The cardiotoxic effect of batrachotoxin.

Abstract

1. The mode of action of batrachotoxin (BTX) in isolated papillary muscles of the guinea-pig heart was investigated using conventional methods for mechanical and electrophysiological measurements. 2. The toxic effect of BTX was due to a slowing of the repolarization of the action potential at negative values of membrane potential which initiated a secondary regenerative depolarization associated with an extrasystole. 3. At low concentrations (0.75–60nM) the prolonging effect on the action potential appeared only after a series of action potentials had been elicited. For a given effect (increase of action potential duration or production of an extrasystole) the product of BTX concentration and number of preceding action potentials in the presence of BTX was approximately constant. The effect on the action potential developed at a progressively increasing rate during repetitive stimulation. 4. On the assumption of a non-linear relation between the degree of BTX receptor occupation and the effect on action potential duration, the effect of BTX is interpreted to be the consequence of an activity-dependent, irreversible one-to-one binding reaction of BTX with sarcolemmal receptors. 5. Removal of BTX during continued stimulation or interruption of stimulation during continued exposure to BTX caused a gradual disappearance of the effect on the action potential; a residual effect was still present after several hours of washing or rest. These findings indicate a time-dependent very slow dissociation of BTX. 6. Tetrodotoxin (TTX) antagonized the effect of BTX indicating the involvement of fast Na channels. Even when the maximum rate of rise of the action potential was markedly suppressed by TTX, a high concentration of BTX was capable to prolong the action potential and to induce secondary depolarizations. A low fractional occupation of BTX receptors apparently suffices for the production of extrasystoles. 7. The transient hyperpolarization of the resting membrane potential present after prolonged stimulation at 1.0 Hz was significantly increased by BTX suggestive of increased Na pumping. 8. Depolarization to-50mV by addition of KCl apparently prevented the binding reaction of BTX. It is tentatively suggested that the activity of the fast Na channels (rather than the activity of the slow channels, depolarization, or the mechanical activity) is responsible for the binding of BTX to its receptors. 9. Concomitantly with its effect on the action potential BTX produced a positive inotropic effect by increasing the rate of force development and prolonged the relaxation time. These effects persisted in preparations from reserpine-pretreated animals, but were absent in the presence of TTX. An increased [Na]i, secondary to a prolonged Na permeability during the action potential, and followed by increased Na/Ca exchange, could explain the positive inotropic effect. 10. The first contraction elicited after incubation of a resting muscle with high concentrations of BTX (0.6–1.2μM) was increased in amplitude and rate of rise. This inotropic effect was absent in muscles from reserpine-pretreated animals. The nerve-depolarizing effect of BTX, causing a release of noradrenaline from adrenergic nerve endings within the muscle, is thought responsible for this effect.