocytes, activating with a time constant ranging from 348 +/- 61 msec (mean +/- SEM) at -20 mV to 129 +/- 25 msec at +60 mV. The reversal potential of tail currents was linearly related to log [K+]o with a slope of 55 mV per decade, and fully activated tail currents showed inward rectification. The potassium selectivity, kinetics, and voltage dependence were similar to those reported for IK in other cardiac preparations. In cells with both Ito and IK, IK greatly exceeded both components of Ito (Ito1 and Ito2) within 50 msec of a voltage step from -70 to +20 mV. Based on the relative magnitude of Ito and IK, three types of cells could be distinguished: type 1 (58% [73/126] of the cells) displayed a large Ito together with a clear IK, type 2 (13% [17/126] of the cells) displayed only IK, and type 3 (29% [36/126] of the cells) was characterized by a prominent Ito and negligible IK. Consistent differences in action potential morphology were observed, with type 2 cells having a higher plateau and steeper phase 3 slope and type 3 cells showing a triangular action potential and lesser phase 3 slope compared with type 1 cells. We conclude that IK is present in a majority of human atrial myocytes and may play a significant role in their repolarization and that previously observed variability in human atrial action potential morphology may be partially due to differences in the relative magnitude of time-dependent outward currents.

A force transducer and a length-ramp generator for mechanical investigations of frog-heart myocytes

An apparatus for studying the mechanics of isolated frog heart myocytes is described. The cells are held horizontal in a through of Ringer solution by means of two suction micropipettes. Myocyte force is measured with an opto-electronic system recording the deflection of the tip of one micropipette, which acts as a cantilever force probe. The force probes are selected for compliance according to the force a myocyte is expected to develop in a given condition, so as to limit myocyte shortening during force development to no more than 1% of the slack cellular length (l0). The other micropipette, which is stiff relative to the forces measured, is mounted on an electromagnetic-loudspeaker motor by which controlled-velocity length changes, of preset size and in either direction, are imposed on myocytes. The force transducer has a sensitivity of 5-10 mV/nN, with a frequency response of 700-900 Hz in Ringer solution and a resolution of 0.5-1 nN. The motor with a suction micropipette can complete controlled-velocity length ramps within 1.5-2.0 ms, across a range of +/- 100 microns at a resolution of 8.0 nm. These values correspond, for frog-heart myocytes 200 microns and 400 microns long, to 25%-50% l0 and 0.002%-0.004% l0 respectively.

The stimulus interval-tension relation in enzymatically isolated single myocytes of the frog heart

1. Apparatus for recording the small tensions developed by electrically stimulated single intact myocytes of frog heart is described. A laser-light optoelectronic transducer was used. The compliance of the force probes was 10-20 nm/nN, with a frequency response of 600-900 Hz in Ringer solution. The myocyte shortening during an ordinary twitch contraction was no greater than 1% of the slack length. The overall sensitivity of the transducer system was 5-10 mV/nN, with a total noise of 0.5-1 nN peak to peak. The experiments were performed at 20-23 degrees C on either atrial or ventricular myocytes at 2.15-2.2 microns sarcomere length, in 1 mM-Ca2+ Ringer solution. 2. Isoprenaline (5 microM), increases in external Ca2+ concentration ([Ca2+]o), and shortening of stimulus interval potentiated the myocyte twitch tension. The dependence of twitch characteristics on these inotropic interventions for all the atrial and ventricular myocytes tested was comparable to that of multicellular preparations under similar experimental conditions. This implies that the enzymatic isolation procedure had not altered the physiological properties of the myocytes. 3. The stimulus interval-tension relation for premature twitches of atrial and ventricular myocytes showed (i) a very steep rising phase in the region of intervals just longer than 0.52 and 0.66 s (the duration of the mechanical refractoriness in atrial or ventricular cells), (ii) a peak, at intervals of 0.7-0.8 s, where the twitch tension was strongly potentiated compared to that of the controls, and (iii) as the stimulus interval was further increased, a progressive return to the control level. The stimulus interval-tension relation for steady-state conditions exhibited similar characteristics. 4. The degree of tension potentiation by isoprenaline was greater in the controls than in the earliest test twitches. The result was that the stimulus interval-tension relations for isoprenaline-treated myocytes showed a gentler rise and a lower peak than for untreated cells. 5. The stimulus interval-tension relation of the heart is a basic property of its cells. It is related to changes in the activation level. The higher the activation level reached in control twitches, the lower the stimulus interval-dependent potentiation capability.

Conduction in bullfrog atrial strands: simulations of the role of disc and extracellular resistance

A number of fundamental properties of intercellular conduction in simulated cylindrical strands of cardiac tissue are examined. The paper is based on recent biophysical information describing the transmembrane ionic currents in bullfrog atrial cells as well as anatomical data on the structures (gap junctions) responsible for the coupling between cells in that tissue. A mathematical model of the single bullfrog atrial cell based on suction microelectrode single-cell voltage clamp data is employed, as well as a modified version of the well-known model of Heppner and Plonsey, to characterized the resistive connections between adjacent cells in a cardiac strand. In addition, the simulated cellular strand is assumed to be encased in a cylindrical, resistive endothelial sheath, thus forming an idealized atrial trabeculum; the trabeculum is immersed in an extensive volume conductor. It is possible to simulate both uniform and discontinuous conduction in this atrial strand model by appropriately changing the resistance of the intercalated discs that occur at cell boundaries. The conduction velocity achieved in the normal or control case is within the range of conduction velocities that have been measured for bullfrog atrial trabeculae using optical methods. Extracellular resistance is shown to have a significant effect on both conduction velocity and the critical value of disc resistance at which discontinuous conduction first occurs. Since the atrial cell model employed in this study is based on experimental data and can accurately simulate the atrial action potential, the transmembrane ionic currents generated by the model are capable of providing detailed information concerning the mechanisms of intercellular current spread, particularly in the region of the intercalated disc.

