Background. Although first isolated from the porcine brain, brain natriuretic peptide (BNP) is also found in the heart, particularly in the atria. Methods and Results. We examined the immunocytochemical localization of BNP in relation to atrial natriuretic peptide (ANP) in five porcine atria using highly specific monoclonal antibodies. With the use of an indirect immunoperoxidase method, serial sections examined by light microscopy showed foci of multihormonal myocytes containing both ANP and BNP localized in the subendocardium. Monohormonal myocytes containing ANP only were observed in the subepicardium and part of the subendocardium. The staining intensity of ANP and BNP showed a transmural gradient from the subendocardium to the subepicardium. At the electron microscopic level, double immunocytochemistry using a two-face immunogold-staining method revealed two types of granules: Type 1 is a monohormonal granule containing ANP alone, and type 2 is a multihormonal granule containing both ANP and BNP. Although most atrial myocytes contained both, type 1 granules were frequently observed in the epicardial side, and type 2 granules were frequently observed in the endocardial side. We observed a few type 2 granules even in the light microscopically monohormonal myocytes. Conclusions. These data suggest that the atrial myocyte population comprises single, multipotential cells able to synthesize both natriuretic peptides in varying proportions. The atrial transmural gradient of BNP and ANP may be related to responses to wall stress.
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