Atrial natriuretic factor in NaCl-sensitive and NaCl-resistant spontaneously hypertensive rats

High salt-induced weakness of anti-oxidative function of natriuretic peptide receptor-C and podocyte damage in the kidneys of Dahl rats

BACKGROUND

Atrial natriuretic peptide (ANP) and its natriuretic peptide receptors A (NPR-A) and C (NPR-C) are involved in the regulation of physiological and pathophysiological process of blood pressure. The present study aimed to determine the role of NPR-C in the development of salt-sensitive hypertension.

METHODS

The Dahl salt-sensitive (DS) and salt-resistant (DR) rats were used in this study. Animals were matched according to their age and weight, and then placed on either a high-salt (HS, 8%) or a normal-salt (NS, 0.4%) diet for 6 weeks randomly using random number table. The systolic blood pressure (SBP), plasmatic sodium concentration (PLNa), urinary sodium excretion (UVNa), and serum creatinine concentration (Scr) were measured. The concentration of ANP in blood and tissues (heart and kidney) was detected by enzyme-linked immunosorbent assay. The expression of ANP, NPR-A, and NPR-C in kidney was evaluated with western blot analysis. Regarding renal redox state, the concentration changes in malondialdehyde (MDA), lipofuscin, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox), and nitric oxide synthase (NOS) in kidney were detected by a spectrophotometric method. The kidney damage was evaluated using pathological techniques and the succinodehydrogenase (SDHase) examination. Furthermore, after an intra-peritoneal injection of C-atrial natriuretic peptide (ANP)4-23 (C-ANP4-23), an NPR-C receptor agonist, the SBP, biochemical values in blood and urine, and renal redox state were evaluated. The paired Student's t test and analysis of variance followed by the Bonferroni test were performed for statistical analyses of the comparisons between two groups and multiple groups, respectively.

RESULTS

The baseline SBP in all groups was within the normal range. At the end of the 6-week experiment, HS diet significantly increased the SBP in DS rats from 116.63 ± 2.90 mmHg to 162.25 ± 2.15 mmHg (t = -10.213, P < 0.001). The changes of SBP were not significant in DS rats on an NS diet and DR rats on an NS diet or on an HS diet (all P > 0.05). The significant increase of PLNa, UVNa, and Scr related to an HS diet was found in both DS and DR rats (all P < 0.05). However, significant changes in the concentration (t = -21.915, P < 0.001) and expression of renal ANP (t = -3.566, P = 0.016) and the expression of renal NPR-C (t = 5.864, P = 0.002) were only observed in DS hypertensive rats. The significantly higher desmin immunochemical staining score (t = -5.715, P = 0.005) and mitochondrial injury score (t = -6.325, P = 0.003) accompanied by the lower SDHase concentration (t = 3.972, P = 0.017) revealed mitochondrial pathologic abnormalities in podocytes in DS rats with an HS diet. The distinct increases of MDA (t = -4.685, P = 0.009), lipofuscin (t = -8.195, P = 0.001), and Nox (t = -12.733, P < 0.001) but not NOS (t = -0.328, P = 0.764) in kidneys were also found in DS hypertensive rats. C-ANP4-23 treatment significantly decreased the SBP induced by HS in DS rats (P < 0.05), which was still higher than NS groups with the vehicle or C-ANP4-23 treatment (P < 0.05). Moreover, the HS-induced increase of MDA, lipofuscin, Nox concentrations, and Nox4 expression in DS rats was significantly attenuated by C-ANP4-23 treatment as compared with those with HS diet and vehicle injection (all P < 0.05).

CONCLUSIONS

The results indicated that the renal NPR-C might be involved in the salt-sensitive hypertension through the damage of mitochondria in podocytes and the reduction of the anti-oxidative function. Hence, C-ANP4-23 might serve as a therapeutic agent in treating salt-sensitive hypertension.