The effects of discrete gap junction coupling on propagation in myocardium

A modified cable theory for a bi-domain model of myocardium that incorporates the effect of gap junctions as discrete objects coupling cardiac cells is derived. The theory is shown to be in agreement with a number of experiments that cannot be explained using standard continuous cable theory, and resolves some apparent contradictions on failure of propagation in two-dimensional anisotropic tissue. In addition, some as yet untested predictions of the theory are mentioned.

Calcium currents in isolated rabbit coronary arterial smooth muscle myocytes

1. Calcium inward currents were recorded from relaxed enzymatically isolated smooth muscle cells from the rabbit epicardial left descending coronary artery using a single-pipette voltage-clamp technique. Outward K+ currents were blocked with CsCl-tetraethylammonium-filled pipette solutions. 2. Relaxed coronary smooth muscle cells had a maximum diameter of 8.6 +/- 0.6 microns and a cell length of 96.7 +/- 3.3 microns when bathed in 2.5 mM [Ca2+]o. The average resting membrane potential at room temperature was -32 +/- 10 mV. The mean cell capacitance was 18.5 +/- 1.7 pF and the input resistance was 3.79 +/- 0.58 G omega. 3. Depolarizing voltage-clamp steps from a holding potential of -80 mV elicited a single time- and voltage-dependent inward current which was dependent upon extracellular [Ca2+]. In 2.5 mM [Ca2+]o, the inward current was activated at a potential of -40 mV and peaked at +10 mV. This current was inhibited by 0.5 mM-CdCl2 and 1 microM-nifedipine and was enhanced with 1 microM-Bay K 8644. No detectable low-threshold, rapidly inactivating T-type calcium current was observed. 4. The apparent reversal potential of this inward current in 2.5 mM [Ca2+]o was +70 mV and shifted by 33.0 mV per tenfold increase in [Ca2+]o. This channel was also more permeable to barium and strontium ions than to calcium ions. 5. Single calcium channel recordings with 110 mM-Ba2+ as the charge carrier revealed a mean slope conductance of 20.7 +/- 0.8 pS. 6. This calcium current (ICa) exhibited a strong voltage-dependent inactivation process. However, the steady-state inactivation curve (f infinity) displayed a slight nonmonotonic, U-shaped dependence upon membrane potential. The potential at which half of the channels were inactivated was -27.9 mV with a slope factor of 6.9 mV. The steady-state activation curve (d infinity) was also well-described by a Boltzmann distribution with a half-activation potential at -4.4 mV and a slope factor of -63 mV. ICa was fully activated at approximately +20 mV. 7. The rate of inactivation was dependent upon the species of ion carrying the current. Both Sr2+ and Ba2+ decreased the rate as well as the degree of inactivation. The tau f (fitted time constant of inactivation) curve displayed a U-shaped relationship in 2.5 mM [Ca2+]o. The reactivation process was voltage dependent and could be described by a single exponential. 8. The current amplitude and the inactivation kinetics were temperature dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

Augmentation of cardiac calcium current by flash photolysis of intracellular caged-Ca2+ molecules

The entry of calcium ions into cells through voltage-activated Ca2+ channels in the plasma membrane triggers many important cellular processes. The activity of these channels is regulated by several hormones and neurotransmitters, as well as intracellular messengers such as Ca2+ itself (for examples, see refs 1-9). In cardiac muscle, myoplasmic Ca2+ has been proposed to potentiate Ca2+ influx, although a direct effect of Ca2+ on these channels has not yet been demonstrated. Photosensitive 'caged-Ca2+' molecules such as nitr-5, however, provide powerful tools for investigating possible regulatory roles of Ca2+ on the functioning of Ca2+ channels. Because its affinity for Ca2+ is reduced by irradiation, nitr-5 can be loaded into cells and induced to release Ca2+ with a flash of light. By using this technique we found that the elevation of intracellular Ca2+ concentration directly augmented Ca2+-channel currents in isolated cardiac muscle cells from both frog and guinea pig. The time course of the current potentiation was similar to that seen with beta-adrenergic stimulation. Thus Ca2+ may work through a similar pathway, involving phosphorylation of a regulatory Ca2+-channel protein. This mechanism is probably important for the accumulation of Ca2+ and the amplification of the contractile response in cardiac muscle, and may have a role in other excitable cells.

