Isolated bovine ventricular myocytes. Characterization of the action potential

Decreasing expression of α1C calcium L-type channel subunit mRNA in rat ventricular myocytes upon manganese exposure

Manganese is an essential trace element found in many enzymes. As it is the case of many essential trace elements, excessive level of manganese is toxic. It has been proven that excessive manganese could cause heart problems. In order to understand the mechanism of manganese toxicity in the heart, the effects of manganese on isolated rat ventricular myocytes were studied. The L-type calcium channel current was measured by whole-cell patch clamp recording mode. In the electrophysiology experiments, both 50 microM Mn2+ and 100 microM Mn2+ could effectively decrease the channel current amplitude density by 35.7% and 68.2%, respectively. Moreover, Mn2+ shifted the steady-state activation curve toward more positive potential and the steady-state inactivation curve toward more negative potential. Investigation by RT-PCR showed that the mRNA expression of alpha1C/Cav1.2 treated with manganese was decreased depending on its concentration, while the mRNA expression of alpha1D/Cav1.3 was almost unchanged. Fluo-3/AM was utilized for real-time free calcium scanning with laser scanning confocal microscopy (LSCM), and the results showed that Mn2+ could elicit a slow and continuous increase of [Ca2+]i in a concentration-dependent manner. These results have suggested that manganese could interfere with the function of the L-type calcium channel, downregulate the mRNA expression of alpha1C/Cav1.2, and thus causing long-lasting molecular changes of L-type calcium channel which have probably been triggered by overloading of calcium in myocytes.

A Simple Method for Isolation of Cardiomyocytes from Adult Rat Heart

A simple, economic, and sparing method for isolation of cardiomyocytes from adult rat heart is proposed. Ultrastructure of cardiomyocytes from suspension of freshly isolated cells was studied by light and transmission electron microscopy. The isolated cardiomyocytes were viable, had characteristic shape and size, and retained their normal structure.

Effects of terpenoid phenol derivatives on calcium current in canine and human ventricular cardiomyocytes

Concentration-dependent (10-1000 microM) effects of terpenoid phenol derivatives were studied on L-type Ca(2+) current in isolated canine and human ventricular cardiomyocytes using the whole-cell configuration of patch clamp technique. Carvacrol, thymol and eugenol suppressed peak Ca(2+) current at +5 mV, having EC(50) values and Hill coefficients of 98+/-11, 158+/-7 and 187+/-15 microM and 1.42+/-0.05, 2.96+/-0.43 and 1.6+/-0.1, respectively, in canine myocytes. Zingerone displayed a weak effect (estimated EC(50): 2+/-0.37 mM, Hill coefficient: 0.73+/-0.07), while vanillin and guaiacol failed to substantially modify Ca(2+) current up to the concentration of 1 mM. In addition to tonic block, thymol and carvacrol, but not eugenol, evoked marked rate-dependent block at 2 Hz. Carvacrol and eugenol accelerated inactivation of Ca(2+) current and caused leftward shift in the voltage dependence of steady-state inactivation without altering activation kinetics. Carvacrol, but not eugenol, increased the time constant of recovery from inactivation. These effects of carvacrol and eugenol developed rapidly and were largely reversible. In myocytes isolated from undiseased human hearts, the effect of carvacrol was similar to that observed in canine cells. It is concluded that suppression of cardiac Ca(2+) currents by phenol derivatives is influenced by the substituent in the benzene ring, and the blocking effect of these drugs may involve interactions with the inactivation machinery of the channel.

Role of individual ionic current systems in ventricular cells hypothesized by a model study

Individual ion channels or exchangers are described with a common set of equations for both the sinoatrial node pacemaker and ventricular cells. New experimental data are included, such as the new kinetics of the inward rectifier K+ channel, delayed rectifier K+ channel, and sustained inward current. The gating model of Shirokov et al. (J Gen Physiol 102: 1005-1030, 1993) is used for both the fast Na+ and L-type Ca2+ channels. When combined with a contraction model (Negroni and Lascano: J Mol Cell Cardiol 28: 915-929, 1996), the experimental staircase phenomenon of contraction is reconstructed. The modulation of the action potential by varying the external Ca2+ and K+ concentrations is well simulated. The conductance of I(CaL) dominates membrane conductance during the action potential so that an artificial increase of I(to), I(Kr), I(Ks), or I(KATP) magnifies I(CaL) amplitude. Repolarizing current is provided sequentially by I(Ks), I(Kr), and I(K1). Depression of ATP production results in the shortening of action potential through the activation of I(KATP). The ratio of Ca2+ released from SR over Ca2+ entering via I(CaL) (Ca2+ gain = approximately 15) in excitation-contraction coupling well agrees with the experimental data. The model serves as a predictive tool in generating testable hypotheses.

