Opposite effects of enalapril and nitrendipine on natriuretic response to atrial natriuretic factor. Renal function evaluated with clearance studies in humans

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.

Influence of angiotensin converting enzyme inhibition and angiotensin II type 1 receptor antagonism on renal sodium and water handling and albuminuria during infusion of atrial natriuretic factor into healthy volunteers

BACKGROUND

Atrial natriuretic factor increases urinary sodium and water excretion. It also causes an increase in albuminuria. Angiotensin converting enzyme inhibition attenuates the effects of atrial natriuretic factor on renal sodium and water handling; however, it is not known whether this effect is mediated by the accompanied decrease in blood pressure or by suppression of the renin-angiotensin system.

OBJECTIVE

To test the hypothesis that atrial natriuretic factor mediates natriuresis and diuresis by inhibiting angiotensin II, by studying the effects of the angiotensin converting enzyme inhibitor enalapril and the angiotensin II type 1 receptor antagonist losartan. In addition, the effects of these drugs on atrial natriuretic factor-induced albuminuria were examined.

DESIGN AND METHODS

We investigated the effects of enalapril and losartan on atrial natriuretic factor-induced changes in urinary excretion of sodium, water and albumin from eight healthy volunteers. Measurements of systemic and renal haemodynamics in these subjects were performed before and during a 2 h infusion of atrial natriuretic factor [0.01 microg/kg per min (low dose) for the first 60 min and 0.02 microg/kg per min (high dose) for the second 60 min]. Measurements were performed after 5 days of pretreatment with placebo, 50 mg losartan or 20 mg enalapril daily.

RESULTS

Mean arterial pressures during the clearance study were 84.6 +/- 1.7 mmHg after placebo, 84.0 +/- 2.2 mmHg after losartan treatment and 80.0 +/- 2.5 mmHg after enalapril treatment (P < 0.05). Plasma renin activity was significantly increased both by losartan and by enalapril treatments. Neither enalapril nor losartan treatment attenuated atrial natriuretic factor-induced changes in renal haemodynamics. After placebo pretreatment, fractional urinary excretion of sodium increased significantly during infusion of atrial natriuretic factor. Losartan treatment did not influence the increase in urinary excretion of sodium during infusion of atrial natriuretic factor, whereas enalapril treatment significantly attenuated this increase (P < 0.01). Atrial natriuretic factor significantly increased albuminuria. Neither losartan nor enalapril treatment reduced atrial natriuretic factor-induced albuminuria.

CONCLUSIONS

Enalapril treatment lowered blood pressure and attenuated the atrial natriuretic factor-induced increase in urinary excretion of sodium. In contrast, the angiotensin II type 1 receptor antagonist losartan, at a dosage that did not lower blood pressure, did not attenuate the increase in urinary excretion of sodium. These data indicate that atrial natriuretic factor increases natriuresis and diuresis independently of angiotensin II. The increase in albuminuria during infusion of atrial natriuretic factor was not influenced by enalapril and losartan treatments.

Effects of urodilatin in the rat kidney: comparison with ANF and interaction with vasoactive substances

We compared the effects of urodilatin (URO) and atrial natriuretic factor (ANF) in normal and hydronephrotic kidneys (HNK) of rats. Furthermore, the impact of blocking different vasoactive hormones on the action of natriuretic peptides on vessels of cortical (C) and juxtamedullary (JM) glomeruli was studied in HNK by using URO. In normal kidneys, effects of URO and ANF (1.2, 2.4, 4.8, 12, and 19.10(-11) mol.kg-1.min-1 i.v.) were not significantly different. At 12.10(-11) mol.kg-1.min-1, URO and ANF increased urine flow 5.4 +/- 1.7 and 3.0 +/- 0.8-fold, increased urinary sodium excretion 20.7 +/- 8.8 and 10.3 +/- 4.0-fold, and decreased blood pressure by 13 +/- 2% and 12 +/- 1%, respectively (mean +/- SEM). In HNK, URO and ANF (0.4, 0.9, and 2.0.10(-11) mol.kg-1.min-1 i.v. and local application of 0.5, 1.0, and 2.0.10(-9) M) dose-dependent dilated preglomerular vessels (max approximately 20%), constricted efferent arterioles (max approximately 15%), and increased glomerular blood flow of C glomeruli in an identical fashion. Comparing URO effects on C and JM arterioles (0.4 and 0.9.10(-11) mol.kg-1.min-1 i.v.), JM responses were about one third of C responses. Angiotensin converting enzyme inhibition (ACEI, 2.10(-6) mol.kg-1 quinapril i.v.), combined ACEI and cyclooxygenase inhibition (CYOI, 2.8.10(-5) M indomethacin), and endothelin (ET) receptor blockade (10(-6) M BQ 123 and IRL 1038) diminished preglomerular vasodilation (C and JM) caused by URO infusion. Efferent vasoconstriction (C and JM) caused by URO was exaggerated by blockade of nitric oxide synthesis (10(-5) M L-NAME) and abolished by combined ACEI and CYOI, by bradykinin receptor blockade (4.10(-8) M Hoe 140), and by ET blockade. CYOI attenuated only JM efferent constriction. Our results show that URO and ANF possess equipotent vascular and similar natriuretic effects in the rat kidney. The magnitude of preglomerular vasodilation, which is directly mediated by these peptides, depends on the basal level of endogenous vasoconstrictors, while efferent vasoconstriction may be mediated by the secondary release of ET.

Atrial natriuretic factor: its (patho)physiological significance in humans

The first human studies using relatively high-doses of ANF revealed similar effects as observed in the preceding animal reports, including effects on systemic vasculature (blood pressure fall, decrease in intravascular volume), renal vasculature (rise in GFR, fall in renal blood flow), renal electrolyte excretion (rises in many electrolytes), and changes in release of a number of different hormones. Whether all these changes are the result of direct ANF effects or secondary to a (single) primary event of the hormone remains to be determined. Certainly, it has been proven that more physiological doses of ANF fail to induce short-term changes in many of these parameters leaving only a rise in hematocrit, natriuresis and an inhibition of the RAAS as important detectable ANF effects in humans. This leads us to hypothesize that ANF is a "natriuretic" hormone with physiological significance. The primary function in humans is to regulate sodium homeostasis in response to changes in intravascular volume (cardiac atrial stretch). Induction of excess renal sodium excretion and extracellular volume shift appear to be the effector mechanisms. The exact mechanism of the natriuresis in humans still needs to be resolved. It appears however, that possibly a small rise in GFR, a reduction in proximal and distal tubular sodium reabsorption, as well as an ensuing medullary washout, are of importance. The pathophysiological role of ANF in human disease is unclear. One may find elevated plasma irANF levels and/or decreased responses to exogenous ANF in some disease states. Whether these findings are secondary to the disease state rather than the cause of the disease remains to be resolved. Therapeutic applications for ANF, or drugs that intervene in its production or receptor-binding, seem to be multiple. Most important could be the antihypertensive effect, although areas such as congestive heart failure, renal failure, liver cirrhosis and the nephrotic syndrome cannot be excluded. Although the data that have been gathered to date allowed us to draw some careful conclusions as to the (patho)physiological role of ANF, the exact place of ANF in sodium homeostatic control must still be better defined. To achieve this, we will need more carefully designed low-dose ANF infusion, as well as ANF-breakdown inhibitor studies. Even more promising, however, is the potential area of studies open to us when ANF-receptor (ant)agonists become available for human use.