Primary cell culture and morphological characterization of ventricular myocytes from the adult newt, Notophthalmus viridescens

Previous work has demonstrated that adult newt cardiac myocytes possess a proliferative ability in response to an experimentally induced injury, in vivo. This study describes an in vitro model in which the proliferative events of the adult cardiac myocyte may be studied. Ventricles were minced and then enzymatically dissociated in a Ca++- and MG++-free salt solution containing 0.5% trypsin and 625 U/ml of CLS II collagenase for 8 to 10 hours at 25 degrees C. Enzyme digests were preplated and then cultured on bovine corneal endothelial-derived basement membrane "carpets" in either serum-free or serum-supplemented modified Leibovitz's medium for up to 30 days. Light and transmission electron microscopic characterization demonstrated that a majority of the myocytes underwent an initial period of disorganization characterized by a "rounding up" of the cell and a loss of myofibrillar organization. Once the myocytes had attached to the culture substratum they began to spread out, underwent a reassembly of their contractile elements, resumed spontaneous contractions, and demonstrated ultrastructural evidence of protein synthesis. Mitosis was observed in several myocytes 8 to 15 days following isolation. In 15-day serum-supplemented and serum-free cultures, 6.5% +/- 0.9% and 8.1% +/- 1.4% of the myocytes were binucleated, respectively. These results demonstrate that adult newt ventricular myocytes can be successfully placed into primary culture and are capable of undergoing mitosis. This work may be considered as a foundation for future investigations which will focus on the mechanisms which control cardiac myocyte proliferation.

A simple device for rapidly exchanging solution surrounding a single cardiac cell

This study describes the design and various physiological applications of a simple device to rapidly change the solution surrounding a single intact cardiac cell. It consists of a short length of double-barreled glass tubing (theta-tubing) attached to a miniature solenoid. A cell is positioned in one of two parallel streams of solution that simultaneously flow from each barrel. Rapid solution switching is achieved by activation of the solenoid that directs the adjacent stream over the cell, changing the bulk solution within 7 ms. Approximately 150 ms were required to change the solution at the membrane surface of guinea pig ventricular cells, judging from potassium-induced changes in resting membrane potential. This delayed response was probably due to, in part, restricted diffusion in the transverse tubular system. The switching speed of this device makes it possible to change extracellular solutions during action potentials and voltage-clamp pulses.

Effects of halothane on membrane potentials and membrane ionic currents in single bullfrog atrial cells

Halothane exerts negative inotropic and negative chronotropic actions on the isolated heart in experimental animals. In order to assess directly the actions of halothane in myocardium, we studied the effects of halothane on membrane potentials and transmembrane ionic currents in single isolated frog atrial cells obtained by the enzymatic dissociation method. The results show: (a) that the action potential is prolonged and its plateau phase and overshoot are depressed, but the resting potential remains unchanged; (b) that there is a significant inhibition of a time- and voltage-dependent outward K+ current and a slow inward Ca2+ current, with a slight decrease of a fast inward Na+ current following halothane (1.0-4.0%) application; and that halothane has no effect on another K+ current, time-independent current.

Selective block of calcium current by lanthanum in single bullfrog atrial cells

A single suction microelectrode voltage-clamp technique was used to study the actions of lanthanum ions (La3+) on ionic currents in single cells isolated from bullfrog right atrium. La3+, added as LaCl3, blocked the "slow" inward Ca2+ current (ICa) in a dose-dependent fashion; 10(-5) M produced complete inhibition. This effect was best fitted by a dose-response curve that was calculated assuming 1:1 binding of La3+ to a site having a dissociation constant of 7.5 x 10(-7) M. La3+ block was reversed (to 90% of control ICa) following washout and, in the presence of 10(-5) M La3+, was antagonized by raising the Ca2+ concentration from 2.5 to 7.5 mM (ICa recovered to 56% of the control). However, the latter effect took approximately 1 h to develop. Concentrations of La3+ that reduced ICa by 12-67%, 0.1-1.5 x 10(-6) M, had no measurable effect upon the voltage dependence of steady state ICa inactivation, which suggest that at these concentrations there are no significant surface-charge effects of La3+ on this gating mechanism. Three additional findings indicate that doses of La3+ that blocked ICa failed to produce nonspecific effects: (a) 10(-5) M La3+ had no measurable effect on the time-independent inwardly rectifying current, IK1; (b) the same concentration had no effect on the kinetics, amplitude, or voltage dependence of a time- and voltage-dependent K+ current, IK; and (c) 10(-4) M La3+ did not alter the size of the tetrodotoxin-sensitive inward Na+ current, INa, or the voltage dependence of its steady state inactivation. Higher concentrations (0.5-1.0 mM) reduced both IK1 and IK, and shifted the steady state activation curve for IK toward more positive potentials, presumably by reducing the external surface potential. Our results suggest that at a concentration of less than or equal to 10(-5) M, La3+ inhibits ICa selectively by direct blockade of Ca channels rather than by altering the external surface potential. At higher concentrations, La3+ exhibits nonspecific effects, including neutralization of negative external surface charge and inhibition of other time- and voltage-dependent ionic currents.

Bupivacaine is an effective potassium channel blocker in heart

The local anesthetic agent bupivacaine increases action potential duration in isolated frog atrial myocytes, and blocks two potassium conductances, IK and IK1. The effective concentrations, particularly for IK, are similar to those which depress the sodium conductance. Potassium channel block may thus contribute to bupivacaine's reported cardiotoxicity.