Effects of thymol on calcium and potassium currents in canine and human ventricular cardiomyocytes

1. Concentration-dependent effects of thymol (1 - 1000 microM) was studied on action potential configuration and ionic currents in isolated canine ventricular cardiomyocytes using conventional microelectrode and patch clamp techniques. 2. Low concentration of thymol (10 microM) removed the notch of the action potential, whereas high concentrations (100 microM or higher) caused an additional shortening of action potential duration accompanied by progressive depression of plateau and reduction of V(max). 3. In the canine cells L-type Ca current (I(Ca)) was decreased by thymol in a concentration-dependent manner (EC(50): 158+/-7 microM, Hill coeff.: 2.96+/-0.43). In addition, thymol (50 - 250 microM) accelerated the inactivation of I(Ca), increased the time constant of recovery from inactivation, shifted the steady-state inactivation curve of I(Ca) leftwards, but voltage dependence of activation remained unaltered. Qualitatively similar results were obtained with thymol in ventricular myocytes isolated from healthy human hearts. 4. Thymol displayed concentration-dependent suppressive effects on potassium currents: the transient outward current, I(to) (EC(50): 60.6+/-11.4 microM, Hill coeff.: 1.03+/-0.11), the rapid component of the delayed rectifier, I(Kr) (EC(50): 63.4+/-6.1 microM, Hill coeff.: 1.29+/-0.15), and the slow component of the delayed rectifier, I(Ks) (EC(50): 202+/-11 microM, Hill coeff.: 0.72+/-0.14), however, K channel kinetics were not much altered by thymol. These effects on Ca and K currents developed rapidly (within 0.5 min) and were readily reversible. 5. In conclusion, thymol suppressed cardiac ionic channels in a concentration-dependent manner, however, both drug-sensitivities as well as the mechanism of action seems to be different when blocking calcium and potassium channels.

Two forms of spiral-wave reentry in an ionic model of ischemic ventricular myocardium

It is well known that there is considerable spatial inhomogeneity in the electrical properties of heart muscle, and that the many interventions that increase this initial degree of inhomogeneity all make it easier to induce certain cardiac arrhythmias. We consider here the specific example of myocardial ischemia, which greatly increases the electrical heterogeneity of ventricular tissue, and often triggers life-threatening cardiac arrhythmias such as ventricular tachycardia and ventricular fibrillation. There is growing evidence that spiral-wave activity underlies these reentrant arrhythmias. We thus investigate whether spiral waves might be induced in a realistic model of inhomogeneous ventricular myocardium. We first modify the Luo and Rudy [Circ. Res. 68, 1501-1526 (1991)] ionic model of cardiac ventricular muscle so as to obtain maintained spiral-wave activity in a two-dimensional homogeneous sheet of ventricular muscle. Regional ischemia is simulated by raising the external potassium concentration ([K(+)](o)) from its nominal value of 5.4 mM in a subsection of the sheet, thus creating a localized inhomogeneity. Spiral-wave activity is induced using a pacing protocol in which the pacing frequency is gradually increased. When [K(+)](o) is sufficiently high in the abnormal area (e.g., 20 mM), there is complete block of propagation of the action potential into that area, resulting in a free end or wave break as the activation wave front encounters the abnormal area. As pacing continues, the free end of the activation wave front traveling in the normal area increasingly separates or detaches from the border between normal and abnormal tissue, eventually resulting in the formation of a maintained spiral wave, whose core lies entirely within an area of normal tissue lying outside of the abnormal area ("type I" spiral wave). At lower [K(+)](o) (e.g., 10.5 mM) in the abnormal area, there is no longer complete block of propagation into the abnormal area; instead, there is partial entrance block into the abnormal area, as well as exit block out of that area. In this case, a different kind of spiral wave (transient "type II" spiral wave) can be evoked, whose induction involves retrograde propagation of the action potential through the abnormal area. The number of turns made by the type II spiral wave depends on several factors, including the level of [K(+)](o) within the abnormal area and its physical size. If the pacing protocol is changed by adding two additional stimuli, a type I spiral wave is instead produced at [K(+)](o)=10.5 mM. When pacing is continued beyond this point, apparently aperiodic multiple spiral-wave activity is seen during pacing. We discuss the relevance of our results for arrythmogenesis in both the ischemic and nonischemic heart. (c) 1998 American Institute of Physics.

Perforated patch-clamp recording in cardiac myocytes using cation-selective ionophore gramicidin

Whole cell currents were recorded in single myocytes dissociated from guinea pig ventricles by the patch-clamp technique. The addition of 0.1 mg/ml gramicidin D, a cation-selective ionophore, into the pipette solution induced a gradual spontaneous perforation of the patch membrane under a conventional cell-attached configuration. The access resistance, measured at approximately 12 min after formation of a gigaohm seal, was 9.2 +/- 1.5 M omega (n = 12). The perforated patch membrane exhibited ionic selectivity for various monovalent cations, with a relative order of Cs+ (1.11) > K+ (1.0) > Na+ (0.65) >> tris(hydroxymethyl)aminomethane+ (approximately 0) but was not permeable for Cl-. Under the gramicidin-perforated patch recording configuration, the cells showed the typical electrophysiological properties for ventricular cells reported previously. The intracellular Cl- concentration, estimated from the reversal potential of the catecholamine-induced Cl- current, was 36.3 +/- 2.9 mM (n = 17). We thus conclude that the gramicidin-perforated patch recording mode provides a useful tool for recording the ionic currents while maintaining the intracellular Cl- concentration.

Electrical restitution in diseased human ventricular myocardium

Action potential configuration and electrical restitution were studied in diseased human ventricular muscle by comparing the characteristics of hypertrophic (HYP) and dilated (DIL) human ventricular preparations. Conventional microelectrode techniques were used to evaluate action potentials evoked at increasingly longer diastolic intervals. The steady-state action potential duration (APD90) was significantly longer in DIL than in HYP preparations (393 +/- 5 ms, n = 4 and 296 +/- 11 ms, n = 4, respectively; P < 0.001, mean +/- SEM). In the dilated preparations studied at long diastolic intervals, the initial period of rapid repolarization (phase 1) was absent, and the rate of final repolarization (phase 3) was reduced. Electrical restitution relations in these preparations were fitted as the sum of two exponentials. The time constant of the fast component was significantly longer in DIL than in HYP preparations (242 +/- 9 ms and 121 +/- 4 ms, respectively; P < 0.001). No difference was observed in the time constants for the slow component of restitution in the two groups. Electrical restitution was also studied in single human ventricular myocytes by using patch clamp techniques. The initial 600 ms period of restitution was fitted in these cells to a monoexponential function. The time constant for this period of the restitution relation was significantly longer, while the estimated amplitude of this early rising phase was significantly lower in human cells obtained from DIL hearts than the respective parameters obtained in the healthy canine and guinea pig cells also examined. The observed changes in the restitution kinetics of the dilated human heart are, likely, the consequence of alterations in the ionic currents that underlie the cardiac action potential.

