Effects of tetraethylammonium and 4-aminopyridine upon canine sinus node

Biphasic effect of tetraethylammonium on canine purkinje fibre action potential configuration

1. Using conventional microelectrode techniques a biphasic effect of tetraethylammonium (5 mmol/l) on the configuration of action potentials recorded from isolated canine Purkinje fibres: action potentials were first shortened (early effect) and then lengthened (late effect) by tetraethylammonium. 2. The early effect of tetraethylammonium also included lengthening of phase 1 duration and elevation of the plateau amplitude. These early effects reached steady-state within the first 3 min of superfusion and were readily reversed within 3 min of initiating washout of the drug. 3. The late effect (gradual lengthening of repolarisation during phase 3) failed to reach steady-state within the initial 60 min of superfusion and was not reversible. 4. The early effects of tetraethylammonium were more marked at slow driving rates and were not affected by blockade of alpha- and beta-adrenoceptors using 1 mumol/l phentolamine and 1 mumol/l propranolol. 5. The early effects of tetraethylammonium were mimicked by 4-aminopyridine (0.5 mmol/l), and in the presence of 4-aminopyridine tetraethylammonium failed to induce further changes in action potential morphology. 6. The early effects of tetraethylammonium may be due to inhibition of the transient outward current. 7. The rapid onset and reversibility of these early effects suggest that tetraethylammonium may act from outside the cell membrane.

Effects of tetraethylammonium on the action potential duration as modified by catecholamine-releasing action in guinea-pig papillary muscles

The mechanism whereby tetraethylammonium ion (TEA) prolongs the action potential (AP) was examined by standard microelectrode techniques in papillary muscles isolated from nonreserpinized and reserpinized guinea-pig hearts. First, effects of 5 and 10 mmol/l TEA on steady-state AP parameters, i.e. the maximum rate of rise of AP (Vmax), AP amplitude, AP duration and resting potential, were examined at stimulation rates of 0.1 to 5 Hz. Second, effects of 5 and 10 mmol/l TEA on nonsteady-state AP parameters, i.e. the recovery processes of Vmax, AP duration and overshoot, were examined by introducing premature responses with various diastolic intervals between the basic stimulation rate of 0.1 Hz. TEA prolonged the AP more in the nonreserpinized preparations than in the reserpinized ones at all stimulation rates. In both preparations, this prolongation of the AP was greater at lower rates (0.1-1 Hz) than at higher rates (2-5 Hz), but the ratio of TEA-treated AP duration to control one was almost the same at all stimulation rates. TEA slowed the recovery process of AP duration in the nonreserpinized preparations, but not in the reserpinized ones. TEA had no effects on the recovery processes of Vmax and overshoot in both preparations. These results suggest that: TEA modifies its intrinsic prolonging action of the AP by releasing norepinephrine from sympathetic nerve terminals; TEA prolongs the AP by reducing the time-independent outward current rather than the time-dependent outward current; and a TEA-sensitive current does not effectively contribute to the total ionic current at the time of Vmax.

The mode of action of 4-aminopyridine on the chronotropic and inotropic responses in the isolated, blood-perfused dog heart

The effects of 4-aminopyridine (4-AP, 0.3 to 300 micrograms) on atrial rate and atrial muscle contractile force were investigated in the isolated, blood-perfused dog atrial preparation. Low doses (less than 30 micrograms) of 4-AP injected into the sinus node artery caused a dose-dependent positive inotropic response and a negligible positive chronotropic response. A higher dose (300 micrograms) of 4-AP induced a transient positive chronotropic response followed by a negative response and positive and negative inotropic responses. The negative cardiac responses to 4-AP were completely inhibited by an adequate dose of atropine and were potentiated by physostigmine. TTX or C6 suppressed partly, but significantly the negative responses induced by 4-AP. On the other hand, the negative responses to electrical stimulation (ES) were inhibited by atropine, C6 and TTX and those to ACh were blocked by atropine but not by C6 or TTX. The positive responses to 4-AP were not affected by propranolol but were potentiated by atropine. The positive inotropic response to 4-AP was significantly suppressed by a calcium channel blocker, diltiazem. From these results, it is concluded that the cardiac responses to 4-AP are composed of direct positive responses and negative responses which are related to activation of parasympathetic ganglionic neurotransmission.

Potentiating effects of 4-aminopyridine on responses to intramural vagal stimulation in the isolated dog atrium

The effects of 4-aminopyridine (4-AP) on the chronotropic and inotropic responses to intramural parasympathetic nerve stimulation and to injection of acetylcholine (ACh) into the sinus node artery were investigated after treatment with propranolol in the isolated, blood-perfused dog atrium. 4-AP (3-100 micrograms) induced positive chronotropic and inotropic effects and 300 micrograms of 4-AP induced biphasic, negative and positive effects. The negative responses to 4-AP were completely blocked by 10 micrograms of atropine. 4-AP (more than 30 micrograms) potentiated significantly the negative chronotropic and inotropic responses to intramural parasympathetic nerve stimulation at a frequency of 3-30 Hz. The potentiation of the responses to stimulation was greater at a low than at a high frequency of stimulation. Potentiation of the chronotropic effect was greater than that of the inotropic one. 4-AP in the doses used did not change the negative chronotropic and inotropic responses to injection of ACh into the sinus node artery. These results suggest that the potentiation by 4-AP of the cardiac responses to parasympathetic nerve stimulation was due to an increase in release of ACh from nerve terminals but not to an increase in responsiveness at the effector site.

Acute effects of amiodarone upon the canine sinus node and atrioventricular junctional region

Amiodarone was selectively perfused into the sinus node artery and atrioventricular node artery of 51 dogs. Amiodarone had an immediate negative chronotropic and dromotropic effect. Threshold concentration was 2.5 micrograms/ml. 25 and 50 micrograms/ml of amiodarone injected into the sinus node artery slowed the heart by 25.6 +/- 3.1 and 33.7 +/- 2.6 beats/min (mean +/- 1 SEM), respectively. Amiodarone 25 and 50 micrograms/ml injected into the AV node artery during AV junctional rhythm slowed the AV junctional pacemaker by 12.2 +/- 1.8 and 17.4 +/- 1.7 beats/min, respectively. Injections of amiodarone into the AV node artery during sinus rhythm regularly increased AV conduction time sometimes causing 2 degrees AV block at the highest concentration used. Impaired conduction was exclusively measured at the level of the A-H interval in the His electrogram. Neither atropine nor propranolol prevented the negative chronotropic effects of amiodarone. Amiodarone had no significant effect on sinus node response to either stellate stimulation or intranodal administration of norepinephrine. The negative chronotropic action of amiodarone was significantly enhanced when amiodarone was administered in a perfusate containing low (0.6 mM) instead of normal calcium. Taken collectively these observations indicate that amiodarone has immediate depressant electrophysiologic effects on both the sinus node and the AV junction and that these early effects might involve the blockade of the slow channel.