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Measurements of the frequency-force relationship were made in left atrial muscle from euthyroid and hyperthyroid guinea pigs and the effects of calcium and norepinephrine on this relationship examined. Isometric contractile force was measured with the muscle suspended in a bath of Krebs-Henseleit solution oxygenated by a gas mixture of 95% O2-5% CO2 at 28°C. Frequency of stimulation was varied from 0.5 to 200/min at suprathreshold voltage delivered by plate electrodes. Low frequency stimulation caused less of a negative inotropic effect in hyperthyroid than in normal controls. The maximum negative and positive inotropic effects in the thyroxine-treated guinea pigs occurred at lower frequencies than in normal controls. These changes altered the shape of the myocardial frequency-force curve of hyperthyroid animals from that of normal controls. Norepinephrine and calcium in the dose levels examined produced a smaller than normal increment in the developed tension of hyperthyroid animals, but the maximum developed tension was similar in the two groups of animals. The lack of effect of reserpine on the frequency-force curve of left atrial muscle from hyperthyroid animals suggests that endogenous norepinephrine stores are not necessary for the production of a change in the curve. The alteration in the frequency-force relationship of atrial muscle from hyperthyroid guinea pigs is further evidence of a direct effect of thyroid hormone on the myocardial contractile mechanism.

Effect of Thyroid Hormone on the Frequency-Force Relationship of Atrial Myocardium from the Guinea Pig