Rb+, Cs+ ions and the inwardly rectifying K+ channels in guinea-pig ventricular cells

Rb+ and Cs+ ion permeability and the effects of these ions from the inside on the inwardly rectifying K+ channel were studied in guinea-pig ventricular cells. A total substitution of either Rb+ or Cs+ for external K+ in the outside-out configuration of the patch-clamp technique abolished the inward current. The outward current carried by K+ was recorded. The unitary amplitude was reduced to about half of the control value with Rb+ but was not changed with Cs+. Internal Rb+ and Cs+, at a concentration of 10-40 mM, reduced the unitary amplitude of the outward current. No substate behaviour was observed. The reversal potential was +18 mV after replacing 105 mM internal K+ with Rb+ at 150 mM external K+. This value gives a permeability ratio of Rb+ to K+ of 0.27. Under a total substitution of Rb+ or Cs+ for internal K+, the outward currents were not measurable. Cs+ induced flickering in the inward current carried by K+. It is thus concluded that Rb+ and Cs+ ions are not measurably permeant at the single-channel level but permit K+ permeation in place of external K+ and that internal Rb+ and Cs+ produce a voltage-dependent block of the channel with fast kinetics.

Voltage-dependent sodium channels in human small-cell lung cancer cells: role in action potentials and inhibition by Lambert-Eaton syndrome IgG

Sodium channels of human small-cell lung cancer (SCLC) cells were examined with whole-cell and single-channel patch clamp methods. In the tumor cells from SCLC cell line NCI-H146, the majority of the voltage-gated Na+ channels are only weakly tetrodotoxin (TTX)-sensitive (Kd = 215 nM). With the membrane potential maintained at -60 to -80 mV, these cells produced all-or-nothing action potentials in response to depolarizing current injection (> 20 pA). Similar all-or-nothing spikes were also observed with anodal break excitation. Removal of external Ca2+ did not affect the action potential production, whereas 5 microM TTX or substitution of Na+ with choline abolished it. Action potentials elicited in the Ca(2+)-free condition were reversibly blocked by 4 mM MnCl2 due to the Mn(2+)-induced inhibition of voltage-dependent sodium currents (INa). Therefore, Na+ channels, not Ca2+ channels, underlie the excitability of SCLC cells. Whole-cell INa was maximal with step-depolarizing stimulations to 0 mV, and reversed at +45.2 mV, in accord with the predicted Nernst equilibrium potential for a Na(+)-selective channel. INa evoked by depolarizing test potentials (-60 to +40 mV) exhibited a transient time course and activation/inactivation kinetics typical of neuronal excitable membranes; the plot of the Hodgkin-Huxley parameters, m infinity and h infinity, also revealed biophysical similarity between SCLC and neuronal Na+ channels. The single channel current amplitude, as measured with the inside-out patch configuration, was 1.0 pA at -20 mV with a slope conductance of 12.1 pS. The autoantibodies implicated in the Lambert-Eaton myasthenic syndrome (LES), which are known to inhibit ICa and INa in bovine adrenal chromaffin cells, also significantly inhibited INa in SCLC cells. These results indicate that (i) action potentials in human SCLC cells result from the regenerative increase in voltage-gated Na+ channel conductance; (ii) fundamental characteristics of SCLC Na+ channels are the same as the classical sodium channels found in a variety of excitable cells; and (iii) in some LES patients, SCLC Na+ channels are an additional target of the pathological IgG present in the patients' sera.

Potassium currents in isolated human atrial and ventricular cardiocytes

The whole-cell configuration of the patch-clamp technique was applied to study and compare ion currents in single ventricular and atrial cardiocytes isolated from human myocardium. In ventricular cardiocytes the K+ inward rectifier current (IK1) was three times larger than in atrial cardiocytes, while its inactivation kinetics were twice as slow when measured at -140 mV. The magnitude of these variables depended on the test potential but was independent of changes in holding potential. A transient outward current (I(to)) was observed in both ventricular and atrial cardiocytes. The amplitude of the inactivating component of Ito was not significantly different in atrial and ventricular cells, but the time course of inactivation was significantly longer in atrial than in ventricular cardiocytes. Steady-state inactivation of Ito in atrial cells was well described by a two-state Boltzmann function having a midpoint potential of -41.4 mV and a slope factor of 6.9 mV-1. No discernible K+ delayed rectifier current (IK) was observed in either cell type. In four of the 12 atrial cells studied, a time dependent inward current was observed at negative test potentials having a 240 +/- 21 ms time constant for activation and an amplitude of 101 +/- 28 pA. This current, which resembled the pacemaker current (I(f)), was not observed in any of the ventricular cells examined.

