A cellular mechanism for the generation of ventricular arrhythmias by acetylstrophanthidin

Transmembrane activity was recorded from canine false tendons bathed with Tyrode's solution at 37°C. Stimulus patterns provided a 3-second pause after every ten beats. Acetylstrophanthidin was infused at concentrations up to 2x10 -7 g/ml. One or two transient depolarizations (TDs) followed the last driven response of each series. The appearance of TDs was associated with depression of normal phase-4 depolarization. The peak of the earliest TD (TD-1) occurred at an interval approximately equal to the basic cycle length. The later TD (TD-2) occurred at about twice the basic cycle length. Coupling intervals were determined primarily by the last cycle length. The amplitude of TD-1 was maximal when the basic cycle length was 600 msec, but TD-2 continued to increase as the basic cycle length diminished further. The amplitude of both TDs increased with the number of beats in the train. Either or both could reach threshold and induce single extrasystoles or trains of extrasystoles. TDs could be induced to reach threshold after each driven response, resulting in sustained bigeminal rhythms with fixed coupling. Possibly TDs provide a mechanism for various clinically observed arrhythmias induced by cardiac glycosides.

Transient depolarizations induced by acetylstrophanthidin in specialized tissue of dog atrium and ventricle

Isolated preparations of atrial specialized conduction fibers ("plateau" fibers) qualitatively resembled ventricular Purkinje fibers (false tendons) in their response to acetylstrophanthidin. Acetylstrophanthidin in concentrations of 1-3 x10 -7 g/ml caused coupled, frequency-dependent transient depolarizations (TDs) in both types of fiber. In free-running strands of plateau fibers the TDs could reach threshold and generate coupled action potentials, but TDs and automatic responses did not occur in ordinary atrial muscle fibers. TDs were suppressed by elevation of the external potassium concentration. Automatic activity and TDs in atrial plateau fibers were abolished by acetylcholine. Automatic activity was also sometimes suppressed in false tendons by acetylcholine, but TDs and related coupled responses were not influenced.

Conduction block associated with transient depolarizations induced by acetylstrophanthidin in isolated canine Purkinje fibers

Transient depolarizations (TDs) appeared in canine Purkinje tissue but not in cardiac muscle during exposure to 1-2 x 10 -7 g/ml of acetylstrophanthidin. At low levels of toxicity, the TDs were associated with a decrease in threshold and an increase in automaticity. As intoxication progressed, the TDs coincided with transient periods of increased threshold and conduction block. Propagation from muscle to Purkinje tissue was blocked during, but not before or after, the crest of the TDs. As a result of the interaction between temporal and spatial (electrotonic) factors, various patterns of block, including unidirectional block, were observed. At moderate frequencies 2:1 block was demonstrated, but at faster rates 1:1 transmission occurred. At a later stage of toxicity, repetitive stimulation led to progressive depolarization in Purkinje tissue followed by a period of block during which slow repolarization eventually restored excitability and conductivity. In the whole heart, conduction block could permit reentry and could be responsible for the ultimate transition from ventricular tachycardia to ventricular fibrillation.

The contribution of membrane hyperpolarization to adaptation and conduction block in sensory neurones of the leech

The factors underlying sensory adaptation and conduction block have been studied in cutaneous mechanoreceptor neurones of the leech. A touch-sensitive cell was activated by applying mechanical or electrical stimuli to its receptive field on the skin. Impulses were recorded extracellularly from its axons and intracellularly from its cell body, which is situated within the C.N.S.1. Activation of the touch cell by mechanical stimuli revealed two distinct types of adaptation with characteristically different time courses. Sustained pressure on the skin caused a brief burst of impulses at the onset of the stimulus. This rapid adaptation to pressure was restricted to the part of the receptive field that had been stimulated mechanically. A second type of adaptation developed more slowly during the course of repetitive mechanical stimulation. It persisted for many seconds after the end of a train of impulses and appeared as an increase in the threshold to mechanical stimuli not only in the region of skin that had been rubbed but throughout the receptive field of the cell.2. Impulses initiated in the cell body propagated antidromically towards the skin and also raised the threshold to touch, indicating that after-effects of impulse activity were responsible for the long-lasting threshold increase.3. Repetitive mechanical stimulation could also produce a reversible conduction block in branches of the touch cell. The block occurred in discrete regions of low safety factor such as axonal branch points both within the ganglion and in the periphery. In some experiments impulses intermittently failed to reach one axonal branch yet continued to invade a separate branch of the same cell.4. Several lines of evidence indicate that both conduction block and the slow component of adaptation are linked to a prolonged hyperpolarization that follows repetitive stimulation of the touch cell. Strophanthidin, which blocks the after-hyperpolarization in touch cells, reduced the adaptation following trains of impulses and also relieved a conduction block previously established by repetitive stimulation. Furthermore, a comparison of the effects of hyperpolarizations produced by current injection and by repetitive firing showed that most of the threshold increase in the cell body after a train of impulses could be attributed directly to the membrane hyperpolarization.5. These experiments suggest several ways in which repetitive activity can have pronounced and long-lasting effects on the performance of a highly branched sensory cell. Thus a relatively small number of impulses in a touch cell can markedly decrease its sensitivity to touch. The functional role of the conduction block observed during vigorous stimulation is not as clear because activity for many seconds or minutes is usually needed to establish a block in the larger branches of the cell.

