Fig. 4. Effect of Drome-DH₄₄ on cyclic nucleotide levels. (A) Drome-DH₄₄ elevates intracellular cAMP levels. Intracellular cAMP levels in wild-type (Oregon R) tubules were measured under control and Drome-DH₄₄ stimulated conditions by radioimmunoassay. The results are expressed as percentage increase of basal rate on peptide stimulation (stimulated rate—mean basal rate)x100/mean basal rate). Values are means ± S.E.M. (N=5-9;): stimulation of cAMP is significant (P<0.05) for concentrations above 10^-8 moll^-1. The mean basal cAMP level was 383±43 fmol/20 tubules. (B) cGMP levels are not affected by Drome-DH₄₄. Intracellular levels of cyclic GMP did not show a significant difference between control and Drome-DH₄₄ (10^-7 moll^-1) stimulated conditions by RIA. The results are expressed as means ± S.E.M., N=4 independent samples, each pooled from 20 tubules. (C,D) Drome-DH₄₄-induced increase in cAMP is confined to principal cells. Immunocytochemistry of whole-mount tubules in control cells (C) and 10 min after application of 10^-7 moll^-1 Drome-DH₄₄ (D). Images were recorded under identical filter and exposure settings, using a matched-pair protocol. A typical stellate cell is outlined in each photograph.