EBM Email Content Delivery
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Lee, K. S.
Right arrow Articles by Kim, S. H.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Lee, K. S.
Right arrow Articles by Kim, S. H.
Experimental Biology and Medicine 229:65-71 (2004)
© 2004 Society for Experimental Biology and Medicine

ORIGINAL RESEARCH ARTICLE

Different Responses of Atrial Natriuretic Peptide Secretion and Its Receptor Density to Salt Intake in Rats

Kyung Sun Lee, So Young Kim, Jeong Hee Han, Yun Ah Kim, Chunhua Cao, Sung Zoo Kim, Kyung Woo Cho and Suhn Hee Kim1

Department of Physiology, Medical School, Institute for Medical Sciences, Chonbuk National University, Jeonju 561-180, Korea

1To whom requests for reprints should be addressed at 2–20 Keum-Am-Dong-San, Department of Physiology, Chonbuk National University Medical School, Jeonju 561-180, Korea. E-mail: shkim{at}moak.chonbuk.ac.kr


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
This study investigated whether high-salt intake influences atrial natriuretic peptide (ANP) system, atrial content, and release rate of ANP as well as receptor density in the kidney were measured in salt intake rats. Male Sprague-Dawley rats received either 0.9% or 2% salt in their drinking water for 10 days. The stretch-induced ANP secretion from isolated perfused nonbeating left atria was accentuated, and the production of cGMP by ANP in renal cortical tissue membranes were pronounced in rats exposed to 0.9% salt for 10 days but not in rats exposed to 2% salt. The levels of ANP receptor density and expression in renal cortex were decreased in 2% salt intake rats but not in 0.9% salt intake rats. No significant differences in atrial and plasma concentrations of ANP and water balance were observed in both salt intakes. Therefore, these results suggest that atrial ANP secretion and its binding sites in the kidney may respond differently to ingested salt concentrations in rats.

Key Words: stretch • receptor • kidney • PCR • cGMP • membrane • atrium


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Atrial natriuretic peptide (ANP) is a cardiac peptide hormone that is secreted primarily by atrial myocytes in response to atrial wall stretch (1–3). ANP elicits a number of physiological effects including diuresis, natriuresis, vasorelaxation, and inhibitions of the release of vasopressor hormones (4, 5). ANP is an antagonistic hormone to renin-angiotensin-aldosterone system in the regulation of blood pressure. The synthesis of ANP is regulated by developmental, hemodynamic, and hormonal factors (5). Atrial myocytes constitute a major site of synthesis and secretion of ANP, and ventricular myocytes also synthesize ANP during the fetal life (6, 7). After birth, the synthesis of atrial ANP gradually increases, whereas that in the ventricles markedly decreases (6). The ventricular synthesis of ANP, however, is reactivated in pathological conditions associated with cardiac hypertrophy (8–10).

A variety of stimuli such as pressure and volume overloads, dexamethasone, thyroid hormone, and endothelin is known to regulate ANP gene expression (4, 5). Especially, the relationship between ANP system and volume overload induced by salt intake has been extensively studied since the discovery of ANP. Despite several investigations, this relationship is still controversial (11–15). It may be due to many possible factors such as the degree of volume load, route of salt intake, salt concentration, duration of exposure, hydration status before exposure, seasonal variation, or species. They may influence differently transcription and translation levels, release rate of ANP, and its binding to the receptor. A few studies have been done using in vitro ANP secretion from rat atria chronically exposed to salt. Recently Wolf and Kurtz (15) suggested that ANP gene expression and secretion are probably not involved in water balance during chronic salt loading. Therefore, the purpose of the present study was to investigate whether high-salt intake influences atrial content of ANP and secretion rate as well as its receptor density in kidney.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Animals.
Male Sprague-Dawley rats weighing 230–250 g were stabilized for 5 days in metabolic cages, and urine was collected for 3 days as a control period. Rats drank either 0.9% or 2% salt solution or tap water for a given number of days, and urine was collected. Water balance was calculated by subtracting urine volume from water intake. Urinary excretion of Na+ was measured using flamephotometry.