Conductance changes, an electrogenic pump and the hyperpolarization of leech neurones following impulses

Following trains of impulses, sensory neurones in the C.N.S. of the leech show a prolonged hyperpolarization, which lasts for seconds or minutes. In the present investigation the mechanisms that underly this hyperpolarization have been studied by recording intracellularly. Two factors have been found to be responsible. One is the activity of an electrogenic pump (see Baylor & Nicholls, 1969b); the other is a long-lasting change in K conductance.1. Additional evidence that an electrogenic pump contributes to a slow after-hyperpolarization of leech sensory neurones is provided by the effects of injecting Na intracellularly. This leads to an increase in membrane potential that is blocked by the cardiac glycoside strophanthidin. Furthermore, after a train of impulses, reducing the K concentration in the external fluid characteristically reduces the hyperpolarizing action of the pump.2. The hyperpolarization following impulses is associated with a reduction of the cell membrane resistance that can persist for several minutes.3. Several lines of evidence suggest that the reduction in input resistance during the hyperpolarization is mainly due to an increased permeability to K. Thus, when the K concentration in Ringer fluid is reduced, the peak amplitude of the hyperpolarization following a train becomes larger. Furthermore, the conductance dependent part of the after-hyperpolarization has a reversal potential close to the equilibrium potential for K (E(K)). Substitution of Cl by SO(4) has little effect either on the after-hyperpolarization or on the conductance change following a train.4. Increased external Ca concentrations lead to a marked increase in the hyperpolarization that follows impulse activity. The enhanced hyperpolarization in high Ca is associated with a corresponding reduction in input resistance. The amplitude and duration of the hyperpolarization following a brief train of impulses can be increased by a factor of 5 or more in Ringer fluid containing 10 mM-Ca instead of the usual 1.8 mM. The hyperpolarization and resistance changes still occur in solutions containing 20 mM-Mg.5. To augment the hyperpolarization the increased concentration of Ca must be present during the train of impulses.6. The relative contributions of the K conductance increase and of the electrogenic pump for generating the hyperpolarization after impulse activity are different in the three types of sensory cell responding to touch, pressure and noxious stimulation.

The influence of external caesium ions on potassium efflux in frog skeletal muscle

1. At a concentration of 2.5 mM in the external solution, Cs ions reduced K efflux in muscles incubated in Na media. This effect was demonstrated in the presence or absence of 2.5 mM-K and in the presence or absence of 10(-4)M ouabain.2. In Ringer solution in which NaCl was replaced by an osmotic equivalent of MgCl(2), external Cs ions increased K efflux if the solution was K-free and decreased K efflux if the solution contained 2.5 mM-K.3. External Cs ions reduced the inward rate of leakage of Na ions into muscle cells by about 25% when the medium was K-free and contained 10(-5)M ouabain.4. The effects of 2.5 mM-K ions and 2.5 mM-Cs ions on the muscle fibre membrane potential were about the same. The influence of Cs ions on K efflux cannot be explained by changes in the resting membrane potential.5. The results suggest that a large part of the K efflux from muscle cells is mediated by a K:K exchange mechanism that is inhibited by external Cs ions.6. The results also suggest that a smaller part of the K efflux is due to K:Na exchange that is also inhibited by external Cs ions.7. In the absence of either external K or Na ions for internal K to exchange with, Cs ions promote a small amount of K exchange, perhaps via both mechanisms.