Gap junctional conductance in ventricular myocyte pairs isolated from postischemic rabbit myocardium

Abnormalities of myocardial gap junction-mediated cell coupling have been implicated in cardiac arrhythmogenesis. The potential role of gap junctional dysfunction in the generation of reperfusion-induced arrhythmias is uncertain. The purpose of this study was to measure the effects of myocardial ischemia and reperfusion on gap junctional conductance (gj) between isolated ventricular myocytes. By using a new experimental model, myocyte pairs were isolated from Langendorff-perfused rabbit hearts 1) after 30 minutes of global normothermic ischemia followed by 30 minutes of reperfusion, 2) after 75 minutes of control perfusion, or 3) immediately after removal of the heart. Myocytes and myocyte pairs were studied using whole-cell recording techniques. Action potential characteristics of cells in all three groups were normal. Despite similar mean gj in all three groups (0.88 +/- 0.27, 1.15 +/- 0.18, and 1.24 +/- 0.25 microS, respectively; p greater than 0.05), the postischemic group was more widely distributed and had a significantly greater proportion of poorly communicating cell pairs than either control group (gj less than 25% of mean in eight of 15 myocyte pairs versus zero of 15 and one of 13, respectively; p less than 0.02). Thus, postischemic myocyte pairs represent a heterogeneous population of electrically coupled cells in which individual deficits in coupling are masked by a normal mean value. In the reperfused intact heart, local disturbances of cell coupling, similarly undetected by gross measures of conduction, could disrupt myocardial conduction and activation on a microscopic scale and thus enhance arrhythmogenicity.

A model of the ventricular cardiac action potential. Depolarization, repolarization, and their interaction

A mathematical model of the membrane action potential of the mammalian ventricular cell is introduced. The model is based, whenever possible, on recent single-cell and single-channel data and incorporates the possibility of changing extracellular potassium concentration [K]o. The fast sodium current, INa, is characterized by fast upstroke velocity (Vmax = 400 V/sec) and slow recovery from inactivation. The time-independent potassium current, IK1, includes a negative-slope phase and displays significant crossover phenomenon as [K]o is varied. The time-dependent potassium current, IK, shows only a minimal degree of crossover. A novel potassium current that activates at plateau potentials is included in the model. The simulated action potential duplicates the experimentally observed effects of changes in [K]o on action potential duration and rest potential. Physiological simulations focus on the interaction between depolarization and repolarization (i.e., premature stimulation). Results demonstrate the importance of the slow recovery of INa in determining the response of the cell. Simulated responses to periodic stimulation include monotonic Wenckebach patterns and alternans at normal [K]o, whereas at low [K]o nonmonotonic Wenckebach periodicities, aperiodic patterns, and enhanced supernormal excitability that results in unstable responses ("chaotic activity") are observed. The results are consistent with recent experimental observations, and the model simulations relate these phenomena to the underlying ionic channel kinetics.

Electrophysiology and ultrastructure of canine subendocardial Purkinje cells isolated from control and 24-hour infarcted hearts

Ventricular arrhythmias that accompany myocardial infarction in dogs may be secondary to the altered electrophysiological properties of the subendocardial Purkinje fibers that survive 24 hours after the coronary occlusion. To better understand the ionic mechanisms that underlie the altered electrical activity of these fibers, we have dispersed, using an enzymatic technique, Purkinje cells from the subendocardium of the infarcted ventricle (IZPCs) and compared their electrical and structural properties to Purkinje cells dispersed from fiber strands (SPCs) and from the subendocardium of the noninfarcted ventricle (NZPCs). Ultrastructural analysis of these cells shows that IZPCs contain an increased number of lipid droplets when compared with the SPCs and NZPCs. In addition, transmembrane action potentials of IZPCs have reduced resting potentials, action potential amplitudes, and upstroke velocity and are increased in duration when compared with either SPCs or NZPCs. Input resistance of IZPCs is increased over that measured in control cells (SPCs and NZPCs). Furthermore, the time course of the process of electrical restitution of action potential duration is altered in IZPCs with long action potentials. Finally, using K+-sensitive microelectrode techniques, we have determined that intracellular free K+ activity (aKi) in IZPCs (93.7 +/- 15 mM) is not significantly different from control aKi measurements (SPC, 106 +/- 13 mM; NZPC, 103 +/- 12 mM). Thus a reduction in aKi does not provide a basis for the reduced resting potentials observed in IZPCs. By studying the relation between the resting potential and log [K+]o we determined that in IZPCs with reduced resting potentials, there is a significant increase in the PNa/PK ratio when compared with control. In summary, to better understand the cellular basis of ventricular arrhythmias postinfarction, we have developed a single cell model that will allow for more rigorous electrophysiological studies of the specific ionic currents that underlie the abnormal electrophysiology.

A study of the electrical characteristics of sodium currents in single ventricular cells of the frog

1. The generation of action potentials elicited from enzymatically dispersed ventricular cells from the frog, Rana catesbeiana, has been shown to be due to the influx of both Na+ and Ca2+. The maximum rate of rise, the amplitude and the duration at 50% repolarization of the action potential were estimated to be 26.4 +/- 5.1 V/s (n = 8), 110 +/- 2.7 mV (n = 8) and 601 +/- 180 ms (n = 8) at 15 degrees C, respectively. 2. Inward Na+ current (INa) was studied in these ventricular cells by the whole-cell patch clamp technique in a medium where Ca2+ current was eliminated by substituting extracellular Mg2+ for Ca2+ and K+ current was suppressed by applying Cs+ intracellularly. All the voltage clamp experiments were carried out at 4 degrees C. 3. INa elicited by single depolarizing steps from a holding potential (VH) of -80 mV had a threshold of -50 mV and was maximal at -20 mV. Peak currents in normal Ringer solution containing 113.5 mM-Na+ were of the order of 0.01-0.02 mA/cm2. Maximum Na+ conductance (gNa) was calculated to be 5.9 mS/cm2. 4. Under normal conditions the reversal potential for INa was determined to be 50 mV, which is close to the value predicted from the Nernst equation. The reversal potential changed by 59 mV per tenfold change in the activity of extracellular Na+ (aNa). 5. The instantaneous relation between INa tail currents and membrane potential is linear, crossing the abscissa at the reversal potential for INa. 6. Reconstructions of INa were made in terms of the parameters of the Hodgkin-Huxley model for the squid axon, using constants obtained from the frog ventricular cells. 7. The falling phase of INa and the development of inactivation measured by the double-pulse method could be well fitted by a single-exponential function. 8. The time course for recovery of INa from inactivation exhibited a single time constant.