A whole-cell patch clamp technique which minimizes cell dialysis

A variant of the whole-cell patch clamp technique is described which allows measurement of whole-cell ionic currents in small cells while minimizing cell dialysis with the pipette solution. The technique involves the application of negative pressure to the inside of small (less than 1 micron) tip diameter pipettes placed on the cell surface to achieve high resistance seals and membrane rupture. The technique has been used successfully in a variety of different types of cells to study membrane currents carried by Ca and K, currents generated by exchange carriers as well as electrical coupling between cells. Overall, the technique seems well suited for the study of ionic currents in small cells, and provides an alternative to conventional patch clamping techniques which necessitate intracellular dialysis.

Two ATP-activated conductances in bullfrog atrial cells

Currents activated by extracellular ATP were studied in single voltage-clamped bullfrog atrial cells. Rapid application of ATP elicited currents carried through two different conductance pathways: a rapidly desensitizing conductance reversing near -10 mV, and a maintained, inwardly rectifying conductance reversing near -85 mV. ATP activated the desensitizing component of current with a K 1/2 of approximately 50 microM and the maintained component with a K 1/2 of approximately 10 microM. Both types of current were activated by ATP but not by adenosine, AMP, or ADP. The desensitizing current was selectively inhibited by alpha, beta-methylene ATP, and the maintained, inwardly rectifying current was selectively suppressed by extracellular Cs. The desensitizing component of current was greatly reduced when extracellular Na was replaced by N-methylglucamine, but was slightly augmented when Na was replaced by Cs. GTP, ITP, and UTP were all ineffective in activating the desensitizing current, and of a variety of ATP analogues, only ATP-gamma-S was effective. Addition of EGTA or BAPTA to the intracellular solution did not obviously affect the desensitizing current. Fluctuation analysis of currents through the desensitizing conductance suggested that current is carried through ionic channels with a small (less than pS) unitary conductance.

Effect of membrane potential changes on the calcium transient in single rat cardiac muscle cells

The mechanism that links membrane potential changes to the release of calcium from internal stores to cause contraction of cardiac cells is unclear. By using the calcium indicator fura-2 under voltage-clamp conditions, changes in intracellular calcium could be monitored in single rat ventricular cells while controlling membrane potential. The voltage dependence of the depolarization-induced increase in intracellular calcium was not the same as that of the calcium current (Isi), which suggests that only a small fraction of Isi is required to trigger calcium release from the sarcoplasmic reticulum. In addition, sarcoplasmic reticulum calcium release may be partly regulated by membrane potential, since repolarization could terminate the rise in intracellular calcium. Thus, changes in the action potential will have immediate effects on the time course of the calcium transient beyond those associated with its effects on Isi.

Currents through ionic channels in multicellular cardiac tissue and single heart cells

Ionic channels are elementary excitable elements in the cell membranes of heart and other tissues. They produce and transduce electrical signals. After decades of trouble with quantitative interpretation of voltage-clamp data from multicellular heart tissue, due to its morphological complexness and methodological limitations, cardiac electrophysiologists have developed new techniques for better control of membrane potential and of the ionic and metabolic environment on both sides of the plasma membrane, by the use of single heart cells. Direct recordings of the behavior of single ionic channels have become possible by using the patch-clamp technique, which was developed simultaneously. Biochemists have made excellent progress in purifying and characterizing ionic channel proteins, and there has been initial success in reconstituting some partially purified channels into lipid bilayers, where their function can be studied.

Passive properties and membrane currents of canine ventricular myocytes

The membrane potential and membrane currents of single canine ventricular myocytes were studied using either single microelectrodes or suction pipettes. The myocytes displayed passive membrane properties and an action potential configuration similar to those described for multicellular dog ventricular tissue. As for other cardiac cells, in canine ventricular myocytes: (a) an inward rectifier current plays an important role in determining the resting membrane potential and repolarization rate; (b) a tetrodotoxin-sensitive Na current helps maintain the action potential plateau; and (c) the Ca current has fast kinetics and a large amplitude. Unexpected findings were the following: (a) in approximately half of the myocytes, there is a transient outward current composed of two components, one blocked by 4-aminopyridine and the other by Mn or caffeine; (b) there is clearly a time-dependent outward current (delayed rectifier current) that contributes to repolarization; and (c) the relationship of maximum upstroke velocity of phase 0 to membrane potential is more positive and steeper than that observed in cardiac tissues from Purkinje fibers.

Sodium current in single cells from bullfrog atrium: voltage dependence and ion transfer properties