Dietary salt supplementation selectively downregulates NPR-C receptor expression in kidney independently of ANP

Atrial natriuretic peptide (ANP) has negative modulatory effects on a variety of pathophysiological mechanisms; i.e., it inhibits hypoxia-induced pulmonary vasoconstriction and vascular remodeling and facilitates natriuresis and vasorelaxation in NaCl-supplemented subjects. We have previously demonstrated organ-selective potentiation of ANP in the pulmonary circulation of hypoxia-adapted animals by local downregulation of its clearance receptor (NPR-C; Li H, Oparil S, Meng QC, Elton T, and Chen Y-F. Am J Physiol Lung Cell Mol Physiol 268: L328-L335, 1995). The present study tested the hypothesis that NPR-C expression is attenuated selectively in kidneys of NaCl-supplemented subjects. Adult male wild-type (ANP+/+) and homozygous mutant (ANP-/-) mice were studied after 5 wk of normal or high-salt diets. Mean arterial pressure (MAP) and left (LV) and right ventricular (RV) mass were greater in ANP-/- mice than in ANP+/+ mice fed the normal-salt diet; salt supplementation induced increases in plasma ANP in ANP+/+ mice and in MAP and LV, RV, and renal mass in ANP-/- mice but not in ANP+/+ mice. NPR-C mRNA levels were selectively and significantly reduced (>60%) in kidney, but not in lung, brain, LV, or RV, by dietary salt supplementation in both genotypes. NPR-A mRNA levels did not differ among diet-genotype groups in any organ studied. cGMP content was significantly increased in kidney, but not in lung or brain, by dietary salt supplementation in both genotypes. These findings suggest that selective downregulation of NPR-C in the kidney in response to dietary salt supplementation may contribute to local elevation in ANP levels and may be functionally significant in attenuating the development of salt-sensitive hypertension.

Vascular, renal, and endocrine responses to low-dose atrial natriuretic peptide in the fluid-balanced New Zealand genetically hypertensive rats with and without endogenous arginine vasopressin

In hypertension, the relationship between atrial natriuretic peptide (ANP) and vasopressin (AVP) is not yet clear, although their renal actions are effectively autoregulation. To examine the possible interaction further, the responses to ANP infusion (75 ng·min-1, i.v.) have been investigated in both hypertensive and normotensive AVP-replete (HT and NT) and AVP-deficient (HTDI and NTDI) rats. This study aimed to assess the renal function and the plasma hormone concentrations of AVP, angiotensin II (AII), ANP, aldosterone, and corticosterone in the conscious, chronically catheterized, fluid-balanced rats, and to examine the cardiovascular, renal, and endocrine responses to a constant infusion of a low-dose ANP. Data gained from the present study showed, for the first time, the hormone profile, plasma electrolyte composition, and detailed renal function of the servo-controlled, fluid-balanced rats. The similarities of plasma electrolyte composition between servo-controlled and untreated rats indicated that the servo-controlled fluid replacement technique maintained the differences between the strains and maintained body fluid balance during the experimental periods. Following ANP administration, there were no changes in glomerular filtration rate (GFR) in all groups, but an enduring diuresis and natriuresis were observed in HT and NT, which were milder in HTDI rats. However, the hypotensive effect of ANP was of a similar magnitude in all rat strains. HTDI rats exhibited an inhibition of the renin-angiotensin system (RAS), which may have participated in the reduced mean arterial blood pressure (MAP) and natriuresis observed in these rats. The renal actions of ANP appear to rely upon renal tubular events, as indicated by increased fractional electrolyte excretions in the AVP-replete rats. This study highlights the importance of AVP to the profile of the renal actions of ANP in normal rats.Key words: atrial natriuretic peptide, arginine vasopressin, renal, hypertension.

Coronary reperfusion enhances recovery of atrial natriuretic peptide secretion. Salvaging endocrine function in patients with acute right ventricular infarction

BACKGROUND

The heart has been demonstrated not only to be a pumping organ but also an endocrine organ secreting atrial natriuretic peptide (ANP). We hypothesized that myocardial ischemia may affect ANP secretion and that reperfusion therapy for acute myocardial infarction can preserve endocrine function of the heart.

METHODS AND RESULTS

Twenty patients with acute right ventricular infarction were examined who underwent reperfusion therapy on admission. These patients had proximal occlusion of the dominant right coronary artery involving the right atrial branches: 9 patients with successful reperfusion (SRP group) and the remaining 11 patients with unsuccessful reperfusion (URP group). Within 24 hours after the onset of infarction, a volume loading test was performed after reperfusion therapy with measurements for plasma ANP levels and hemodynamics. Before the volume loading test, the plasma ANP level and mean right atrial pressure were similar between these two groups. However, in the URP group, percent increase in ANP in response to volume loading was strikingly smaller (URP, 45 +/- 18% versus SRP, 133 +/- 25%; P < .01) despite similar percent increase in mean right atrial pressure (URP, 100 +/- 46% versus SRP, 86 +/- 23%). The peak ANP level occurred significantly later in the URP group (69 +/- 16 hours) than in the SRP group (28 +/- 9 hours, P < .001) after the onset of infarction.