Contribution of two types of calcium currents to the pacemaker potentials of rabbit sino-atrial node cells

1. Two types of calcium currents, the transient type and long-lasting type, were examined by both whole-cell and cell-attached patch-clamp modes in single isolated sino-atrial node cells of the rabbit. 2. In the whole-cell clamp mode, in response to a depolarizing pulse to -40 mV from a holding potential of -80 mV, a transient type calcium current with an amplitude of 2.1 +/- 0.7 pA/pF (mean +/- S.D.; n = 15) was recorded. The threshold potential was approximately -50 mV. 3. Nickel (40 microM) and tetramethrin (0.1 microM) blocked the transient type calcium current without appreciable effects on the long-lasting type. Nifedipine and D600 blocked the long-lasting type, but did not affect the transient type. Cadmium (20 microM) and cobalt (2 mM) inhibited both types of calcium currents equally. 4. Both types of calcium currents showed an increased amplitude with increasing extracellular calcium concentration. The values of the Michaelis constant, Km, were 0.95 mM for the transient type and 3.92 mM for the long-lasting type, indicating that these types represent two different classes of channels. 5. In the cell-attached patch-clamp mode, the single-channel conductance of the transient type calcium current was 8.5 pS, by using 100 mM-BaCl2 in the pipette, whereas that of the long-lasting type was 16.0 pS, under the same conditions. Each of these values was similar to those found in other cells, respectively. 6. In the whole-cell clamp mode, the transient type current began to inactivate at -70 mV and was fully inactivated at -40 mV. The steady-state inactivation curve of the transient type current was approximately 50 mV negative to that of the long-lasting type. The overlap of the membrane potential between the activation and inactivation curves was small. The time constant of the inactivation shortened from 20 to 5 ms as the potential became progressively positive over the range from -80 to +30 mV. 7. Isoprenaline (1 microM) increased the amplitude of the long-lasting type Ca2+ current, but was not effective on the transient type, suggesting that the long-lasting type calcium current may be responsible for the positive chronotropic effect of isoprenaline. 8. While recording spontaneous electrical activity of the cell, application of 40 microM-nickel induced bradycardia and this effect was enhanced when the membrane was constantly hyperpolarized.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

On the relationship between V max of slow responses and Ca-current availability in whole-cell clamped guinea pig heart cells

The relationship between Ca current availability and maximum rate of rise (V max) of slow responses was determined in the same single guinea pig ventricular heart cell under voltage and current clamp conditions (whole-cell clamp technique). The results are as follows. (1) Cell capacitance measured in 32 cells from the current response to a fast ramp voltage-clamp pulse (119.6 +/- 4.6 pF, mean +/- SE) or from Vmax values at a holding potential of -50 or -40 mV (118.6 +/- 5.3 pF) are identical. (2) In control conditions ([Ca]o 1.8, [K]o 4 and [Cs]i 140 mM), voltage-dependence of steady-state inactivation of Ca current (ICa) or Vmax are similar up to -35 mV. However, Vmax significantly (P less than 0.005) underestimates ICa availability at more positive potentials. At -30 mV, ICa and Vmax amplitudes represent respectively 35.6 and 22.4% (n = 14) of their maximum value. (3) In the presence of 50 nM isoprenaline, Vmax and the underlying ICa are respectively increased by 79.2 +/- 13.8% and 71.2 +/- 13.8% (n = 15). No statistically significant deviation from linearity is then observed. (4) When Vmax amplitude is expressed as a function of ICa density, an almost linear relationship is observed for Vmax values between 0 and 25 V/s. Vmax is then best described by the equation: Vmax (V/s) = 1.043 ICa (pA/pF) -0.514 (46 cells). (5) We conclude that, under conditions that minimize outward currents, Vmax of slow responses accurately measures ICa amplitude, except when ICa is decreased to less than 40% of its maximum control amplitude (i.e., below 4 pA/pF). At that point, Vmax underestimates ICa.

Tetrodotoxin exerts a large frequency-dependent depression of the maximum rate of rise of action potentials in guinea pig ventricular myocytes

The action potential configuration in guinea pig ventricular myocytes was unaffected by low concentrations (0.3-1 microM) of tetrodotoxin (TTX); high concentrations (10-30 microM) depressed both the overshoot (5-10 mV) and duration (5-10%). Although the control Vmax was unaffected by stimulation rate (0.1-5 Hz), the depression of Vmax by TTX was greatly potentiated at rates above 1 Hz: on dose-response curves, 50% control Vmax occurred at 4.3 microM (5 Hz) versus 22 microM (less than or equal to 1 Hz). The frequency dependent component of the Vmax depression reported here is much larger than the "extra" block of Na channels observed by others in voltage clamp studies on Purkinje strands. This is not a discrepancy; rather it is a consequence of a non-linear relation between Vmax and available Na conductance.