1. Whole-cell and patch-clamp techniques (Hamill, Marty, Neher, Sakmann & Sigworth, 1981) have been used to make quantitative measurements of the transient inward sodium current (INa) in single cells from bullfrog atrium. This preparation is particularly suitable for the study of INa: (i) the current density is relatively low, (ii) the cells lack a transverse tubule system, (iii) isolated myocytes can be maintained at reduced temperatures (approximately 8-12 degrees C); therefore kinetics can be studied quantitatively. 2. INa was pharmacologically and kinetically isolated from other transmembrane currents by blocking ICa with CdCl2 (0.2-0.5 mM) or LaCl3 (5 x 10(-6) M), and by using only relatively short voltage-clamp depolarizations which did not activate IK (the delayed rectifier). 3. The voltage dependence of INa in bullfrog atrium is similar to that in amphibian node of Ranvier or fast skeletal muscle. The threshold for activation is approximately -50 mV. The peak of the INa vs. membrane potential relation is near -5 to -10 mV. The reversal potential in 'normal' (115 mM-Na+) Ringer solution is +59.0 mV (S.D. +/- 3.4, n = 10). Reduction of external Na+ concentration to one-third of normal resulted in an approximately -27 mV shift of the reversal potential, close to that expected for a highly Na+-selective conductance. 4. Steady-state inactivation of INa (h infinity), measured with a conventional two-pulse voltage-clamp protocol, spanned the membrane potential range from -90 to -50 mV. The potential dependence of h infinity was well described by a single Boltzmann function with half-inactivation at -71 mV and maximum slope of 6.0 mV. 5. Steady-state activation of INa (m infinity) was determined from fits of INa records to a Hodgkin-Huxley model. The potential dependence of m infinity was fitted to a Boltzmann function with half-activation at -33 mV and maximum slope of 9.5 mV. Thus at temperatures around 10 degrees C there was very little overlap of the m infinity and h infinity curves, and only very small steady-state 'window' currents are predicted. 6. The activation time constant, tau m, had a 'bell-shaped' dependence on membrane potential. The peak value of tau m was about 4.2 ms, at a membrane potential of -35 mV (9 degrees C). 7. The time course of inactivation of INa was consistently better described by the sum of two exponentials than by one exponential.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolated muscle cells as a physiological model

Summary of a symposium presented by the American Physiological Society (Cell and General Physiology Section and Muscle Group) at the 70th Annual Meeting of the Federation of American Societies for Experimental Biology, St. Louis, Missouri, April 15, 1986, chaired by M. Lieberman and F. Fay. This symposium reflects a growing interest in seeking new technologies to study the basic physiological and biophysical properties of cardiac, smooth, and skeletal muscle cells. Recognizing that technical and analytical problems associated with multicellular preparations limit the physiological significance of many experiments, investigators have increasingly focused on efforts to isolate single, functional embryonic, and adult muscle cells. Progress in obtaining physiologically relevant preparations has been both rapid and significant even though problems regarding cell purification and viability are not fully resolved. The symposium draws attention to a broad, though incomplete, range of studies using isolated or cultured muscle cells. Based on the following reports, investigators should be convinced that a variety of experiments can be designed with preparations of isolated cells and those in tissue culture to resolve questions about fundamental physiological properties of muscle cells.

Cellular and subcellular heterogeneity of [Ca2+]i in single heart cells revealed by fura-2

Digital imaging of calcium indicator signals (fura-2 fluorescence) from single cardiac cells has revealed different subcellular patterns of cytoplasmic calcium ion concentration ([Ca2+]i) that are associated with different types of cellular appearance and behavior. In any population of enzymatically isolated rat heart cells, there are mechanically quiescent cells in which [Ca2+]i is spatially uniform, constant over time, and relatively low; spontaneously contracting cells, which have an increased [Ca2+]i, but in which the spatial uniformity of [Ca2+]i is interrupted periodically by spontaneous propagating waves of high [Ca2+]i; and cells that are hypercontracted (rounded up) and that have higher levels of [Ca2+]i than the other two types. The observed cellular and subcellular heterogeneity of [Ca2+]i in isolated cells indicates that experiments performed on suspensions of cells should be interpreted with caution. The spontaneous [Ca2+]i fluctuations previously observed without spatial resolution in multicellular preparations may actually be inhomogeneous at the subcellular level.

Isolation of cardiac myocytes from the adult newt, Notophthalmus viridescens. an electron microscopic and quantitative light microscopic analysis

This report describes a technique which permits a high yield of viable adult cardiac myocytes from the adult newt using enzymatic separation techniques at low temperature and high enzyme concentrations. Observations by light microscopy showed the isolated myocytes to have a distinctively slender morphology which consisted of a variable number of arm-like appendages radiating from the center of cells which were predominantly mononucleated. Atrial myocytes were typically observed to have two to three arm-like appendages while ventricular myocytes typically had three to six appendages. The majority of myocytes displayed normal fine structure when examined by transmission electron microscopy. Computerized image analysis revealed that atrial cells were significantly greater in cell length (192.9 +/- 53.4 microns) and in nuclear length (25 +/- 5.3 microns) and perimeter (59.2 +/- 10.7 microns) than were ventricular cells (162.8 +/- 39 microns, 23.6 +/- 5.1 microns and 57.4 +/- 11.1 microns, respectively), while cell widths and areas were greater in ventricular cells (16.5 +/- 4.7 microns and 1839.8 +/- 585.0 microns, respectively) than in atrial cells (13.2 +/- 3.1 microns and 1520.3 +/- 527.6 microns, respectively). Comparison of these data with previous descriptions of isolated amphibian and mammalian cardiac myocytes emphasizes species-related differences.