Blood Collection and Tissue Preparation.
On the day of experiments, rats were sacrificed by decapitation, and blood was collected into a prechilled tube containing aprotinin (200 kallikrein inhibitory units [KIU]/ml), soybean trypsin inhibitor (SBTI, 50 N-a-benzoyl-L-arginine ethyl ester units/ml), phenylmethylsulfonyl fluoride (PMSF, 600 µM/ ml), and EDTA (2.7 mM/ml). Blood was centrifuged at 10,000 g for 15 mins at 4°C, and plasma was kept at -70°C. Plasma ANP was extracted using Sep-Pak C18 cartridge (Waters Associates, Milford, MA) as described previously (16, 17). Both atria were separated, weighed, and kept in 2 ml of 0.1 N acetic acid at 4°C. Tissues were boiled for 10 mins, homogenized with Polytron homogenizer, and centrifuged at 10,000 g for 15 mins at 4°C. The concentrations of ANP in plasma extracts and supernatants of tissue homogenates were measured by specific radioimmunoassay as described previously (16).

Isolated Perfused Atrial Preparation.
An isolated perfused atria was prepared by the method described previously (18). Briefly, the left atrium was rapidly inserted with a Tygon cannula and secured by a ligature. The cannulated atrium was fixed into the organ chamber with water-tight silicone rubber cap and perfused with oxygenated HEPES buffer solution (pH 7.4) at a rate of 0.4 ml/min using a peristatic pump. The composition of HEPES buffer solution was as follows: NaCl 118 mM, KCl 4.7 mM, CaCl2 2.5 mM, MgSO4 1.2 mM, NaHCO3 25 mM, HEPES 10 mM, glucose 10 mM, and bovine serum albumin (BSA) 0.1%. The pericardial HEPES buffer solution, which contained [3H]-inulin for the measurement of translocation of extracellular fluid (ECF), was oxygenated by silicone tubing coils located inside the organ chamber. The pericardial space of the organ chamber was sealed and connected with a calibrated microcapillary tube, through which atrial volume was monitored. After stabilization for 30 mins, perfusates were collected two times at 2-min intervals, and then atrial distension was induced for 2 mins by elevating the position of outflow catheter tip to 1 cmH2O. Atrial contraction was induced by lowering the position of outflow catheter tip to the basal level. Intra-atrial pressure was subsequently increased from 0 to 2, 4, 6, or 10 cm H2O every 8 mins. A radioimmunoassay (RIA) for ANP (19) in perfusates was done on the day of experiments, and all samples in an experiment were analyzed in a single assay.

Measurement of ECF Translocation.
The ECF translocated from atria was measured as described previously (20). Radioactivities of [3H]-inulin in atrial perfusate and pericardial buffer solution were measured with a liquid scintillation counter, and the amounts of ECF translocated through the atrial wall were calculated by total radioactivity in perfusate (counts per minute/minute) divided by radioactivity in pericardial reservoir (counts per minute/milliliter) and atrial wet weight (milligrams).

Activation of Particulate Guanylyl Cyclase (GC) in Renal Tissue Membranes and RIA of cGMP.
Particulate GC activity was measured by determination of cGMP generated in protein aliquots of renal tissue membranes according to a method described previously (21, 22). Briefly, renal cortex and medulla were separately homogenized at 4°C in 30 mM phosphate buffer (pH 7.2) containing 120 mM NaCl and 1 mM phenanthrolene by three 30-sec bursts of 27,000 rpm. The homogenates were centrifuged at 8,000 g for 10 mins at 4°C, and the supernatants were recentrifuged at 36,000 g for 60 mins at 4°C. The membrane pellets were washed three times with 50 mM Tris-HCl (pH 7.6) and resuspended in this solution. Five-microgram protein aliquots of the membrane suspension were incubated at 37°C for 15 mins in 50 mM Tris-HCl (pH 7.6) containing 1 mM isobutyl-1-methylxanthine, 1 mM GTP, 0.5 mM ATP, 15 mM creatine phosphate, 80 µg/ml creatine phosphokinase, and 4 mM MgCl2 and various concentrations of ANP. The reaction was stopped by the addition of 375 µl of cold 50 mM sodium acetate (pH 5.8) and boiling for 5 mins.

