THE INFLUENCE OF THYROID HORMONE ON MYOCARDIAL METABOLISM OF NOREPINEPHRINE IN THE GUINEA PIG

¹ The Departments of Internal Medicine and Physiology, Yale University, School of Medicine, New Haven, Connecticut, The Division of Cardiology, Philadelphia General Hospital, and The Department of Internal Medicine, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania

The uptake of H³-norepinephrine (H³NE) in paired normal and hyperthyroid guinea pigs was determined in atrial and right and left ventricular myocardium after the intravenous administration of 10 or 20 µc/100 g b.wt. of the isotope. Atrial myocardium from hyperthyroid animals had a higher H³NE concentration than normal controls 10 min after injection of 20 µc of isotope. The pattern of distribution of myocardial H³NE in normal and hyperthyroid guinea pigs followed the same distribution as sympathetic innervation to the heart. Myocardial H³NE concentration remained stable from 1 to 30 min after injection of H³NE. Endogenous myocardial norepinephrine (NE) stores were measured in normal and thyroxine-treated guinea pigs and rats. In the guinea pig, endogenous NE distribution followed that of myocardial sympathetic innervation. NE concentration in thyrotoxicosis was increased to the greatest degree in myocardium receiving the largest supply of sympathetic nerves. In contrast to the increased myocardial NE concentration found in hyperthyroid guinea pigs, thyroxine treated rats had a decrease in ventricular myocardial NE and no change in atrial stores. Administration of reserpine to guinea pigs was associated with a reduction of endogenous myocardial NE to almost undetectable amounts. The rate of replenishment of the stores was similar in hyperthyroid and paired euthyroid animals, and control concentrations of NE were achieved within 8 days after reserpine administration. The results demonstrate that, in the guinea pig, thyroxine administration is associated with an increased myocardial uptake of circulating NE and an increase in endogenous NE stores.