CARDIAC β-ADRENORECEPTORS IN THE THYROXINE-TREATED DOG

SUMMARY 1. Myocardial β-adrenoreceptors, assessed in membrane preparations of left ventricle by 3 H-dihydroalprenolol binding, were compared in dogs following thyroxine administration (n= 6), aortocaval fistula (n= 12), and in normal dogs (n= 7). 2. Left ventricular hypertrophy occurred in response to both aortocaval fistula and thyroxine treatment. Yield of myocardial membrane between the different groups was not significantly different. 3. Binding site concentration (pmol/mg membrane protein) in dogs with aortocaval fistulae was similar to that in normal dogs and was not significantly influenced by the presence or absence of cardiac failure, degree of hypertrophy or duration of fistula. 4. Thyroxine-treated dogs had an increased concentration of binding sites (c. 1.8-fold) compared with both fistula and normal groups, while binding affinities were similar in all groups. 5. The data suggest that β-adrenoreceptor concentrations in myocardium increase as a direct result of thyroid hormone action rather than as a result of secondary haemodynamic changes or ventricular hypertrophy.