We used the secretion of the novel salmon cardiac peptide (sCP) as a model to examine the mechanisms of ventricular hormone release. Mechanical load increased dose dependently the secretion of immunoreactive sCP from isolated perfused salmon ventricle, with 3. 3-fold increase when a load of 13 cmH(2)O was applied. Endothelin-1 (5 nmol/l) was also able to rapidly increase the secretion of sCP. The released peptide corresponded to the biologically active sCP-29, whereas the large ventricular storage consisted of pro-sCP-sized material. With the use of immunoelectron microscopy, a large number of granules containing immunoreactive sCP could be detected in salmon ventricle. As judged by RNA blot analysis, there was very active basal expression of the sCP gene in the ventricle, which was not increased by mechanical load of up to 2-h duration. Our results show that the ventricle actively expresses the gene of sCP, stores the prohormone in secretory granules, and releases the peptide in response to mechanical load and endothelin-1. Thus the salmon ventricle uses the regulated pathway to produce and release a hormone structurally related to the mammalian natriuretic peptides.

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...

The natriuretic peptide system plays a major role in the fluid and electrolyte homeostasis in vertebrates, based on results from physiological studies (reviewed in Ruskoaho, 1992;Thibault et al. 1999) and experiments with transgenic animals (John et al. 1995;Lopez et al. 1995;Matsukawa et al. 1999). The regulation of the synthesis and secretion of cardiac A-and B-type natriuretic peptides, is, however, still incompletely understood. The ability of salmon (Salmo salar) to maintain volume and electrolyte balance despite the highly challenging osmoregulatory conditions of sea and fresh water prompted us to use it as a model for our studies on the regulation of natriuretic peptides. As an initial step, we recently cloned and sequenced from salmon heart the cDNA of a novel peptide hormone, salmon cardiac peptide (sCP) (Tervonen et al. 1998), which is stored in secretory granules and released by mechanical load, analogous to the mammalian natriuretic peptides (Kokkonen et al. 2000). Homologues of the novel peptide can be found in several teleost species . The evolutional position of fish make them ideal for studying the conserved characteristics of the natriuretic peptide system (Powers, 1989;Postlethwait et al. 1998;Wittbrot et al. 1998). They also make excellent models for studies on the general factors regulating the expression and secretion of natriuretic peptides. In keeping with this we have recently found that the promoter of the sCP gene is very active not only in salmon heart but also in mammalian cardiomyocytes, suggesting that, despite the large phylogenetic distance, the same transcription factors are responsible for the heart-specific expression of the peptides in mammals and salmon (MajalahtiPalviainen et al. 2000). Since sCP transcripts or peptide cannot be found in any tissues outside the heart (MajalahtiPalviainen et al. 2000) it can be used as a specific model for the endocrine function of the heart.In fish and other ectothermic animals, thermal balance is governed by external sources of heat, and therefore ambient temperature has a major influence on most physiological processes. Many fishes are exposed seasonally to considerable changes in water temperature. Temperature acclimation is accompanied with compensatory changes in metabolic, contractile and morphological properties of muscle, which significantly offset the negative impact of Temperature has a major influence on cardiac natriuretic peptide in salmon 1. Natriuretic peptides have a major role in fluid and electrolyte homeostasis in vertebrates. Ambient temperature has a major influence on physiological processes in ectothermic animals.Here we have studied the mechanisms of regulation of a natriuretic peptide, sCP (salmon cardiac peptide), in salmon (Salmo salar) acclimatised and acclimated to varying temperatures.2. The circulating and cardiac levels of sCP were found to be markedly upregulated in warmacclimatised and warm-acclimated salmon. The release of sCP from isolated in vitro perfused salmon ventricle was, however, not increased by ...

The aim was to find out the effects of endothelin-1 (ET-1) in salmon (Salmo salar) cardiac contractile and endocrine function and its possible interaction with beta-adrenergic regulation. We found that ET-1 has a positive inotropic effect in salmon heart. ET-1 (30 nM) increased the contraction amplitude 17+/-4.7% compared with the basal level. beta-Adrenergic activation (isoprenaline, 100 nM) increased contraction amplitude 30+/-13.1%, but it did not affect the contractile response to ET-1. ET-1 (10 nM) stimulated the secretion of salmon cardiac natriuretic peptide (sCP) from isolated salmon ventricle (3.3+/-0.14-fold compared with control) but did not have any effect on ventricular sCP mRNA. Isoprenaline alone (0.1-1,000 nM) did not stimulate sCP release, but ET-1 (10 nM) together with isoprenaline (0.1 nM) caused a significantly greater increase of sCP release than ET-1 alone (5.4+/-0.07 vs. 3.3+/-0.14 times increase compared with control). The effects on the contractile and secretory function could be inhibited by a selective ETA-receptor antagonist BQ-610 (1 microM), whereas ETB-receptor blockage (by 100 nM BQ-788) enhanced the secretory response. Thus ET-1 is a phylogenetically conserved regulator of cardiac function, which has synergistic action with beta-adrenergic stimulation. The modulatory effects of ET-1 may therefore be especially important in situations with high beta-adrenergic tone.

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