Isolated rat cardiac myocytes as an experimental model to study calcium overload: the effect of calcium-entry blockers

Calcium overload and the effect of a series of calcium-entry blockers were studied in isolated adult cardiac myocytes from the rat challenged with veratrine. The isolation procedure resulted in a high yield of individual rod shaped, calcium tolerant myocytes. After incubation with veratrine, an alkaloid which induces both sodium and calcium influx, 93% of the myocytes became calcium intolerant: the quiescent rod shaped cells vigorously contracted after 30 sec of contact with veratrine and contracture (round cells) ensued within 1 min. Exposure for 30 min to various doses of calcium-entry blockers prior to veratrine addition resulted in the prevention of contracture, the degree of protection depending on the type and the concentration of calcium-entry blocker. Among the different calcium-entry blockers tested, the diarylalkylpiperazines lidoflazine, cinnarizine and flunarizine were protective from the 10(-7) M concentration onwards. Nicardipine was protective at the 10(-6) M and 10(-5) M concentrations, verapamil at 10(-5)M only while other blockers of the "slow channel" type (diltiazem and nifedipine) were not protective in the concentration range tested. This study shows that isolated myocytes represent a valid model for pharmacological investigations. The results with the calcium-entry blockers stress the heterogeneity of the different series of calcium-entry blockers.

Calcium currents of cesium loaded isolated smooth muscle cells (urinary bladder of the guinea pig)

Single smooth muscle cells isolated from the urinary bladder of the guinea-pig were studied at 35 degrees C in a solution composed of 150 mM NaCl, 3.6 mM CaCl2, 1.2 mM MgCl2, 5.4 mM KCl, 20 mM TEA-Cl, 5 mM glucose, 10 mM HEPES/NaOH (pH 7.4). Whole cells were clamped with a single patch electrode. The clamp settled a step from -65 to -5 mV within 260 microseconds, and afterwards the voltage inhomogeneities were less than 2 mV (measured at the cell edge with a second electrode). The calcium inward current iCa was dissected from net currents by blocking potassium outward currents by means of patch electrodes filled with 130 mM CsCl (Klöckner and Isenberg 1985 a). Pyruvate, succinate and oxalacetate in the patch electrode stabilized iCa and prevented its "run down". 140 ms long clamp steps from -65 to -5 mV evoked a net inward current which could be reversibly blocked by 5 mM NiCl2. The "Ni-sensitive" difference current iCa peaked within 2-4 ms to about 1 nA per cell. Afterwards it completely inactivated; the inactivation could be fitted with three exponentials (time constants of 4, 30, and 250 ms, respectively). The half decay time of 16 ms suggests that most of the inactivation resulted from the fast exponential process. The reference current in the presence of Ni was nearly time independent and almost zero; therefore, iCa could be approximated from the net inward current using the zero current as a reference line.(ABSTRACT TRUNCATED AT 250 WORDS)

Electric current flow in cell pairs isolated from adult rat hearts

Cell pairs were isolated from ventricles of adult rat hearts so as to study cell-to-cell coupling. Both cells of each pair were impaled with micro-electrodes connected to balanced bridge circuits. Rectangular current pulses were passed and the resulting voltage deflexions monitored. The data were analysed in terms of a delta configuration of three resistive elements, the resistances of the non-junctional membrane of cell 1 and cell 2 (rm, 1 and rm, 2), and the resistance of the nexal membrane (rn). The nexal membrane resistance was found to be insensitive to voltage gradients across the non-junctional membrane (range examined: -70 to -10 mV) and direction of current flow. The mean value of rn was 2.12 M omega ([K+]o = 12 mM). Taking into account morphological parameters, this corresponds to a specific nexal membrane resistance (Rn) of 0.1 omega cm2. Spontaneous uncoupling in which one cell remained polarized while the other one depolarized was never observed. The current-voltage relationship of the non-junctional membrane was found to be bell-shaped. The specific resistance (Rm) at the resting membrane potential (approximately -50 mV) was 3.2 k omega cm2 ([K+]o = 12 mM). Comparative studies performed on single cells revealed a similar relationship Rm versus Vm. Rm at the resting membrane potential (Vm approximately -50 mV) was 2.5 k omega cm2 ([K+]o = 12 mM). The specific capacitance of the non-junctional membrane (Cm) was determined from experiments on single cells. Cm was found to be independent of Vm (voltage range: -80 to 0 mV). The mean value of Cm was 1.66 microF/cm2 ([K+]o = 12 mM). For comparison, experiments on cell pairs and single cells were also carried out with [K+]o = 4 mM. The values obtained for Rn, Rm and Cm did not deviate significantly from those found with [K+]o = 12 mM.

Effect of imipramine on calcium and potassium currents in isolated bovine ventricular myocytes

Isolated bovine ventricular myocytes were investigated with a two-microelectrode voltage clamp technique. The clamp currents were analyzed in terms of ICa and IK. Possible effects on INa were avoided by superfusing the cells with a Na-free medium. Imipramine (IMI) was applied at a concentration of 3.6 microM. Within the initial 3 min (early phase), IMI reduced peak ICa by 38 +/- 9% but IMI did not change the time constants of inactivation, the voltage dependence of peak ICa or its reversal potential. Therefore, we conclude that IMI reduced calcium conductance. After 10 min of exposure (late phase), IMI can also reduce the reversal potential of ICa. The inward rectifying potassium current (IK1) was transiently enhanced by 15 +/- 8% but later (8-10 min) reduced by 19 +/- 4%. Washout of IMI completely reversed all the effects within 10 min. Reduction of ICa diminished the rate of rise and the overshoot of the slow action potential and can explain the shortening of the AP seen in both Na-free and Na-containing media. Possible clinical implications are discussed.

