The effect of hypertrophy on mRNA of rat liver

This thesis describes a study of hypertrophy in rat liver and presents data on the effects of hypertrophy on liver mRNA populations. Results are presented on : i. The effects of partial hepatectomy on liver polysomal Poly(A)+RNA populations. ii. Effects of adrenalectomy on liver polysomal Poly (A)+ RNA populations. iii. Effects of glucocorticoids on the liver polysomal Poly(A)+RNA populations of adrenalectomized rat livers. Liver mR,NA was prepared from total polysomal 'RNA and ' was fractionated on O1igo(dT)-cellulose to obtain polysomal Poly(A)-containing species. The purified mRNA was extensively characterized under denaturing and non-denaturing conditions. The diversity and complexity of mRNA populations were examined by cDNA hybridization and unique DNA hybridizations. Differences between the populations were also compared by heterologous hybridization to total and fractionated cDNA and unique DNA preparations. Normal liver was found to contain 12,400 diverse Poly(A)+RNA sequences when analysed by cDNA hybridization and 29,000 sequences when analysed by unique DNA hybridizations. These results compare favourably with those of others and illustrate the well documented findings that analysis by unique DNA hybridization tend to produce values 2 to times greater than those derived by cDNA hybridization. Reasons are discussed for believing that the latter method produces underestimates. Analysis of mRNA populations of liver responding to partial hepatectomy showed little change when compared with normal rat liver. Both cDNA and unique DNA hybridization revealed a total complexity similar to that of the intact tissue. Heterologous hybridizations, however, showed that in the later stages of the hypertrophy phase of regeneration some changes + in the relative abundances of the Poly(A) RNA species could be detected. Complementary DNA hybridization analysis of the mRNA population of adrenalectomized rat liver revealed a massive change in the complexity such that total number of sequences obtained was reduced to one quarter. This unlikely result was found to be repeatable but is thought to reflect the changes in the low abundance, high complexity class of mRNA species. Thus, the large number of sequences which appear to disappear from the mRNA population of the adrenalectomized rat liver have probably just become much rarer and are therefore difficult to detect by Rot hybridization. These concepts are supported by unique DNA hybridizations which revealed substantial but very much smaller differences in the complexities of the two mRNA populations. Within 4-6 hrs of the treatment of adrenalectomized rats with glucocorticoids, the mRNA complexity became very similar to that of normal rat liver. Prolonged treatment with glucocorticoids did not result in a significant increase in the mRNA complexity. The possible significance of these findings is discussed