Effect of Thyroxine on the Cardiac Uptake of Catecholamines

The influence of thyroxine upon n the cardiac uptake of catecholamines was investigated in rabbits. A single injection of thyroxine(1.0m/kg) into rabbits did not affect the concentration of myocardial catecholamines. However, this dose of thyroxine greatly increased the cardiac uptake of catecholamine following injection of 2.0mg of norepinephrine as compared to that of untreated normal animals and it remained elevated for several hours. Similarly thyroxine also enhanced the accumulation of myocardial catecholamines following administration of dopa(60-80mg/kg) and epinephrine(1.0-1.5mg/kg).

Cardiac Catecholamines in Rabbits: Seasonal Changes and Norepinephrine Effect

In an attempt to determine whether myocardial catecholamines vary from season to season, their concentration in rabbits was measured throughout the whole year by the spectrophotofluorometric method. The highest concentration of cardiac catecholamine was observed in summer. Measurement of the atrial response to norepinephrine revealed no significant alteration during the entire period of the experiment.

The Effect of Beta-Adrenergic Receptor Blockade on the Atrial Refractory Period of Hyperthyroid Rabbits

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.

The Influence of Rotatory Movement on the Tissue Catecholamines in Rats

The present study was designed to examine the possible relationship between the function of the labyrinth and the role of the sympathetic nervous system In experimental motion sickness produced by rotatory movement(8O r.p.m.). The catecholamines in the brain, the heart and the adrenal gland of rats were rapidly reduced to one half of normal values following exposure to rotatory movement. The pretreatment with streptomycin and dramamine completely prevented the depletion by the rotatory movement of the catecholamines in the brain, the heart and the adrenal gland, but scopolamine did not prevent the decrease. Bretylium or chlorpromazine signifcantly inhibited reduction of the catechol-amines in both of the brain and the heart. However they did not influence the decrease in the adrenal gland. The reduction of the tissue catecholamines in rotatory movement is presumed to be caused largely by activation of the sympathetic nervous system mediated through labyrinthine stimulation.

Tissue Catecholamines in Hypercholesteremic Rabbits

Hypercholesteremia was induced by cholesterol feeding of rabbits for 10 weeks. Gross examination of aorta of these animals showed an evidence of atheromatous lesions. The endogenous catecholamines in heart, adrenal gland, spleen, brain, liver and kidney of these hypercholesteremic rabbits were markedly reduced as compared to those of normal animals, respectively. There may exist some correleration between the serum cholesterol and tissue catecholamines.