The effects of the Anemonia sulcata toxin (ATX II) on membrane currents of isolated mammalian myocytes

The effects of Anemonia sulcata toxin (ATX II) on action potentials and membrane currents were studied in single myocytes isolated from guinea-pig or bovine ventricles. Addition of ATX II (2-20 nM) prolonged the action potential duration without a significant change in resting membrane potential. Concentrations of 40 nM-ATX II or more induced after-depolarizations and triggered automaticity. The effects were reversible after washing or upon addition of 60 microM-tetrodotoxin (TTX). 5 mM-Ni did not modify the effects. The single patch-electrode voltage-clamp technique of Hamill, Marty, Neher, Sakmann & Sigworth (1981) was applied to record membrane currents in response to 8.4 S long depolarizations starting from a holding potential of -90 mV. Currents flowing later than 5 ms after the depolarizing step were analysed. The fast events could not be considered because of insufficient voltage homogeneity. After 2 min of exposure to ATX II (20 nM) the changes in net membrane currents were measured. The difference between the currents in the presence of ATX II and during control was defined as the 'ATX-II-induced current' (iATX). After 4 min of wash iATX disappeared. Within 10 S of exposure to 60 microM-TTX, iATX was blocked completely. At potentials positive to -60 mV, iATX was inwardly directed and decayed slowly but incompletely during the 8.4 S long depolarizing pulse. The rate of decay was faster during clamp pulses to more positive potentials. A high amplitude noise was superimposed on the current trace; its amplitude decreased with more positive potentials. We analysed the voltage dependence of iATX with 'isochronous' current-voltage relations. The 0.1 S isochrone of iATX was characterized by a 'threshold' for negative currents at -60 mV, a branch with a negative slope (k = -7 mV, potential of half-maximal activation (V0.5) = -38 mV, bovine cells) leading to a maximum inward current at -20 mV, and an ascending branch which led to an apparent reversal potential (Erev) around +40 mV. The values measured in guinea-pig myocytes were similar though not identical (k = -5.5 mV, V0.5 = -30 mV, maximum of inward current at -5 mV, Erev = +50 mV). Erev shifted to less positive potentials in later isochrones. Holding the membrane at -45 mV prevented the induction of extra current by ATX II. When the holding potential was then changed to -85 mV, iATX developed within some 2 min. Returning the holding potential to -45 mV blocked iATX with a similar slow time course.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation of calcium current and its sensitivity to monovalent cations in dialysed ventricular cells of guinea-pig

The ion selectivity of the Ca2+ channels in single ventricular cells of guinea-pig was studied using a 'giga-ohm seal' patch electrode for voltage clamp and internal dialysis. To isolate the Ca2+ channel current, currents through the Na+ channel and K+ channels were minimized by replacing external Na+ with Tris+ and removing K+ from both sides of the membrane. With 5 mM-ATP and 5 mM-EGTA in the pipette solution, the Ca2+ current was well maintained for more than 30 min in K+- and/or Na+-free external solution. Substitution of Cs+ for intracellular K+ eliminated the region of negative slope conductance in the steady-state current-voltage curve and shifted the zero-current potential or resting potential from -80 to -31 mV. After Cs+ substitution, a large inward current still flowed via inwardly rectifying K+ channels, but was abolished by removing external K+, which resulted in reduction of the resting membrane slope conductance to 1% of the control value. A decaying outward current attributable to the inwardly rectifying K+ channel was observed on depolarization in 5.4 mM-external K+ solution with Cs+-rich internal solution after blocking Ca2+ current. The induction of that current caused an apparent decrease of Ca2+ channel current when the K+-rich internal solution was switched to the Cs+-rich one at an external K+ concentration of 5.4 mM. When inwardly rectifying K+ current was suppressed by exposure to K+-free external solution, replacement of intracellular K+ with Cs+ caused no significant change in the Ca2+ current. With Cs+-rich solution in the electrode, the decaying outward current was responsible for an apparent depression of the Ca2+ current observed when extracellular K+ was increased. When the K+ current was abolished by 0.2 mM-extracellular Ba2+, changes in external K+ concentration did not affect the Ca2+ current, excluding the possibility of a direct inhibitory action of K+ on the Ca2+ channel. A time- and voltage-dependent outward current attributed to Cs+ was observed at potentials above +30 mV in Na+-, K+-free external solution with Cs+-rich internal solution. This current persisted in the presence of 20 mM-intracellular TEA Cl and 5 mM-extracellular 4-aminopyridine. Inorganic Ca2+ channel blockers, such as Co2+ or Cd2+, not only suppressed the inward Ca2+ current but also caused some reduction in outward current. Thus the blocker-sensitive peak current reversed at around +75 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic basis for the antagonism between adenosine and isoproterenol on isolated mammalian ventricular myocytes

