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Action potentials and ionic currents were measured by patch-clamp in single right ventricular myocytes isolated enzymatically from rabbits and guinea pigs at room temperature. Based upon the shape of their action potentials, the rabbit myocytes were divided into two groups. In group 1 (n = 15), a prominent phase of initial rapid repolarization (phase 1) occurred following the upstroke of the action potential. Action potentials recorded in the second group (n = 9) did not have a noticeable phase 1. In group 1, voltage-clamp protocols revealed a transient outward current (Ito) activated upon depolarization from a holding potential of -50 mV to potentials positive to -20 mV. This outward current inactivated rapidly (tau = 50 ms) in a voltage-independent manner. The transient outward current was not apparent in group 2. In all the rabbit myocytes, the inward calcium current (ICa) was activated between -30 and -20 mV, with a peak at 0 mV. The inactivation kinetics of the inward calcium current were dependent on the voltage. In 4 out of 11 cells, inactivation was best described by a single exponential relation, and in the remaining 7 cells by a double exponential relation. The recovery of the inward calcium current from inactivation and the restitution of the duration of the action potential showed similar time-courses of 132 +/- 11.6 ms (n = 8) and 163.7 +/- 23.0 ms (n = 6), respectively. In all the rabbit myocytes the relation between current and voltage, established at the end of 400 ms command pulses, displayed a negative slope conductance. In rabbit myocytes, unlike those of guinea pig, pulses as long as 5 s revealed the delayed rectifier outward current to be weak or absent. Our results suggest that two populations of ventricular myocytes exist in the rabbit; neither has a prominent delayed rectifier, at least at room temperature, while one group has a transient outward current and the other does not. The major currents controlling the duration of the action potential at room temperature in myocytes from the right ventricle of the rabbit, therefore, are the inward calcium current and the inward rectifier potassium current (IK1).

1. The frequency-dependent electrophysiological effects of UK-68,798 in concentrations of 1, 3, 10 and 30 nM were examined in isolated cardiac Purkinje fibres of the dog at both a number of constant rates of stimulation and following abrupt changes in pacing cycle length. 2. In all concentrations evaluated, UK-68,798 lengthened action potential duration in a concentration- and rate-dependent manner (e.g., at a cycle length = 500 ms, control APD90 = 234.0 +/- 3.3 ms, while after 10 nM UK-68,798, APD90 = 315.0 +/- 5.9 ms). 3. The duration of action potentials evoked following abrupt changes in pacing rate were also increased in a concentration-dependent manner at all diastolic intervals tested. 4. The fast and slow time constants for restitution of APD were not altered by UK-68,798. However, the amplitude terms for this relation were increased. 5. In addition, the maximum upstroke velocity (Vmax) was not significantly affected by exposure to UK-68,798 at any concentration or diastolic interval. The kinetics for recovery of Vmax were thus unaffected. 6. These findings are similar to those previously reported for recognized class III antiarrhythmic agents (e.g., bretylium, clofilium, and sotalol); however, UK-68,798 was 1,000 to 10,000 times more potent. 7. The combined potency and selectivity of this agent seem to make it an ideal tool for the investigation of cardiac potassium channels believed responsible for controlling the duration of the action potential. 8. This potent and highly selective compound may prove extremely useful in the control of cardiac arrhythmias.

The effects of sotalol, a beta-adrenoceptor blocker and class III antiarrhythmic agent, on transmembrane ionic currents were examined in single rabbit and guinea pig ventricular myocytes using whole-cell voltage-clamp techniques. In neither of these species did 60 microM sotalol appreciably effect the inward rectifier, the transient outward or the inward calcium currents. In addition, sotalol did not elicit a slowly inactivating component of the sodium current as did 1 microgram/ml veratrine. In guinea pig ventricular myocytes, sotalol also significantly depressed the outward delayed rectifier current. An outward delayed rectifier current was not observed in rabbit ventricular myocytes examined at room temperature; and, under these conditions sotalol did not lengthen action potential duration. Sotalol induced lengthening of cardiac action potential duration can, therefore, be explained by depression the outward delayed rectifier current.

