Actions of aldosterone on rRNA and Na+ transport in the toad bladder

Aldosterone increased methylation by [methyl-3H]methionine of nuclear ribosomal ribonucleic acid (rRNA) sedimenting at 18S, 28S, and 40S within 90 min and of 28S cytoplasmic RNA within 240 min of continuous exposure to the precursor and the hormone, in the toad bladder. In addition, incorporation of [14C]uridine into cytoplasmic 4S transfer RNA, and 18S and 28S rRNA was enhanced after 240 min of continuous exposure to the precursor and the hormone. Aldosterone had minimal effects on the 3H or 14C-labeled, acid-soluble pools. These results suggest that aldosterone augments the synthesis of rRNA at the transcriptional level. Ribosomes isolated 240 min after treatment of the toad bladder with aldosterone showed increased incorporation of [3H]-phenylalanine into peptides with both endogenous messenger RNA (mRNA) and exogenous mRNA (i.e., poly(uridylic acid) in an in vitro assay. Inhibition of reinitiation with NaF or poly (AUG) reduced the rate of amino acid polymerization by 45% but the aldosterone to control ratio remained significantly high. These results imply an increase in active ribosomes and perhaps in endogenous mRNA content. Our findings, however, do not distinguish between a steroid-dependent increase in the total number of active ribosomes and an increase in translational activity per ribosome.