Isolation and characterization of single myocardial cells from the quail, Coturnix coturnix japonica

1. The enzymatic cell isolation technique was applied to the bird heart resulting in myocytes of which 10-50% maintained their spindle-shaped morphology, excluded the vital dye, Evans blue and tolerated physiological concentration of Ca2+ ions. 2. The length of spindle-shaped myocytes was on average 289 +/- 7 microns, and the maximum width was 10.2 +/- 0.3 microns. The mean length of the sarcomeres was 2.18 +/- 0.03 microns. 3. In electron micrographs the fine structure of the spindle-shaped myocytes looked normal--regular sarcomeric organization with clear A and I bands, mitochondria with tightly located cristae and well-developed sarcoplasmic reticulum (SR). 4. Most (80%) of the spindle-shaped myocytes were quiescent in physiological calcium concentration and practically all of them could be induced to twitch by electric field stimulation. Some beat spontaneously showing mostly slowly-propagating (135 +/- 6 microns/sec at 20 degrees C) contraction waves, so-called phasic contractions. Sometimes spontaneous twitch-type contractions could also be seen.

Direct control of contraction force of single frog atrial cells by extracellular ions

We describe a method by which the ionic surround of an isolated frog heart cell can be changed within a small fraction of a contraction cycle while continuously measuring contraction force. With this method, we have investigated the effect on force development of changing the extracellular concentrations of Ca [( Ca]o) and Na [( Na]o) in the period between electrically driven contractions and during the rising phase of a contraction. Raising or lowering either [Ca]o or [Na]o more than 300 ms prior to a stimulus caused peak force of the next contraction to be changed 100% of the way to the steady-state value characteristic of the new ionic concentrations. Similar maneuvers at later times relative to the stimulus caused progressively smaller changes. Lowering [Ca]o from 2 to 1 mM or raising [Na]o from 78 to 110 mM 100 ms after stimulation brought twitch force 35 and 67% of the way to the new steady states, respectively. We conclude that extracellular Ca is the source of activator Ca in these cells and that extracellular Na plays a role in regulation of the intracellular Ca concentration early in the contraction cycle.

Membrane electrical properties of vesicular Na-Ca exchange inhibitors in single atrial myocytes

Na-loading single frog atrial cells produce changes in membrane currents that are similar to the creep currents originally observed in Na-loaded cardiac Purkinje fibers. Exposure to the Na ionophore, monensin, was used to induce creep currents in isolated atrial cells. The sensitivity of myocardial creep currents to three compounds that have been shown to be inhibitors of Na-Ca exchange flux activity in isolated sarcolemmal vesicles was assessed. Dodecylamine, quinacrine, and the amiloride analog, 3',4'-dichlorobenzamil block creep currents at concentrations well below those required to block Na-dependent Ca uptake in sarcolemmal vesicles. The estimated Ki's for inhibition of myocardial creep currents were 3 microM for dodecylamin, 10 micron for quinacrine, and 4 microM for 3',4'-dichlorobenzamil. The sensitivity of creep currents to these compounds is consistent with the hypothesis that creep currents may represent the electrogenic activity of a Na-Ca exchange carrier. In an additional series of experiments, the relative specificity of these compounds was tested by examining their effects on myocardial membrane channels. Both dodecylamine and 3',4'-dichlorobenzamil were found to inhibit myocardial Ca and K currents over the same range of concentrations in which block of exchange activity occurs. These results seriously question the use of these exchange carrier inhibitors as selective experimental probes for defining the role of Na-Ca exchange in various physiological processes.

Voltage-tension relations in single frog atrial cardiac cells

Voltage-clamp experiments were performed on isolated single frog (Rana catesbeiana or Rana pipiens) atrial cells to determine the voltage-contraction relations of the single cardiac cell. The contractile responses of the single cell associated with long duration (3 second) depolarizing steps consisted of a rise to peak (phasic) followed by a decay to a sustained contraction (tonic). These phasic-tonic type contractile responses could be obtained under conditions where membrane potential was well controlled along the entire length of the cell. Thus, the data obtained on the single cell indicate that the phasic-tonic contractile response is the characteristic contractile response of frog atrial tissue. The voltage dependence of the extent of relaxation to the tonic component following the peak of the contraction was affected dramatically by the intracellular sodium concentration. This result indicates that both the relaxation following peak contraction as well as the tonic contraction are related to calcium control via the sodium-calcium exchanger. The data also indicate that calcium entry via the inward calcium current is required for the contractile response to have a phasic component. These data indicate that calcium entry via the inward calcium current followed by the sodium-calcium exchanger first reducing and then maintaining the intracellular calcium level produces the characteristic phasic-tonic contractile response.

K+, Na+, and Cl- activities in ventricular myocytes isolated from rabbit heart

Intracellular K+, Na+, and Cl- activities (aiK, aiNa and aiCl) were measured in ventricular myocytes enzymatically isolated from adult rabbit heart. The activities in normal Tyrode solution containing 2.5 mM Ca2+ were the following (in mM): aiK = 100.0 +/- 3.5 (n = 9); aiNa = 8.4 +/- 1.5 (n = 6); and aiCl = 17.9 +/- 1.5 (n = 11) (mean +/- SE). Membrane potential was -81.6 +/- 0.7 mV (n = 26). These values were determined after correction for changes of junction and tip potential at the reference electrode, estimated to be 4.9 +/- 0.6 mV (n = 7) for 0.15 M KCl-filled electrodes; and intracellular interference detected by the Cl- ion-selective electrode, 11.2 +/- 0.6 mM (n = 4). Extended-tip shunting was avoided by fabricating Na+ ion-selective microelectrodes from aluminosilicate rather than borosilicate glass. These results show that isolated cardiac cells can maintain normal intracellular ion activities. Diffusion of electrolyte from the reference electrode can rapidly alter the intracellular milieu, however. After 10 min of impalement with 0.15 M KCl-filled microelectrodes (resistance approximately equal to 25 M omega), aiK increased by 8.7 +/- 2.0 mM and aiCl by 10.3 +/- 3.1 mM. In contrast, aiNa did not significantly change during the double impalement.

