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Figure 5 Photomicrographs showing in situ hybridization of ANP mRNA and BNP mRNA. Hybridization signals were dark precipitates in the cytoplasm. Panels A and B: serial sections. No hybridization signal was found in any connective tissue or vasculature. The BNP mRNA signals were found only in the subendocardial myocytes of the atria with mRAP less than 5 mm Hg (panel A), whereas they were found diffusely and transmurally in the atria with mRAP of 5 mm Hg or more (panels C and E). The ANP mRNA signals were also found in a similar distribution; they were found either in the subendocardial myocytes of the atria with lower mRAP (panel B) or diffusely and transmurally in those with higher mRAP (panel D). A control section (a serial section of panel E) incubated with digoxigenin-labeled sense riboprobes of BNP showed no signals (panel F). Asterisks and arrowheads indicate epicardial and endocardial side, respectively. Original magnification was x200 (panels A through E).