Rat atrial natriuretic peptides inhibit oxygen consumption by rat kidney

The effects of the atrial natriuretic peptide deficiency on renal cortical mitochondrial bioenergetics in the Dahl SS rat

Atrial Natriuretic Peptide (ANP) plays an important role in blood pressure regulation. Low levels of ANP correlate with the development of salt-sensitive hypertension (SS-HTN). Our previous studies indicated that ANP deficiency exacerbated renal function decline in SS-HTN. In the heart and fat tissue, ANP was reported to affect lipid peroxidation and mitochondrial bioenergetics but the effects of ANP on mitochondrial function in the kidney are unexplored. We hypothesized that ANP deficiency in SS-HTN causes renal bioenergetic shift, leading to disruption of mitochondrial network and oxidative stress. To address the hypothesis, we placed Dahl SS wild-type (SSWT) and ANP knockout (SSNPPA-/-) rats on 4% NaCl high salt (HS) diet to induce HTN or maintained them on 0.4% NaCl normal salt (NS) diet and assessed mitochondrial bioenergetics and dynamics using spectrofluorimetry, Seahorse assay, electron paramagnetic resonance (EPR) spectroscopy, Western blotting, electron microscopy, PCR and cytokine assays. We report that under high salt conditions, associated with hypertension and renal damage, the SSNPPA-/- rats exhibit a decrease in mitochondrial membrane potential and elevation in mitochondrial ROS levels compared to SSWT. The redox shift is also evident by the presence of more pronounced medullar lipid peroxidation in the SSNPPA-/- strain. We also revealed fragmented, more damaged mitochondria in the SSNPPA-/- rats, accompanied by increased turnover and biogenesis. Overall, our data indicate that ANP deficiency causes disruptions in mitochondrial bioenergetics and dynamics which likely contributes to aggravation of the renal damage and hypertension in the Dahl SS rat; the major pathological effects are evident in the groups subjected to a combined salt and ANP deficiency-induced mitochondrial stress.

Regulation of mitochondria function by natriuretic peptides

Natriuretic peptides (NPs) are well known to promote renal Na⁺ excretion, counteracting the effects of the renin-angiotensin-aldosterone system. Thus, NPs serve as a key component in the maintenance of blood pressure, influencing fluid retention capabilities via osmoregulation. Recently, NPs have been shown to affect lipolysis and enhance lipid oxidation and mitochondrial respiration. Here, we provide an overview of current knowledge about the relationship between NPs and mitochondria-mediated processes such as reactive oxygen species production, Ca²⁺ signaling, and apoptosis. Establishing a clear physiological and mechanistic connection between NPs and mitochondria in the cardiovascular system will open new avenues of research aimed at understanding and potentially using it as a therapeutic target from a completely new angle.

The mitochondrial component of intracrine action

In recent years the actions of intracellular-acting, extracellular signaling proteins/peptides (intracrines) have become increasingly defined. General principles of intracrine action have been proposed. Mitochondria represent one locus of intracrine action, and thus far, angiotensin II, transforming growth factor-beta, growth hormone, atrial natriuretic peptide, Wnt 13, stanniocalcin, other renin-angiotensin system components, and vascular endothelial-derived growth factor, among others, have been shown to be mitochondria-localizing intracrines. The implications of this mitochondrial intracrine biology are discussed.

Atrial natriuretic peptide inhibits renal glutamine extraction

The effect of administered atrial natriuretic peptide, ANP, on renal glutamine extraction, oxygen consumption and ammoniagenesis was determined in the intact functioning kidney of nonacidotic and chronically acidotic rats. Chronic acidosis shifted the metabolic fuel dependency towards glutamine, reflected by a 3.7 fold increase in extraction. Bolus injection of ANP, 1 microgram/100g BW, results in increased GFR, massive diuresis and an acid urine. Glutamine uptake fell in both groups, reversing from uptake to release in nonacidotic animals and dropping nearly 50 percent in acidotic rats. In contrast, oxygen consumption fell only 20 percent. Inhibition of glutamine extraction appears to be an indirect effect of ANP dependent upon the elevated GFR and elimination of glutamine uptake from the blood, but not from the filtrate. Efficacy of ANP, unlike classical diuretics, was not affected by the prevailing acid base condition nor a large shift in the fuel utilized.