1. Conventional microelectrode techniques were used to measure simultaneous changes in membrane potential (Vm) and conductance (Gm) induced by single electrical stimuli in muscles bathed in Cl(-)-free solution containing 40 mM of tetraethylammonium (TEA+). 2. Stimulation induced slow transient depolarizations (slow response) accompanied by increased calcium conductance, while the potassium conductance was first elevated and later reduced. 3. Stepwise elevation of [K+]0 from 2.5 to 5 or 10 mM during the slow response evoked an abrupt repolarization of 42.3 +/- 8.9 mV (n = 4; p less than 0.001), and 24.8 +/- 3.5 +/- mV (n = 5; p less than 0.001), respectively, while Gm was increased to 1.45 +/- 0.25-fold (n = 5; p less than 0.05). Neither the slow response nor K(+)-induced changes in Vm or Gm were sensitive to tetrodotoxin (3 microM), however, nifedipine (10 microM) abolised the slow response. 4. It was concluded that beyond the increase of calcium conductance, the ionic conductance of the inward rectifier K+ channel was reduced during the slow response, which could be restored by the elevation of [K+]0. The results suggest the possible contribution of these mechanisms to the electrical instability of myotonic muscles. Potential therapeutic consequences are discussed.

1. Conventional microelectrode techniques were used to study the effects of antiarrhythmic drugs (quinidine, 2-20 microM; lidocaine, 5-50 microM; verapamil, 2-20 microM), 4-aminopyridine (4-AP, 50-100 microM), and tetrodotoxin (TTX, 1.5-6 nM) on repetitive electrical discharges induced in the muscle membrane in the presence of 1 microM cevadine. 2. Antiarrhythmic drugs and 4-AP produced progressive reduction of the maximum upstroke velocity (V+ max) of the discharges, while the cycle length was prolonged by each drug except 4-AP. 3. The efficacy in depression of V+ max and prolongation of cycle length was not proportional in the case of individual drugs. 4. A simple model of the repetitive activity incorporating drug-effects was presented. The cevadine-modified Na+ channels could be blocked by antiarrhythmic agents, however, the efficacy of these drugs on the normal and cevadine-modified Na+ channels were found to be different.

1. The electrophysiological effects of veratridine, cevadine and aconitine (10(-8)-2 x 10(-4), 2 x 10(-7)-2 x 10(-6) and 2 x 10(-6)-10(-4) mol/l, respectively) were compared on frog muscle membrane using conventional microelectrodes. 2. Veratridine and aconitine were equally effective in depolarizing the resting membrane with the threshold concentration of 5 x 10(-5) mol/l. 3. Volleys of repetitive discharges and slow transient depolarizations were observed when single electrical stimuli were applied in the presence of veratridine (5 x 10(-8)-2 x 10(-5) mol/l), but not aconitine. Volleys with aconitine could be evoked only by repetitive stimulation; however no tendency of repolarization was observed following these volleys. Two orders of magnitude more aconitine than veratridine was required to induce volleys with similar parameters. 4. The effects of cevadine were similar to those of the corresponding concentrations of veratridine. 5. The observed differences between the electrophysiological actions of aconitine and veratrum alkaloids may be explained in part with differences in Na+ channel inactivation produced by these toxins, in addition to differences in their use-dependent behavior.

1. Conventional microelectrode techniques were used to study the effect of quinidine (10 microM), lidocaine (20 microM), and verapamil (3-10 microM) on action potential upstroke (V+ max) in frog skeletal muscle and dog Purkinje fiber. 2. The frequency-dependent nature of V+ max depression induced by these drugs was similar in both preparations, however, quinidine was more potent in skeletal muscle while lidocaine was in Purkinje fibers. 3. In skeletal muscle tetrodotoxin (3 and 15 nM) and low concentrations of antiarrhythmic drugs proportionally reduced the maximum velocity of depolarization and repolarization (V+ max and V- max, respectively), whereas V- max was more depressed than V+ max by high concentrations (50-200 microM) of antiarrhythmics. Decreases in the overshoot potential were proportional to the V+ max block in the case of each drug. 4. These results indicate that therapeutically relevant concentrations of quinidine and lidocaine inhibit skeletal muscle Na+ channels in a use-dependent manner similar to heart, while at higher concentrations the K+ channels may also be blocked. Therapeutic implications of the results are discussed.

