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Ultrastructure and Atrial Natriuretic Peptide(ANP)‐like Immunoreactivity of Cardiocytes in the Larval, Metamorphosing and Adult Specimens of the Japanese Toad, Bufo japonicus formosus

The distribution of an atrial natriuretic peptide (ANP)‐like material in the cardiocytes of larval, metamorphosing, and adult specimens (both breeding and non‐breeding) of the toad, Bufo japonicus formosus, was studied immunohistochemically, ultrastructurally and immunocytochemically. Histochemically, ANP‐immunoreaction was positive in the atrium and ventricle in stage 33 larvae, while negative in the ventricle in stage 40 larvae. In adult toads, the reaction was stronger in the right than in the left atrium but quite weak in the ventricles, particularly those of non‐breeding specimens. Electron microscopy indicated a very small number of secretory granules in the atrial and ventricular cardiocytes of embryos as early as the limb‐bud stage (stage 28), and as development proceeded, the number of these granules increased rapidly in atrial but not in ventricular cardiocytes. In metamorphosing animals, a small population of larger granules (200–250 nm) was noted next to those of ordinary size (the median, 110 nm) in the same cell. In adult toads, granules of about 120 nm and 200 nm in median size were found in the same cell. Postembedding immunogold staining consistently indicated ANP‐immunoreactivity in these granules in atrial and ventricular cardiocytes. The plasma content of immunoreactive ANP was considerably higher in breeding (20.5 ± 5.9 pg/ml) than in non‐breeding toads (5.4 ± 1.7 pg/ml). These results are discussed in relation to presently available data on the physiological role of ANP.