ABSTRACT
Treatment of rabbits with thyroxine 1 mg/kg daily for three to six days produced a hyperthyroid state as evidenced by increase in the heart rate and PBI. The hyperthyroid animals exhibited a significant shortening of the atrial refractory period. Beta adrenergic receptor blocking agents, propranolol (l0(-6)M pronethalol (5 X 10(-5)M) or MJ-1999 (5 X 10(-4)M), completely blocked the shortening of the atrial refractory period produced by the treatment with thyroxin, and with norepinephrine. Consequently, the shortening of the atrial refractory period of rabbits treated with thyroxin appears to bs mediated through stimulation of the beta adrenergic receptor in the heart. From these results, it may be concluded that the stimulation of the beta adrenergic receptor plays an important role in the alteration of cardiac function found in hyperthyroidism, and that the beta adrenergic blocking agents may be useful in treatment of the cardiac complications of hyperthyroidism.