Localization of salmon cardiac peptide (sCP) in the heart of salmon (Salmosalar L.)

We have previously cloned and characterized a novel cardiac hormone from th e salmon (Salmo salar) which has a uniquely heart-specific distribution and a low structural similarity with any other known natriuretic peptides. Spe cific antibodies were raised in goat against the salmon cardiac peptide. Fo r localization and quantification, four different methods were applied: imm unohistochemistry (avidin-biotin peroxidase), transmission electron microsc opy, cryoimmunoelectron microscopy (protein A-gold), and a specific radioim munoassay. Both atrial and ventricular myocytes stained immunohistochemical ly. The staining was similar in all myocytes and no specific myoendocrine c ells were found. Within a single myocyte, both atrial and ventricular, the staining was stronger near the nucleus. Transmission electron microscopy re vealed that both the atrium and the ventricle contained small sarcoplasmic granules of similar type with a diameter of 100 to 200 nm and an electron-d ense core with a clear halo. The granules were typical vesicles which can b e found in secretory cells utilizing the regulatory pathway. The highest nu mber of granules was found near the nucleus, but granules were located also near the Golgi apparatus, between myofilament bundles, and in subsarcolemm al positions. Gold particles, conjugated to antibodies raised against the s almon cardiac peptide, were deposited on similar sarcoplasmic granules foun d in transmission electron microscopy. Among the sarcoplasmic granules with gold particles there were granules which did not show any cardiac peptide immunoreactivity. A significantly (Student's t test, P < 0.05) higher conce ntration of cardiac peptide was found in the heart atrium than in the ventr icle, 16.2 +/- 3.5 pmol/mg tissue (n = 8) and 4.5 +/- 1.7 pmol/mg tissue (n = 8), respectively. The findings show that the salmon cardiac peptide is l ocalized in secretory granules in both compartments of the heart. The morph ology of the granules suggests that both the atrium and the ventricle utili ze the regulatory pathway to release salmon cardiac peptide. (C) 2000 Acade mic Press.