Comparative effects of bradykinin and atrial natriuretic factor on neuronal and non-neuronal noradrenaline uptake in the central nervous system of the rat

Angiotensin converting enzyme inhibition improves cardiac neuronal uptake of noradrenaline in spontaneously hypertensive rats

OBJECTIVES

It has been shown that a diminished sympathetic activity contributes to the hypotensive and cardioprotective actions of angiotensin converting enzyme (ACE) inhibitors (ACEI). Besides an inhibition of central sympathetic tone and peripheral noradrenaline release, we hypothesized that the interactions of ACEI with the sympathetic system may include a modulation of neuronal catecholamine uptake by peripheral nerves.

DESIGN

We investigated the influence of fosinopril on noradrenergic uptake into cardiac neurones in vitro and in vivo in acute and chronic models.

METHODS AND RESULTS

Acute administration of fosinoprilat to isolated perfused rat hearts increased the extraction of [3H]-noradrenaline from the perfusate by 39%. Treatment (14 days) of spontaneously hypertensive rats (SHR) with fosinopril (20 mg/kg per day) enhanced the cardiac uptake of i.v. administered [3H]-noradrenaline by 28%. The endogenous left ventricular content of noradrenaline was increased by 49% after an antihypertensive treatment of SHR with fosinopril (20 mg/kg per day). Identical increases in cardiac noradrenaline stores (53%) were observed in SHR treated with a blood pressure ineffective dose of fosinopril (0.2 mg/kg per day). The myocardial content of adrenaline was increased in parallel to noradrenaline after both dose regimes.

CONCLUSIONS

It is concluded that ACEI increases neuronal uptake of catecholamines in SHR in a blood pressure-independent manner. This effect occurs acutely and is independent of central sympathetic activity. Therefore, we hypothesize that ACEI modulate the activity of the cardiac noradrenaline transporter by direct activation. The improved uptake of noradrenaline may contribute to the antihypertensive and cardioprotective effects of ACEI.

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.