To find out the significance of the newest member of the natriuretic peptide family, salmon cardiac peptide (sCP), we have determined the distribution of the peptide and its mRNA as well as the tissue and plasma molecular forms in several teleosts. Using probes based on the salmon sCP cDNA in Northern blot analysis we found mRNA homologous to that of sCP to be present in the heart of 15 fish species representing nine different genera. We developed a specific RIA for the salmon 29 amino acid peptide to be able to study the distribution of the peptide in the heart and plasma of different fish species. Despite the probable interspecies differences in the peptide sequence, large quantities of immunoreactive sCP were found in the atrium, ventricle and plasma of most of the fish species studied, suggesting that a cardiac hormone homologous to sCP has an endocrine function in a large variety of teleost species. The molecular form of the hormone secreted and stored in the tissue was determined by gel filtration high pressure liquid chromatography. In salmon, as in most of the other fish species studied, the predominant immunoreactive sCP in plasma corresponded to the low molecular weight form, with a size similar to that of the biologically active 29 amino acid sCP (sCP-29), whereas the form stored in the heart corresponded to the high molecular weight pro-sCP-sized material. The form secreted by isolated perfused salmon ventricle, in the basal state as well as when mechanically loaded, was the sCP-29-sized peptide, thus ruling out the possibility that the conversion from high to low molecular weight material is caused by plasma proteases. In conclusion, sCP-like peptides are produced and secreted from the heart of a large number of different fish species. Their post-translational processing appears to be remarkably similar to that of mammalian atrial natriuretic peptide.

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