Prazosin attenuates the natriuretic response to atrial natriuretic factor in man

ANP in regulation of arterial pressure and fluid-electrolyte balance: lessons from genetic mouse models

The recent development of genetic mouse models presenting life-long alterations in expression of the genes for atrial natriuretic peptide (ANP) or its receptors (NPR-A, NPR-C) has uncovered a physiological role of this hormone in chronic blood pressure homeostasis. Transgenic mice overexpressing a transthyretin-ANP fusion gene are hypotensive relative to the nontransgenic littermates, whereas mice harboring functional disruptions of the ANP or NPR-A genes are hypertensive compared with their respective wild-type counterparts. The chronic hypotensive action of ANP is determined by vasodilation of the resistance vasculature, which is probably mediated by attenuation of vascular sympathetic tone at one or several prejunctional sites. Under conditions of normal dietary salt consumption, the hypotensive action of ANP is dissociated from the natriuretic activity of the hormone. However, during elevated dietary salt intake, ANP-mediated antagonism of the renin-angiotensin system is essential for maintenance of blood pressure constancy, inasmuch as the ANP gene "knockout" mice (ANP -/-) develop a salt-sensitive component of hypertension in association with failure to adequately downregulate plasma renin activity. These findings imply that genetic deficiencies in ANP or natriuretic receptor activity may be underlying causative factors in the etiology of salt-sensitive variants of hypertensive disease and other sodium-retaining disorders, such as congestive heart failure and cirrhosis.

Atrial natriuretic peptide is involved in renal actions of moxonidine

Moxonidine, an antihypertensive imidazoline compound, reduces blood pressure by selective activation of central imidazoline I(1)-receptors and inhibition of sympathetic nerve activity and by direct actions on the kidney, with both mechanisms resulting in diuresis and natriuresis. We hypothesized that the hypotensive and renal actions of moxonidine may be mediated by atrial natriuretic peptide (ANP), a cardiac peptide involved in pressure and volume homeostasis through its vasodilatory, diuretic, and natriuretic actions. Renal parameters were measured on an hourly basis over a period of 4 hours in conscious rats that received bolus intravenous injections of moxonidine (1 to 150 microg/300 microL saline). During the first hour, moxonidine dose-dependently stimulated diuresis, natriuresis, kaliuresis, and urinary cGMP, the index of ANP activity. Moxonidine (50 microg) significantly (P<0.001) stimulated urinary volume (0.35+/-0.04 versus 1.05+/-0.09 mL/h per 100 g), sodium (14. 3+/-2.5 versus 51.8+/-6.5 micromol/h per 100 g), potassium (10.5+/-2. 3 versus 32.3+/-3.2 micromol/h per 100 g), and cGMP (325+/-52 versus 744+/-120 pmol/h per 100 g). Pretreatment with a selective imidazoline receptor antagonist, efaroxan, dose-dependently inhibited moxonidine-stimulated renal parameters. Efaroxan (25 microg per rat) significantly inhibited moxonidine-stimulated diuretic and natriuretic effects and urinary cGMP excretion (744+/-120 versus 381+/-137 pmol/h per 100 g, P<0.02). The alpha(2)-adrenoceptor antagonist yohimbine (50 microg per rat) partially yet significantly inhibited moxonidine-stimulated diuresis and natriuresis but not cGMP excretion. Plasma ANP was dose-dependently increased by moxonidine and was inhibited by pretreatment with efaroxan (220.8+/-36.9 versus 100.3+/-31.7 pg/mL, P<0.03) but not by yohimbine. In conclusion, selective in vivo activation of imidazoline receptors by moxonidine is associated with dose-dependent diuresis, natriuresis, and kaliuresis as well as stimulated plasma ANP and urinary cGMP excretion, thus implicating ANP in the renal actions of moxonidine.

