Age-related changes in 28S and 18S RNA in antigen-stimulated and non-stimulated rat spleens

The possible changes in rRNA amounts in the spleens of immunized and non-immunized rats during aging were investigated. The cytoplasmic RNA was extracted from intact spleens or spleen cell suspensions and 28S_o/ 18S_o, 28S_y/18S_y, 28S_o/28S_y, 18S_o/18S_y ratios were calculated, the most significant change (P < 0.01) occurring at the level of 28S_o/28S_y ratio; i.e., there was a six-fold increase in the ratio of 28S rRNA in old rats as compared to young rats suggesting a preferential digestion of 26S subunit by neutral and alkaline RNAase whose activities were found to decrease in old non-immunized rats. In immunized rats the situation was different, as there was a considerable decrease (over twenty fold) in rRNA from old animals as compared to young ones, the most significant results being obtained with spleen cell suspensions. As age-related changes in the immune system begin at the time of sexual maturity, we propose the use of rRNA as a marker of rat senescence.