Sodium currents in single cardiac Purkinje cells

The sodium (Na) channel is the fundamental unit of excitability in heart muscle. This channel has been very difficult to study in detail, because the major experimental tool, the voltage clamp, has been difficult to use in multicellular tissue. In the absence of more direct studies in the heart, it has been assumed that the sodium channel in the heart was the same as that in nerve tissue, where it could be studied quantitatively. However, the sodium channel is not likely to be the same as in nerve, because it responds differently to local anesthetics and to other drugs such as tetrodotoxin. It is essential to learn the details of the cardiac sodium channel, because it is the membrane process that underlies many lethal cardiac arrhythmias, and it is the molecular site of action of the most effective antiarrhythmic drugs. Single cardiac Purkinje cells were dialyzed at room temperature through a large bore pipette, and their Na+ currents were studied under voltage clamp control. The peak currents were 0.5 to 1.0 mA/cm2, assuming a 1 mu farad/cm2 membrane. Peak currents near 0 mV were achieved in less than 1 ms. The decay of the Na+ current did not correspond to a single exponential process. This result and the observation that recovery from inactivation occurred with a latency are inconsistent with the original Hodgkin-Huxley model, but they qualitatively fit a model with two sequential inactivated states or a model with two kinetically different types of Na+ channels. The steady state inactivation curve shifted in the negative direction after initiation of intracellular dialysis, stabilizing with a half-availability voltage of -115 mV.

Comparison of steady-state electrophysiological properties of isolated cells from bullfrog atrium and sinus venosus

Single electrode whole cell voltage-clamp experiments and frequency domain analyses have been used to study and compare the K+ currents in enzymatically dispersed single cells from the atrium and the sinus venosus (pacemaker region) of the bullfrog heart. Admittance measurements made near the "resting' or zero-current potential yield data from which the equivalent circuit of each cell type may be obtained. Data from both atrial and pacemaker cells are well-fitted by a model consisting only of parallel resistance-capacitative elements, as predicted from their micro-anatomy. Neither of these amphibian cardiac cells contain a transverse tubule system (TT) and both have very little sarcoplasmic reticulum (SR). These results complement and extend two earlier investigations: (i) Moore, Schmid and Isenberg (J. Membrane Biol. 81:29-40, 1984) have reported that in guinea pig ventricle cells (which do contain an internal membrane system consisting of transverse tubules and a substantial SR) the SR may be electrically coupled to the sarcolemma; (ii) Shibata and Giles (Biophys. J. 45:136a, 1984) have shown that although bullfrog atrial cells have an inwardly rectifying background K+ current, IK1, pacemaker cells from the immediately adjacent sinus venosus do not. Data from admittance measurements also provide evidence that a TTX-insensitive inward Ca2+ current is activated in the pacemaker range of potentials.

Isolation and characterization of single myocardial cells from the perch, Perca fluviatilis

In applying the enzymatic cell isolation technique to the fish heart about 40% of the dispersed myocytes maintained their spindle-shaped morphology, and about half of them tolerated physiological concentration of Ca2+ and excluded the vital dye, Evans blue. The length of spindle-shaped myocytes was on average 133 +/- 3 micron and the maximum width was 4.2 +/- 0.1 micron. The mean length of the sarcomeres was 2.1 +/- 0.1 micron. The sizes of the myocytes did not vary significantly with the weights of the fish. Electron microscopic examinations showed typical fish myocardial cell structure; absence of transverse tubule system, a sparse network of sarcoplasmic reticulum and from a few up to eight or more myofibrils. The cells were mononuclear. Most of the Ca2+-tolerant myocytes were quiescent, but the contraction in them could be induced by electric field stimulation. Both the spontaneous and electrically triggered contractions were of twitch type. The slowly propagating contraction waves, so-called phasic contractions common in isolated mammalian cardiac myocytes, could not be seen at all.

Voltage clamp of bull-frog cardiac pace-maker cells: a quantitative analysis of potassium currents

