Effects of 3'-deoxycytidine on rRNA synthesis in toad bladder: analysis of response to aldosterone

Insulin-stimulated sodium transport in toad urinary bladder

Mammalian and teleost insulins increase active sodium transport by the toad urinary bladder at subnanomolar concentrations. This stimulation is evident within 15 min and persists for hours. Porcine proinsulin and a cross-linked derivative of bovine insulin are less effective than porcine insulin in stimulating the short-circuit current (SCC), indicating the specificity appropriate for activation of sodium transport through an insulin receptor. The initial stimulation by insulin of the SCC is not blocked by pretreatment with actinomycin D, puromycin, cycloheximide, or tunicamycin. However, in the presence of any one of these inhibitors the sustained increase in SCC is blocked and the rise is short-lived, lasting only 45 to 90 min. In amphotericin-treated bladders, the addition of insulin did not further stimulate SCC.

Effects of 3'deoxyadenosine and actinomycin D on RNA synthesis in toad bladder: analysis of response to aldosterone

Previous studies have shown that aldosterone increases transepithelial active Na+ transport after a latent period of about 60 min and incorporation of 3H-uridine into polyadenylated RNA (poly(A)(+)RNA) (putatively poly(A)(+)mRNA) as early as 30 min after aldosterone addition. To assess the physiological importance of this pathway, the effects of 3'deoxyadenosine and actinomycin D were compared in studies on the urinary bladder of the toad Bufo marinus. 3'deoxyadenosine (30 microgram/ml) only partially, though significantly, inhibited the aldosterone-dependent increase in Na+ transport measured as short-circuit current (scc). The incorporation of 3H-uridine into poly(A) (+)RNA was inhibited by 70 to 80%. In contrast, Actinomycin D (2 microgram/ml) totally inhibited the aldosterone-dependent increase in scc, and the incorporation of 3H-uridine into poly(A)(+)RNA by 68 to 75%. 3'deoxyadenosine or actinomycin D alone had no significant effects on baseline scc, while inhibiting poly(A)(+)RNA to the same extent. The differential effects of deoxyadenosine and actinomycin on aldosterone-dpendent Na+ transport may be related to their different sites of action on RNA synthesis: both drugs inhibited, to a similar extent, cytoplasmic poly(A)(+)mRNA: however, 3'deoxyadenosine, in contrast to Actinomycin D, failed to inhibit poly(A)(-)RNA, sedimenting between 4S and 18S (putatively poly(A)(-)mRNA). We conclude that the mineralocorticoid action of aldosterone during the first three hours depends on the synthesis of both poly(A)(+)mRNA and poly(A)(-)mRNA.

Role of RNA in the action of aldosterone on Na+ transport

Recent data describing the effects of aldosterone on the induction of messenger RNA (= mRNA) and ribosomal RNA (= rRNA) are reviewed. In the urinary bladder of the toad, aldosterone induces a few specific polyadenylated mRNAs (= poly(A)(+)mRNA) during the latent period, i.e., 30 to 60 min after hormone addition. Later i.e., 90 to 240 min after aldosterone addition, 18S and 28S cytoplasmic rRNA subunits are also induced. The effect of poly(A)(+)mRNA is mineralocorticoid-specific and correlates well with the aldosterone-dependent Na+ transport. Actinomycin D which inhibits both poly(A)(+)mRNA and nonpolyadenylated mRNA (= POLY(A)(-)mRNA) totally abolishes the response to aldosterone on Na+ transport. 3'deoxyadenosine (cordycepin), which inhibits poly(A)(+)mRNA but not poly(A)(-)mRNA, only inhibits 50 to 60% of the physiological response. These differential effects suggest that an intact poly(A)(-)mRNA pathway is also an important factor in mediating the action of aldosterone. In contrast, 3'deoxycytidine, which inhibits rRNA but not mRNA, does not impair the mineralocorticoid response, at least during the first 3 hr of aldosterone action.