Immunoreactive localization of vasoactive hormones (Atrial natriuretic peptide and endothelin) in the heart of Protopterus annectens, an African lungfish

The present study demonstrated, by immuno- histochemistry and Western blotting, the presence of im- munoreacting atrial natriuretic peptide (ANP) and endo- thelin in the heart of Protopterus annectens by both light and electron microscopy. The distribution of ANP gran- ules was investigated. ANP granules were localised in myocytes from the atrium, ventricle and conus arterio- sus; endothelin-1 (ET-1) was demonstrated in subendo- cardial myocytes of the atrium and the conus. No ET-1 immunoreactivity was observed in the ventricle wall. At the light-microscopical level, ET-1 appeared to occur in the endocardium, but at the electron-microscopical level no immunogold labelling was seen on the granules of the endocardial cells. It is suggested that ET-1 is pro- duced and stored in the subendocardial cells and re- leased into the subendocardial space to reach the ANP- producing myocytes and the endothelial cells.