Renal interaction of atrial natriuretic peptide with angiotensin II: glomerular and tubular effects

Natriuretic Peptides and Normal Body Fluid Regulation

Natriuretic peptides are structurally related, functionally diverse hormones. Circulating atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are delivered predominantly by the heart. Two C-type natriuretic peptides (CNPs) are paracrine messengers, notably in bone, brain, and vessels. Natriuretic peptides act by binding to the extracellular domains of three receptors, NPR-A, NPR-B, and NPR-C of which the first two are guanylate cyclases. NPR-C is coupled to inhibitory proteins. Atrial wall stress is the major regulator of ANP secretion; however, atrial pressure changes plasma ANP only modestly and transiently, and the relation between plasma ANP and atrial wall tension (or extracellular volume or sodium intake) is weak. Absence and overexpression of ANP-related genes are associated with modest blood pressure changes. ANP augments vascular permeability and reduces vascular contractility, renin and aldosterone secretion, sympathetic nerve activity, and renal tubular sodium transport. Within the physiological range of plasma ANP, the responses to step-up changes are unimpressive; in man, the systemic physiological effects include diminution of renin secretion, aldosterone secretion, and cardiac preload. For BNP, the available evidence does not show that cardiac release to the blood is related to sodium homeostasis or body fluid control. CNPs are not circulating hormones, but primarily paracrine messengers important to ossification, nervous system development, and endothelial function. Normally, natriuretic peptides are not powerful natriuretic/diuretic hormones; common conclusions are not consistently supported by hard data. ANP may provide fine-tuning of reno-cardiovascular relationships, but seems, together with BNP, primarily involved in the regulation of cardiac performance and remodeling. © 2017 American Physiological Society. Compr Physiol 8:1211-1249, 2018.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

Effects of human atrial natriuretic peptide in patients after coronary artery bypass surgery

OBJECTIVES

To determine the effects of synthetic human atrial natriuretic peptide (ANP) on renal function, hemodynamics, and levels of vasoactive peptides when infused in the immediate postoperative period after coronary bypass surgery in patients with normal kidney function.

DESIGN

A prospective, randomized, double-blind, placebo-controlled study.

SETTING

The Department of Cardiothoracic Anaesthetics and Intensive Care of a university hospital.

PARTICIPANTS

Thirty patients with normal kidney function scheduled for elective coronary bypass surgery.

INTERVENTIONS

During the first 3 hours postoperatively, patients received an infusion of either ANP 7.5 pMol/kg/min (ANP) or vehicle alone (C).

MEASUREMENTS AND MAIN RESULTS

No differences were found between the two groups in respect to sex, degree of coronary disease, preoperative medical treatment, or duration of cardiopulmonary bypass and aortic occlusion. Plasma ANP levels increased nearly 10-fold from a mean of 7.0 +/- 1.1 pMol/L in ANP and remained at baseline levels in C, (p < 0.001). In ANP, there occurred significant increases in urine flow (p < 0.001), inulin clearance (p < 0.001), filtration fraction (p = 0.007), and fractional clearance of sodium (p < 0.001) and of osmoles (p < 0.001) compared with C. During the study, no differences in mean arterial pressure, heart rate, and right atrial or pulmonary capillary wedge pressure were detected between the groups. Cardiac index decreased by 5% in ANP compared with a 9% increase in C (p = 0.027). Vasopressin levels significantly increased in C but remained at baseline levels in ANP (p = 0.031). There were no changes in levels of catecholamines or angiotensin II.

CONCLUSIONS

The results of this study show that ANP increases diuresis, natriuresis, and glomerular filtration in the immediate postoperative period after coronary bypass surgery.

Effects of atrial natriuretic factor on norepinephrine release in the rat hypothalamus

The effects of atrial natriuretic factor on the mechanisms involved in norepinephrine release were studied 'in vitro' in slices of Wistar rat hypothalamus. Atrial natriuretic factor (10, 50 and 100 nM) decreased spontaneous [3H]norepinephrine secretion in a concentration dependent way. In addition, the peptide (10 nM) also reduced acetylcholine induced output of norepinephrine. The atrial factor (10 nM) was unable to alter the amine secretion when the incubation medium was deprived of calcium or when a calcium channel blocker such as diltiazem (100 microM) was added. In conclusion, atrial natriuretic factor reduced both spontaneous and acetylcholine evoked [3H]norepinephrine release in the rat hypothalamus. These findings suggest that the atrial natriuretic factor may alter catecholamine secretion by modifying the calcium available for the exocytotic process of catecholamine output.

Involvement of endogenous dopamine and DA-1 receptors in the renal effects of atrial natriuretic factor in rats

The precise mechanism of the diuretic and natriuretic effects of Atrial Natriuretic Factor (ANF) is still unclear. The present study was undertaken with the aim of elucidating the mechanism of interaction between ANF, dopamine (DA) and DA receptors in the renal effects of ANF in rats. In pentobarbital anesthetized rats, ANF infusion (10 micrograms/kg/hour) produced marked diuresis and natriuresis which was accompanied by significant hypotension, bradycardia and also a modest increase in glomerular filtration rate. However, there was no accompanying increase in urinary DA excretion during ANF infusion. Pretreatment with SCH 23390, a selective DA-1 receptor antagonist caused significant attenuation of the diuretic and natriuretic effects of ANF whereas the hemodynamic changes produced by ANF were still evident in SCH 23390 treated animals. Plasma ANF levels were elevated to the same magnitude in both the control and SCH 23390 treated groups. Pretreatment with carbidopa, a dopa decarboxylase inhibitor also significantly blunted the diuretic and natriuretic effects of ANF. The antagonism of the ANF-induced diuresis and natriuresis by SCH 23390 and carbidopa was comparable in magnitude. These results suggest that endogenous DA, via activation of DA-1 receptors plays a permissive role in the renal effects of ANF, perhaps by enhancing tubular responsiveness to ANF. However, there does not appear to be a stimulatory effect of ANF on renal DA production or release.