Natriuretic peptides and the control of catecholamine release in two freshwater teleost and a marine elasmobranch fish

Experiments were carried out in situ and in vivo to investigate the relationship between natriuretic peptides (NPs) and humoral catecholamine secretion in the American eel (Anguilla rostrata), rainbow trout (Oncorhynchus mykiss) and spiny dogfish (Squalus acanthias). In situ perfusion of the chromaffin tissue of A. rostrata with homologous atrial NP (ANP; 10–9 mol l–1) or ventricular NP (VNP; 10–9 mol l–1), or O. mykiss with either rat ANP (10–9 mol l–1), eel VNP (10–9 mol l–1), or trout VNP (10–9 mol l–1), did not significantly affect basal or carbachol-elicited (10–5 mol kg–1) catecholamine release. Bolus injections of homologous ANP (10–9 mol kg–1) or VNP (10–9 mol kg–1) in A. rostrata in vivo elicited a rapid and prolonged reduction in arterial blood pressure and an increase in heart rate (fH); circulating plasma catecholamine levels were unaffected. In O. mykiss, bolus injections of rat ANP (10–9 mol kg–1) or trout VNP (10–9 mol kg–1) elicited a significant bi-phasic pressor- depressor response and a marked increase in fH. Neither the acute pressor or the longer-term depressor effects of NPs in O. mykiss were associated with any change in circulating plasma catecholamine levels. In S. acanthias, bolus injections of homologous C-type natriuretic peptide (CNP; 10–9 mol kg–1) elicited a bi-phasic pressor-depressor response, an increase in systemic resistance, a decrease in cardiac output and stroke volume, but no change in fH. Plasma noradrenaline levels were elevated 15- fold after CNP injection while circulating adrenaline levels remained unchanged. These results show that NPs of systemic origin do not directly or indirectly affect basal or cholinergically-mediated catecholamine release in A. rostrata and O. mykiss and that the initial pressor response to NP's in trout cannot be attributed to an elevation of circulating catecholamines. Conversely, CNP appears to be a potent secretagogue (direct or indirect) of noradrenaline release in S. acanthias and thus there is likely to be a significant humoral adrenergic component to the cardiovascular effects of NPs in this species.