Hepatic messenger ribonucleic acid activity profiles in experimental azotemia in the rat. Relationship to food intake and thyroid function.

We have studied the hepatic messenger RNA (mRNA) activity profile in chronically azotemic rats and sought to determine whether the observed changes could be mediated either by reduced food intake or diminished thyroid function at the tissue level. mRNA activity profiles were produced by two-dimensional gel electrophoretic separation of radioactively labeled products of an in vitro reticulocyte lysate system which had been programmed by hepatic RNA. Of the approximately 240 translational products identified in this system, seven sequences were consistently altered in azotemia. In pair-fed animals six of these also decreased, but the alterations in three were depressed to a significantly lesser extent in the pair-fed group. Moreover, analysis of covariance suggested that food intake could account for the differences in only one sequence. The possibility that the mRNA activity profile in azotemia could represent the effects of diminished thyroid function was minimized by the finding that the reductions in plasma thyroxine (T4) and triiodothyronine (T3) levels observed were due largely to reduced plasma protein binding, with maintenance of the mean free T4 and free T3 concentrations within the normal range. The changes in only one mRNA sequence could be related to free T3 levels alone. Our findings, therefore, indicate that although diminished food intake and reduced thyroid function may contribute to some of the observed changes in the mRNA activity profiles, the bulk of alterations in azotemia appear to be mediated by other mechanisms. The striking overlap between the sequences affected by azotemia and pair-feeding raises the speculation that altered gene expression in azotemia may reflect an impaired hepatic response at the pretranslational level to metabolic signals associated with food intake.