The effects of phlorizin and phloretin on the cable properties were investigated in frog sartorius muscle by conventional cable analysis. Actions of phloretin on voltage-dependent ionic conductances were also studied by analysis of the phase plane trajectories. Both drugs evoked a significant decrease in specific membrane resistance (Rm) in chloride-containing Ringer's solution. The linear membrane capacitance increased by about 30%. On the contrary, in the presence of the non-penetrating anion, glutamate, a slight increase in Rm was induced by phlorizin. It is suggested that these drugs may increase the chloride conductance in the muscle membrane. Under the effect of phloretin the resting membrane potential remained unchanged but the amplitude of the action potential was lowered and the rate of repolarization was significantly reduced. The rate of depolarization during the "foot" of the action potential and the conduction velocity calculated from the rate constant of depolarization decreased. The maximum Na conductance was not altered by phloretin but K conductance was reduced. The time constant (tau K) reflecting the kinetic properties of K conductance was increased about seven-fold. It is suggested that great importance may be attributed to the dipole properties of these drugs in the actions presented above.

The effect of 10(-5) mol/l bencyclane on the repetitive electrical activity of muscle membrane was studied with the conventional microelectrode technique. Electrical activity was induced by repetitive stimulation in normal Ringer solution (train) or by a single depolarizing current pulse in the presence of 10(-6) mol/l cevadine (volley). Bencyclane decreased, in a use-dependent manner, the maximum rates of depolarization and repolarization (Vmax+ and Vmax-, resp.) of the action potentials both of the train and the volley. The inhibition of Vmax+ and Vmax- was proportional; however, it was stronger for the volleys than for the trains. The cycle length (mean interspike interval) of the volley was increased by bencyclane; the prolongation was progressive during consecutive cycles. The dissociation of bencyclane from the Na channel was studied by applying trains of different durations with equal pulse numbers. Bencyclane at a higher concentration (5 x 10(-5) mol/l) caused a reversible tonic block: the overshoot potentials, Vmax+ and Vmax- were markedly reduced. The reduction of Vmax- was slightly stronger than that of Vmax+. Slow membrane potential oscillation (SMPO) was evoked by treating the muscle with 10(-4) mol/l of cevadine. The administration of 5 x 10(-6) mol/l bencyclane decreased the frequency of SMPO, while 10(-5) mol/l bencyclane terminated the slow oscillation activity without changing its baseline potential. The present results indicate that bencyclane induces use-dependent inhibition of Na channels in muscle, similarly as do class 1 antiarrhytnmic drugs. Inhibition was observed with both normal and cevadine-modified Na channels.

Applying conventional microelectrode technique, the effect of bencyclane was studied on the maximal rate of rise (Vmax) of the transmembrane action potential in frog skeletal muscle and canine cardiac Purkinje fiber. Bencyclane (10 microM) decreased the Vmax from 333.7 +/- 6.9 V/sec to 302.7 +/- 10.2 V/sec (n = 6, p less than 0.05) in skeletal muscle without changing the resting membrane potential. If repetitive stimulation with different constant cycle lengths was applied, a further, frequency-dependent decrease of Vmax developed in both tissues with similar frequency-dependence. In skeletal muscle bencyclane increased the time of 50% repolarization by 33.3 +/- 2.5% (n = 7, p less than 0.01) and decreased the overshoot potential by 11.3 +/- 0.72 mV (n = 7, p less than 0.01) measured at 250 msec cycle length. In cardiac Purkinje fiber bencyclane shortened the action potential duration (APD90) from 258.3 +/- 15.4 msec to 241.7 +/- 12.1 msec (n = 6, p less than 0.05) without changing the resting membrane potential and action potential amplitude measured at 500 msec cycle length. The comparable size of the Vmax-block at the same cycle lengths observed in skeletal muscle (short APD) and Purkinje fiber (long APD) suggests that the inhibition may by mainly attributed to the open sodium channel population. It was concluded that the antiarrhythmic action of bencyclane, based on the use-dependent blockade of sodium channels, might be an important component of the therapeutic effect of bencyclane.