Immunoreactive Atrial Natriuretic Peptide and Dopamine β-Hydroxylase in Myocytes and Chromaffin Cells of the Heart of the African Lungfish, Protopterus aethiopicus

Abstract

The heart of the African lungfish, Protopterus aethiopicus, was examined for immunoreactive atrial natriuretic peptide (ANP) and dopamine β-hydroxylase (DβH) as markers for hormone secreting myocytes and chromaffin cells, respectively. Specific antibodies raised against rat α-ANP and rat DβH were used for immunofluorescence microscopy and immunogold electron microscopy. DβH-immunoreactive cells were restricted to subendocardial areas of the atrium whereas ANP immunoreactivity occurred throughout both the atrial and the ventricular myocardium, showing particularly strong staining intensity in the atrial myocytes. The granular ANP immunostaining in the atrial myocytes was frequently accumulated in the sarcoplasm. In the ventricular myocytes ANP immunoreactivity occurred as scattered granular staining throughout the sarcoplasm. ANP and DβH immunofluorescence staining coincided with the presence of immunoreactive specific granules and secretory vesicles in the cardiac myocytes and chromaffin cells, respectively, as revealed by electron microscopy. The number of ANP-containing specific granules was generally high in the atrial myocytes, and they were frequently observed in clusters in subsarcolemmal areas. Granular frequency was considerably lower and the mean granular diameter was smaller (0.142 ± 0.045 μm versus 0.213 ± 0.049 μm) in the ventricular than in the atrial myocytes. The present results indicate that ANP and DβH are phylogenetically highly conserved proteins from the dipnoi to the rat. The large amounts of ANP and of specific granules are consistent with an endocrine myocardium in the Protopterus heart. The presence of DβH and secretory vesicles in the subendocardial chromaffin cells of the atrium suggests a local production of catecholamines from dopamine in the heart of this dipnoan.