Natriuretic peptide immunoreactivity in nerve structures and Purkinje fibres of human, pig and sheep hearts

Atrial natriuretic peptide is a well-described peptide in cardiac Purkinje fibres and has been shown to interfere with the autonomic regulation in the heart of various species, including man. Recently, we detected immunoreactivity for the peptide in intracardial ganglionic cells and nerve fibre varicosities of bovine hearts, by the use of a modified immunostaining technique that induced an improved detection of natriuretic peptides. These findings raised the question as to whether natriuretic peptides are detectable in these tissues in man and other species. The conduction system from human, pig and sheep hearts was dissected processed with antisera against atrial natriuretic peptide and the closely related brain natriuretic peptide. Immunostaining for the brain natriuretic peptide was detected in some Purkinje fibres in all of these species. Interestingly, in pig, sheep and human hearts, some ganglionic cells and nerve fibres showed atrial natriuretic peptide immunoreactivity, particularly in the soma of human ganglionic cells. This is the first study showing immunoreactivity for the atrial natriuretic peptide in nerve structures and for the brain natriuretic peptide in Purkinje fibres of the human heart. The results give a morphological correlate for the documented effects of atrial natriuretic peptide on the heart autonomic nervous system and for the presumable effects of brain natriuretic peptide in the conduction system of man.

Atrial natriuretic peptide regulation of noradrenaline release in the anterior hypothalamic area of spontaneously hypertensive rats

In spontaneously hypertensive rats (SHR), high NaCl diets increase arterial pressure and sympathetic nervous system activity by decreasing noradrenaline release in the anterior hypothalamic area (AHA), thereby reducing the activation of sympathoinhibitory neurons in AHA. Atrial natriuretic peptide (ANP) can inhibit the release of noradrenaline, and ANP concentration is elevated in the AHA of SHR. The present study tests the hypothesis that in SHR, local ANP inhibits noradrenaline release from nerve terminals in AHA. Male SHR fed a basal or high NaCl diet for 2 wk and normotensive Wistar Kyoto rats (WKY) fed a basal NaCl diet were studied. In SHR on the basal diet, microperfusion of exogenous ANP into the AHA elicited a dose-related decrease in the concentration of the major noradrenaline metabolite 3-methoxy-4-hydroxy-phenylglycol (MOPEG) in the AHA; this effect was attenuated in the other two groups. In a subsequent study, the ANP-C (clearance) receptor agonist c-ANP was microperfused into the AHA to increase extracellular concentration of endogenous ANP in AHA. c-ANP reduced AHA MOPEG concentration in SHR on the basal NaCl diet but not in the other two groups. These data support the hypothesis that local ANP inhibits noradrenaline release in the AHA and thereby contributes to NaCl-sensitive hypertension in SHR.

Raised plasma levels of atrial natriuretic factor in cardiac allograft recipients: evidence of increased cardiac secretion and decreased renal clearance

The mechanism(s) causing high levels of plasma atrial natriuretic factor (ANF) in cardiac allograft recipients is(are) unclear. The kidney is important for the clearance of ANF and renal function may decline with cyclosporin A therapy in these patients. The relationship between plasma ANF level and renal function and also the pharmacokinetics of a continuous infusion of ANF (15.5 ng.kg-1.min-1 for 60 min) was examined in 6 cardiac allograft recipients on cyclosporin A therapy. Resting plasma ANF levels were significantly higher in these patients than in 8 healthy subjects (71 vs. 21 ng.l-1). Both effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) were significantly lower in these patients than in healthy subjects (215 vs. 617 ml.min-1 and 55 vs. 102 ml.min-1 respectively). There was a significant inverse correlation between plasma ANF and ERPF (r = -0.86) and between plasma ANF and GFR (r = -0.81). During the period of ANF infusion, steady state plasma ANF levels were significantly higher in cardiac allograft recipients. Total body clearance of ANF was marginally lower in these patients than in healthy subjects (60 vs. 10.0 l.min-1) although this difference did not reach statistical significance. Derived endogenous secretion rate of ANF was threefold higher in patients when compared to healthy subjects (633 vs. 208 ng.min-1). We have therefore shown that cardiac allograft recipients on cyclosporin A have elevated plasma ANF levels and also decreased renal function. Pharmacokinetic analysis have shown that this increase in plasma ANF levels is due more to increased ANF secretion than to decreased ANF clearance in these patients.