Spontaneously active single cells have been obtained from the sinus venosus region of the bull-frog, Rana catesbeiana, using an enzymic dispersion procedure involving serial applications of trypsin, collagenase and elastase in nominally 0 Ca2+ Ringer solution. These cells have normal action potentials and fire spontaneously at a rate very similar to the intact sinus venosus. A single suction micro-electrode technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981; Hume & Giles, 1983) has been used to record the spontaneous diastolic depolarizations or pace-maker activity as well as the regenerative action potentials in these cells. This electrophysiological activity is completely insensitive to tetrodotoxin (TTX; 3 X 10(-6) M) and is very similar to that recorded from an in vitro sinus venosus preparation. The present experiments were aimed at identifying the transmembrane potassium currents, and analysing their role(s) in the development of the pace-maker potential and the repolarization of the action potential. Depolarizing voltage-clamp steps from the normal maximum diastolic potential (-75 mV) elicit a time- and voltage-dependent activation of an outward current. The reversal potential of this current in normal Ringer solution [( K+]0 2.5 mM) is near -95 mV; and it shifts by 51 mV per tenfold increase in [K+]0, which strongly suggests that this current is carried by K+. We therefore labelled it IK. The reversal potential of IK did not shift in the positive direction following very long (20 s) depolarizing clamp steps to +20 mV, indicating that 'extracellular' accumulation of [K+]0 does not produce any significant artifacts. The fully activated instantaneous current-voltage (I-V) relationship for IK is approximately linear over the range of potentials -130 to -30 mV. Thus, the ion transfer mechanism of IK may be described as a simple ohmic conductance in this range of potentials. Positive relative to -30 mV, however, the I-V exhibits significant inward rectification. A Hodgkin-Huxley analysis of the kinetics of IK, including a demonstration that the envelope of tails quantitatively matches the time course of the onset of IK during a prolonged depolarizing clamp step has been completed. The steady-state activation variable (n infinity) of IK spans the voltage range approximately -40 to +10 mV. It is well-fitted by a Boltzmann distribution function with half-activation at -20 mV. The time course of decay of IK is a single exponential. However, the activation or onset of IK shows clear sigmoidicity in the range of potentials from the activation threshold (-40 mV) to 0 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

A transient outward current in isolated cells from the crista terminalis of rabbit heart

Voltage-clamp experiments were carried out with the objective of identifying and characterizing the time- and voltage-dependent properties of a transient outward current recorded in single myocytes from the crista terminalis region of the rabbit heart. A collagenase enzymic dispersion procedure similar to that described by Desilets & Horackova (1982) was used to obtain these viable individual myocytes. Transmembrane ionic currents were recorded using a single micro-electrode voltage-clamp technique. In experiments aimed at studying a tetrodotoxin-resistant transient inward current, (ICa); a transient outward current was consistently recorded following blockade of ICa with Cd2+ (5 X 10(-4) M). The time and voltage dependence of the activation and inactivation of this current were measured. Its steady-state inactivation curve spans the voltage range -70 to -10 mV, and it is activated between -20 and +10 mV. The reversal potential of this transient outward current is approximately -75 mV in [K+]O 5 mM, suggesting that it is carried mainly by K+. This transient outward current can be inhibited completely by external application of 4-aminopyridine (4-AP, 3 mM). The time- and voltage-dependent properties, the reversal potential, and the sensitivity to 4-AP of this transient outward current are all very similar to those of a transient outward current first identified in molluscan neurones. Hence, we have labelled it, IA. Selective inhibition of IA and knowledge of its voltage- and time-dependent properties yield specific predictions concerning its role in the action potential of isolated crista terminalis cells. Consistent with these predictions, a decrease in stimulus rate is found to decrease the duration of the action potential and vice versa; and application of effective doses of 4-AP results in a substantial lengthening of the action potential. These results are discussed in terms of the possible physiological role of IA in subsidiary or follower pace-maker tissue, and the anatomical and physiological heterogeneity of the sino-atrial node region of the rabbit heart.

Ionic currents that generate the spontaneous diastolic depolarization in individual cardiac pacemaker cells

An enzymatic dispersion procedure has been developed to obtain viable, spontaneously active single myocytes from cardiac pacemaker tissue: the bullfrog (Rana catesbeiana) sinus venosus. Recordings of time- and voltage-dependent Ca2+ and K+ currents have been made by using a single suction-microelectrode technique. The results show that two time- and voltage-dependent currents interact to modulate the slope of the pacemaker potential. These are: (i) the decay of a delayed rectifier K+ current and (ii) the activation of a Ca2+ current. In addition, the data strongly suggest that cardiac pacemaker tissue does not have an inwardly rectifying background K+ current.

Uncoupling of cardiac muscarinic and beta-adrenergic receptors from ion channels by a guanine nucleotide analogue

Guanine nucleotide binding proteins, interchangeably called N or G proteins, seem to be the primary signal-transducing components of various agonist-induced cell membrane functions. In the heart, G proteins have been implicated in beta-adrenergic modulation of the slow inward Ca2+ current. We have investigated the role of G proteins in muscarinic activation of an inwardly rectifying, acetylcholine (ACh)-induced K+ current (IACh), and beta-adrenergic activation of an (isoprenaline)-induced Ca2+ current (Isi). Here we report that intracellular application of the non-hydrolysable GTP analogue 5'-guanylylimidodiphosphate (GppNHp) brought about an agonist-induced, antagonist-resistant, persistent activation of IACh and Isi. This functional uncoupling of channel from receptor suggests that the muscarinic receptor and the IACh channel are separate molecular structures. Membrane conductance responses to sequential activation of muscarinic and beta-adrenergic receptors demonstrate that in contrast to the muscarinic inhibition of Isi, muscarinic stimulation of IACh is mediated by a G protein via a pathway that does not involve adenylate cyclase. Taken together, the results support the notion that agonist is required to induce GppNHp binding and/or activation of the G proteins. Once triggered by agonist, the control system remains maximally activated, thereby transforming the cell so that it no longer responds to subsequent homologous receptor-mediated signals.