We recently characterized a novel heart-specific hormone from salmon (salmon cardiac peptide, sCP). We have now prepared a recombinant plasmid expressing the NH2-terminal fragment of pro-sCP (NT-pro-sCP) and used it to set up a specific RIA for the peptide. Because of the sensitivity of the assay and the high circulating levels, NTpro-sCP can be measured from as little as 2 l of serum. This enables repeated sampling from the same animal in different experimental setups. Mechanical load increased the release of NT-pro-sCP from isolated perfused salmon ventricle, in parallel with sCP. Bolus injection of human endothelin-1 (ET-1; 1 g) in the dorsal aorta of salmon resulted in an extensive increase of serum NT-pro-sCP (from 0.99 Ϯ 0.11 to 4.6 Ϯ 1.5 nmol/l). The response was abolished by pretreatment with a specific type A ET (ETA) receptor antagonist (BQ-123) but not with a type B ET receptor antagonist (BQ-788).The NT-pro-sCP levels had a good correlation with those of sCP (r 2 ϭ 0.75). Our results demonstrate the practical usefulness of circulating NT-pro-sCP as a marker of the endocrine function of salmon heart. They also suggest that ET-1 has an important role in regulating sCP release from teleost heart by an ETA receptor-mediated mechanism. natriuretic peptide; endothelin; recombinant protein; radioimmunoassay; study in vivo NATRIURETIC PEPTIDES are cardiac hormones with an important role in regulating cardiovascular and fluid homeostasis (2). The release of A-and B-type natriuretic peptides (ANP and BNP) from the heart is stimulated by increased load (11,14,18) and by various paracrine factors, such as endothelin-1 (3, 16, 30). They reduce the cardiac load by causing natriuresis, diuresis, and relaxation of the vascular smooth muscle and by inhibiting the renin-angiotensin-aldosterone system (21). Although blood volume expansion is an important stimulus for ANP secretion in mammals, in nonmammalian eel, ANP secretion has been reported to be more sensitive to osmotic than volemic stimuli. In this teleost, ANP causes excretion of Na ϩ , thereby promoting adaptation to seawater (9).In mammals, ANP is stored in the secretory granules of atrial myocytes as a large-molecular-weight prohormone (proANP). It is processed to low-molecularweight ANP during exocytosis (33), presumably by the membrane-bound serine protease corin (36). The products are the 28-amino-acid biologically active ANP and the inert 98-amino-acid NH 2 -terminal fragment NTproANP (8,31). The elimination of ANP from the circulation is very rapid, resulting in low and labile plasma concentrations. The half-life of rat NT-proANP in rat circulation has been reported to be eight times longer than that of ANP (32). Therefore, and because its plasma concentrations are much higher, the measurement of NT-proANP is preferred over that of ANP in the assessment of cardiac function, e.g., in patients with congestive heart failure (22). The same appears to apply for .We have recently cloned and characterized from salmon a novel peptide hormone, salmon cardiac pep...

Volume excess and mechanical load lead to the induction of the endocrine activity of the heart. The increased production and secretion of A- and B-type natriuretic peptides (ANP and BNP), in turn, unload the heart due to their physiological effects. To find out the mechanisms of cardiac-specific expression and sensitivity to mechanical stimuli of the natriuretic peptide genes, we have used salmon (Salmo salar) as our model organism, because osmoregulating fish have a particularly well developed defense mechanism against volume excess. We have previously cloned a complementary DNA from salmon heart encoding a novel vasorelaxant cardiac hormone, salmon cardiac peptide (sCP). Its production is restricted to the heart, and its release is very sensitive to mechanical load. We have now cloned the gene encoding sCP. The structure of the gene suggests that sCP may represent an ancestral form of the mammalian natriuretic peptides. Remarkably, despite the large phylogenetic distance, the sCP promoter is as effective as mammalian ANP promoters in cultured neonatal rat atrial cardiomyocytes. Therefore, structural and functional comparisons of the promoters of sCP and ANP provide an excellent means of identifying the elements and transcription factors required for atrial-specific gene expression and the regulation of the endocrine function of the heart. Isolation of the protein product of sCP gene from salmon atrium demonstrated that the storage form of sCP is the prohormone of 126 amino acids. The final processing of the prohormone appears to take place during exocytosis of the secretory granules, as the released and circulating form is the biologically active 29-amino acid sCP.

No abstract