1. A group of rabbits was made hypothyroid by thyroidectomy, and another group was injected daily with L-thyroxine. After an appropriate interval respective alterations in thyroid state were confirmed by measurement of heart weight and of plasma iodine, and the animals' atria were isolated for recording.2. Measurements were made of atrial contractions, conduction velocity, spontaneous heart rate and maximum driven frequency, and action potentials were recorded with intracellular micro-electrodes.3. The resting potential and action potential heights were not affected by differences of thyroid state.4. Atrial arrhythmias are common in hyperthyroidism, rare in myxoedema. The possibility that hypothyroidism might reduce the rate of rise of the action potential, as do anti-arrhythmic drugs, and hyperthyroidism increase it, was investigated. Although the rate of rise was slower in hypothyroid atria at some driving frequencies, this could not alone account for an anti-arrhythmic effect, because at frequencies near the spontaneous heart rate the rate of rise of the action potential was not reduced.5. The duration of the repolarization phase of the action potential was greatly prolonged in atria from thyroidectomized rabbits, and was shortened in hyperthyroid atria. These changes could account for a reduced probability of arrhythmias in hypothyroidism, and the converse in hyperthyroidism.

Action Potentials↗

PubMed1969

Carotid sinus and angina pectoris.

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Angina Pectoris↗

PubMed1969

The relevance of beta-receptor blockade to ouabain-induced cardiac arrhythmias.

1. (+)-propranolol and (+/-)-propranolol are comparable in their potency as a local anaesthetic on the intact and desheathed frog sciatic nerve.2. (+/-)-propranolol is much more potent than (+)-propranolol as a beta-receptor blocking agent and also more effective than the latter in protecting guinea-pigs against ouabain-induced ventricular fibrillation.3. In isolated rabbit atria (-)-, (+)- and (+/-)-propranolol, and I.C.I. 50172, which has hardly any local anaesthetic activity, greatly reduce the rate of rise of the intracellularly recorded action potential at concentrations which have no significant effect on electrical threshold, contractions, spontaneous frequency, maximum driving frequency, repolarization time or conduction velocity.

Action Potentials↗