We studied the effects of adenosine and isoproterenol on membrane currents of isolated bovine and guinea pig ventricular myocytes with a two-microelectrode voltage clamp technique. Adenosine (50 microM to 0.2 mM) alone had no effect on any of the membrane currents measured, but it antagonized the effects induced by 10 nM isoproterenol. Peak calcium membrane current was augmented by isoproterenol from a control of 4.8 +/- 0.6 to 8.6 +/- 0.8 nA and adenosine reduced it to 5.7 +/- 0.7 nA (mean +/- SEM of six cells). The inactivation time constant was not altered by isoproterenol alone or isoproterenol plus adenosine, and neither was the voltage dependence of peak calcium membrane current. Thus, the changes caused by isoproterenol could be described as an increase in maximal calcium conductance from 0.86 +/- 0.7 to 1.55 +/- 0.04 mS/cm2 and partially antagonized by adenosine to 0.97 +/- 0.04 mS/cm2. Isoproterenol also increased the non-inactivating component of calcium membrane current from 17 +/- 1 to 24 +/- 4%, and adenosine reduced it to 18 +/- 2% (n = 4). The steady state activation and inactivation variables remained unchanged. Consistent with these effects on calcium membrane current, adenosine completely antagonized the isoproterenol-induced increase of the slow action potentials obtained in sodium-free medium. Isoproterenol increased the steady state outward currents at potentials between -90 and -30 mV (i.e., probable iK1). Adenosine alone had no effect on potassium membrane current, but it antagonized the effects of isoproterenol. Slow action potentials in 25 mM potassium were enhanced by isoproterenol, but were only moderately attenuated by adenosine. Accordingly, in 25 mM potassium the isoproterenol-induced changes in membrane currents were not antagonized by adenosine. This lack of inhibition by adenosine of the isoproterenol effects in 25 mM potassium could not be mimicked by 1-minute-long conditioning prepulses to -45 mV. The results indicate that adenosine by itself (absence of isoproterenol) has no effect on maximal calcium conductance, that the isoproterenol-induced increase in cyclic adenosine 3',5'-monophosphate, which leads to an increase in maximal calcium conductance, is antagonized by adenosine, and that such action can account for the ability of adenosine to attenuate the stimulatory effects of isoproterenol.

Single cardiac Purkinje cells: general electrophysiology and voltage-clamp analysis of the pace-maker current

Single Purkinje cells from dog, sheep and cow hearts were isolated by injecting a Ca-free collagenase containing Tyrode solution in the space between the connective tissue sheath and the Purkinje cells. A small proportion of these cells survived the isolation procedure and these cells were used for further investigation. The cells showed electrophysiological properties similar to intact Purkinje fibres as indicated by the following results. Maximum diastolic potentials between -70 and -85 mV and specific membrane resistances of 21-32 k omega cm2 indicated that the single cells were not leaky or hyperpermeable . The action potential showed a rapid upstroke, with a maximum rate of rise, Vmax' between 150 and 750 V/s, and two phases of fast repolarization separated by a plateau phase with a duration of about 200 ms. Each action potential was followed by a spontaneous depolarization with an amplitude between 1 and 10 mV. The upstroke of the action potential could be blocked by tetrodotoxin (TTX) in a dose-dependent manner. The rate of depolarization of the action potential was sensitive to changes in membrane potential; the resulting S-shaped curve showed a half-maximum potential of -65 mV and a steepness of 0.46 mV-1. The duration of the action potential was sensitive to external K concentrations, catecholamines and TTX in a way similar to intact Purkinje fibres. Both application of catecholamines and lowering the external K concentration induced spontaneous activity. The cells were used to study the ionic nature of the pace-maker current under voltage-clamp conditions using the two-micro-electrode technique. This pace-maker current was blocked in a voltage-dependent manner by 1 mM-Cs, and was not affected by 1 mM-Ba. The steady-state activation curve was shifted in the depolarizing direction by application of adrenaline. In contrast to voltage-clamp data obtained on the pace-maker current of intact Purkinje fibres, the pace-maker current in a single cell did not reverse near the presumed equilibrium potential for K ions; no reversal could be seen in the voltage range negative to -50 mV. These observations together with preliminary results on the Na and K dependence of the pace-maker current are strong arguments in favour of the hypothesis that the pace-maker current in cardiac Purkinje fibres is an inward current carried by Na and K ions and activates upon hyperpolarization.

Strontium, nifedipine and 4-aminopyridine modify the time course of the action potential in cells from rat ventricular muscle

Action potentials, initiated by brief depolarizing pulses, were recorded from single cells isolated from rat ventricular muscle. These action potentials showed a rapid upstroke to about +30 mV, followed by two phases of repolarization referred to as the early and late phases of the action potential. Nifedipine (1 microM), which blocks the second inward current (Isi) carried by Ca in these cells, shortened the early phase. Substitution of strontium for calcium in the solution bathing the cells, a procedure which prolongs Isi, prolonged the early phase. 4-Aminopyridine (1 mM), which inhibits transient outward current, prolonged the early phase with either calcium or strontium in the external solution. It is concluded that both Isi and transient outward current contribute to the early phase of the action potential in rat ventricular muscle. It is also suggested that Isi does not directly contribute to the late phase, since the characteristics of the late phase are not compatible with such a role, and the possibility of additional inward current is investigated in the accompanying paper (Mitchell et al., 1984).

Linear electrical properties of isolated cardiac cells

A frequency domain equivalent circuit analysis of isolated ventricular cells indicated the presence of an internal membrane structure which has a total capacitance four- to sixfold larger than the surface membrane. The internal membrane was mainly attributed to the sarcoplasmic reticulum since other morphological studies have shown that its area is many-fold larger than that of the surface membrane. Corresponding estimates from the transverse tubular system indicate an area less than that of the surface; thus this structure is not a likely candidate for the observed internal capacitance. Measurements in hypertonic solutions showed that the access resistance to the internal membrane reversibly increased as the tonicity was elevated. Freeze-fractured electron microscopic studies confirmed that hypertonic solutions increased the volume of transverse tubular system, which thus appears to have little relation to the access resistance. The most probable source of the access resistance is the diadic junction to the sarcoplasmic reticulum, which therefore would electrically couple it to the surface membrane.