Comparative effect of ANF and various diuretics on isolated nephron segments

The effect of the synthetic form of ANF 0.1 to 10 microgram/ml (peptide 101-126), a diuretic and natriuretic peptide isolated from rat heart atria, on the metabolism of dog and rat kidney tubules was studied in vitro and compared to that of furosemide (0.1 to 1 mM), hydrochlorothiazide (0.5 mM) or amiloride (0.1 mM). In order to pinpoint eventual site(s) of ANF action along the nephron, proximal tubules, thick ascending limbs and papillary collecting ducts were isolated from dog kidneys as well as proximal tubules from rat kidneys. The substrate uptake (O2, lactate, glutamine, glucose) and production of metabolites (glutamate, ammonium, alanine, glucose) by these nephron segments were measured in absence or presence of the diuretic agents or the vehicle for ANF (acetate 1 mM). The total ATP turnover and the contribution of identified metabolic pathways for this turnover was calculated. It was expected that a molecule with diuretic properties reducing the permeability of cell membranes to NaCl would secondarily reduce the Na-K-ATPase activity, and therefore the oxygen and substrate utilization by affected cells. It was shown: that each nephron segment used presented the expected specific metabolic characteristics; that furosemide markedly inhibits the oxidative metabolism of thick ascending limbs; that acetate (the vehicle used for ANF) displaces the oxidation of glutamine and lactate in nephron segments with aerobic metabolism; that ANF had no effect on the metabolism of the studied segments despite the presence of specific c'GMP-generating receptors in the distal nephron. It is concluded that ANF must exert its natriuretic effect by a mechanism different from that of classical diuretics.

The atrial natriuretic factor

In less than three years since the rapid and potent natriuretic response to intravenous injection of atrial myocardial extract in rats was reported the factor responsible for the diuretic, natriuretic, and vasodilating activity of the atrial homogenates was isolated, its chemical structure elucidated, and its total synthesis achieved. Also the cDNA and the gene encoding for the atrial natriuretic factor in mice, rats, and man have been cloned and the chromosomal site identified. The major effects of this hormone are vasodilatation, prevention and inhibition of the contraction induced by noradrenaline and angiotensin II, diuresis, and natriuresis associated in most instances with a pronounced increase in glomerular filtration rate and filtration fraction, inhibition of aldosterone secretion, and considerable stimulation of particulate guanylate cyclase activity. High density specific binding sites have been demonstrated in the zona glomerulosa of the adrenal cortex, in the renal glomeruli, and in the collecting ducts, and in the brain areas involved in the regulation of blood pressure and of sodium and water (AV3V region, subfornical organ, nucleus tractus solitarius, area postrema).

Atrial natriuretic hormones--thirty years after the discovery of atrial volume receptors

Twenty-five years after the discoveries of the existence of atrial granules and of volume receptors in the heart atria the search for natriuretic hormones has led to the isolation and identification of the atrial natriuretic factors (ANF) now considered as a hormonal system. These peptides are probably synthesized and stored in the Golgi apparatus of cardiac myocytes and are released in response to atrial wall stretch following acute plasma volume expansion and increased central blood volume, e.g., during head-out water immersion, in arterial hypertension, or increased left and/or right atrial pressure in cardiac failure, but also possibly in response to increased frequency of myocardial contractions, e.g. in paroxysmal tachycardia. The mechanisms of the renal action of these potent natriuretic hormones are not yet precisely known. Increased GFR may contribute to the initial rise in urinary sodium excretion and increased renal medullary blood flow to the later phase of natriuresis. The proximal tubule, the thin descending and the ascending limb of Henle's loop and especially the medullary collecting tubule were so far incriminated as tubular sites of action of ANF. Finally, recycling of sodium in medullary tissue and secretion of sodium via back-flux from the interstitium into the medullary collecting tubule are postulated to result in the hypernatric urine observed after ANF administration. Direct suppression of the secretion of renin, aldosterone, vasopressin, and vasopressin-stimulated cAMP synthesis may also contribute to its diuretic, natriuretic, and antihypertensive effects. The renal hemodynamic and tubular as well as the adrenal and systemic vascular effects are related to enhanced cGMP synthesis in medium-sized arterial vessels, in glomeruli and specific tubular segments, and in adrenal tissue, and may be calcium dependent. Specific ANF-binding sites were detected in these target organs. Although increased ANF release was observed in response to atrial distension in various disease states, which may contribute to renal sodium elimination in human hypertension and congestive heart failure, further studies are needed to identify its precise physiological and pathophysiological significance.