CHANGES IN BETA-1-ADRENERGIC AND BETA-2-ADRENERGIC RECEPTOR MESSENGER-RNA LEVELS IN BROWN ADIPOSE-TISSUE AND HEART OF HYPOTHYROID RATS

The aim of the present work was to study the effect of hypothyrodism on the expression of the beta-adrenergic receptor (beta-AR) in interscapular brown adipose tissue and heart. The total density of plasma membrane beta-AR per tissue is decreased by 44% in hypothyroid rat interscapular brown adipose tissue and by 55% in hypothyroid rat heart compared with euthyroid controls. The effects of hypothyroidism on the density of both beta-1- and beta-2-AR subtypes were also determined in competition displacement experiments. The densities of beta-1- and beta-2-AR per tissue are decreased by 50% and 48% respectively in interscapular brown adipose tissue and by 52% and 54% in the heart. Northern blot analysis of poly(A)+ RNA from hypothyroid rat interscapular brown adipose tissue demonstrated that the levels of beta-1- and beta-2-AR mRNA per tissue are decreased by 73 % and 58 % respectively, whereas in hypothyroid heart, only the beta-1-AR mRNA is decreased, by 43 %. The effect of hypothyroidism on the beta-1-AR mRNA is significantly more marked in the interscapular brown adipose tissue than in the heart. These results indicate that beta-AR mRNA levels are differentially regulated in rat interscapular brown adipose tissue and heart, and suggest that the decrease in beta-AR number in interscapular brown adipose tissue and heart of hypothyroid animals may in part be explained by a decreased steady-state level of beta-AR mRNA.