CONCLUSIONS

The response of ANP release to volume loading is attenuated in patients with right ventricular infarction without coronary reperfusion. However, successful reperfusion induces a rapid recovery of cardiac endocrine function as well as its mechanical function. A sufficiently elevated plasma ANP level may be a useful predictor of hemodynamic improvement in patients with right ventricular infarction.

Antihypertensive effect of cicletanine is exaggerated in NaCl-sensitive hypertension

Cicletanine (CIC), a furopyridine derivative, lowers blood pressure in hypertensive animals and humans. We have previously identified an NaCl-sensitive substrain of spontaneously hypertensive rat (SHR-S) that displays enhanced sensitivity to the depressor effects of exogenous atrial natriuretic peptide (ANP) when fed a high NaCl diet. The current study tested the hypotheses that CIC has an exaggerated antihypertensive effect in NaCl-supplemented SHR-S and that this effect might be ANP dependent. CIC (40 mg/kg/day) or vehicle was administered by gavage in a single daily dose for three weeks beginning immediately prior to initiation of 1% or 8% NaCl diets in seven-week-old male SHR-S. CIC significantly decreased mean arterial pressure (MAP) and the ratio of left ventricular and septum weight to body weight (LV + S/BW) in both 8% NaCl- and 1% NaCl-fed SHR-S. The depressor effect of CIC was greater in the 8% NaCl group (-26 mmHg) than in the 1% NaCl group (-13 mmHg). CIC was associated with significant reduction in RAP in the 8% NaCl group but not in the 1% NaCl group. Neither CIC treatment nor 8% NaCl significantly altered plasma ANP or cyclic guanosine monophosphate (GMP) levels in plasma, aorta, or kidney. CIC was associated with significant decreases in plasma norepinephrine (NE) levels in the 1% NaCl group but not in the 8% NaCl group. The data demonstrate that the antihypertensive effect of CIC is exaggerated in NaCl-sensitive hypertension. The antihypertensive effect of CIC appears not to be related to ANP or cyclic GMP but may be related to a combination of a sympatholytic and natriuretic/diuretic effects in SHR-S.

High NaCl diet enhances arterial baroreceptor reflex in NaCl-sensitive spontaneously hypertensive rats

Previous studies from our laboratory have shown that arterial baroreceptor reflex control of lumbar sympathetic nerve activity is blunted in the NaCl-sensitive spontaneously hypertensive rat (SHR-S) compared with either the NaCl-resistant spontaneously hypertensive rat (SHR-R) or the normotensive Wistar-Kyoto (WKY) rat. In the current study, the effect of dietary NaCl supplementation on arterial baroreceptor reflex control of lumbar sympathetic nerve activity and heart rate was assessed in SHR-S and control SHR-R and WKY rats. Male SHR-S, SHR-R, and WKY rats were fed diets containing either 1% or 8% NaCl beginning at 7 weeks of age and were studied at age 9-10 weeks. Arterial baroreceptor reflex-mediated changes in lumbar sympathetic nerve activity and heart rate were recorded in conscious, unrestrained rats during phenylephrine-induced (15-40 micrograms/kg/min) and nitroprusside-induced (15-300 micrograms/kg/min) changes in mean arterial pressure. SHR-S maintained on a 1% NaCl diet had blunted baroreceptor reflex control of lumbar sympathetic nerve activity during acute increases in MAP compared with SHR-R and WKY rats (p less than 0.05). After ingestion of the 8% NaCl diet, this blunting was absent, indicating enhancement of baroreceptor reflex control of lumbar sympathetic nerve activity. SHR-S maintained on a 1% NaCl diet also had blunted arterial baroreceptor control of lumbar sympathetic nerve activity during nitroprusside-induced decreases in mean arterial pressure compared with WKY rats, but this was not significantly altered during ingestion of the 8% NaCl diet.(ABSTRACT TRUNCATED AT 250 WORDS)