Prolonged fasting reduces rat hepatic beta 1 thyroid hormone receptor protein without changing the level of its messenger ribonucleic acid.

The level of hepatic nuclear T3-binding capacity falls in rats subjected to fasting. To define the mechanism underlying these changes, we have assayed in liver the concentration of the mRNA coding for the beta 1-receptor (beta 1-TR) isoform, the total nuclear T3-binding capacity, and the fraction of the total binding capacity that can be specifically immunoprecipitated with an anti-beta 1-TR immunoglobulin G preparation. Although no changes in beta 1-TR mRNA concentration were noted, we observed a 60% fall in total binding capacity. beta 1-TR mRNA levels were preserved despite a 50% fall in total poly(A)+ RNA. The fall in beta 1-TR protein, however, was consistent with a generalized decrease in total hepatic protein content. This study provides yet another instance in which measurement of receptor mRNA is not consonant with the behavior of the nuclear T3 receptor protein.