In Vitro Autoradiography.
For in vitro receptor autoradiography, the kidney was rapidly removed and snap frozen in liquid nitrogen. Transverse plane sections (20 µm) of kidney were cut in a cryostat at -20°C, thaw mounted on gelatin-chrome-alum–coated slides, and then dried in a desiccator overnight at 4°C. Binding conditions of [125I]-ANP to the kidney section were prepared according to a method described elsewhere (23). To measure binding constants, the competitive inhibition of [125I]-ANP binding of the kidney was determined in consecutive sections by coincubating with various concentrations of unlabeled ANP and 250 pM of [125I]-ANP. Subsequently, the sections were rinsed three times with cold distilled water at 4°C and quickly dried under a stream of cold air. Autoradiographic images were generated by exposure of renal sections to Hyperfilm-3H (Amersham International, Little Chalfont, UK) with [125I]-labeled polymer standard strips (Amersham International) at room temperature for 3 days. Autoradiographic images were viewed with a Leica Wild M420 macroscope (Heerbrugg, Switzerland) and captured using a Sony video camera (Tokyo, Japan) with CCD iris and a Hamamatsu AC adaptor (Tokyo, Japan) connected to a Power Macintosh 8100/80AV computer (Uxbridge, UK). The number of ligand-binding sites of different affinities, the apparent Kd, and the maximal binding capacities (Bmax) on renal glomerulus and medulla were derived separately in each individual using the LIGAND iterative model-fitting computer program.

Semiquantitative Reverse Transcription–Polymerase Chain Reaction (RT-PCR) of Messenger RNAs (mRNAs) for Natriuretic Peptide Receptors (NPRs).
Semiquantitative RT-PCR was performed as described previously (23). One microgram of RNA was suspended in 20 µl RT buffer containing 50 mM Tris (pH 8.3), 75 mM KCl, 3 mM MgCl2, 0.125 mM each of deoxynucleotide triphosphates (dNTPs), 20 U ribonuclease inhibitor, 5 µM random hexamers, and 200 U SuperScript II reverse transcriptase (Invitrogen, Carlsbad, CA). Messenger RNA was reverse transcribed at room temperature for 10 mins and 42°C for 50 mins. The reaction was stopped by heat inactivation for 15 mins at 99°C and then chilled on ice. Complementary DNA products were amplified by PCR with sense and antisense primers.

NPR-A sense 5'-CGCATTGAGTTGACACGA-3'(8925–8942)

NPR-A antisense 5'-CCCGTCTACCACACAGTT-3'(9837–9854)

NPR-C sense 5'-GGGCAGTGAGCGAGTGGT-3' (757–774)

NPR-C antisense 5'-CCCCATCCTTCTTGCTGT-3' (1140–1157)

Twenty microliters of PCR buffer contained 2 mM Tris (pH 8.3), 10 mM KCl, 1.5 mM MgCl2, 125 µM each of dNTPs, 5 U Taq polymerase, and 10 pM each of sense and antisense primers. The temperature profile of amplification consisted of 1 min of denaturation at 95°C, 1 min annealing at 65°C, and 2 min extension at 72°C for 40 cycles. PCR products were separated with 2% agarose gels, and bands were visualized by ethidium bromide staining and photographed using Polaroid 665 film (Polaroid, Cambridge, MA). Intensity of the bands was quantified using a densitometer, and the relative amount of mRNAs for NPRs was calculated using glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as an internal standard gene.

Statistical Analysis.
The results are given as means ± SEM. The statistical significance of the differences was analyzed by repeated-measures analysis of variance (ANOVA) (Fig. 1A–DGo) or one-way ANOVA (Tables 1Go and 2Go) followed by Bonferroni’s multiple-comparison test. Two-way ANOVA analysis for repeated measures was also used. Student’s unpaired t test was used for analysis of two groups. The critical level of significance was set at P < 0.05.



View larger version (34K):
[in this window]
[in a new window]
  		Figure 1. Changes in water intake (A), water balance (B), urine volume (C), urinary excretion of sodium (D), and ANP concentrations in atria and plasma (E) by 0.9% and 2% salt intake. Persistent water retention, diuresis, and natriuresis were observed, but atrial and plasma concentrations of ANP did not change during 0.9% salt intake. The average of each parameter obtained form individual rat for 6 days after exposure of salt was calculated and shown in Figure 1aGo–d. No significant differences in water intake, water balance, urine volume, and sodium excretion were found between two salt intake groups. Arrow indicates the start of salt intake. UNaV, urinary excretion of sodium; LA, RA, left and right atria, respectively; PANP, plasma ANP concentration; 1D, 3D, 6D, first, third, and sixth days of 0.9% salt intake. *, **, significantly different from control group, P < 0.05 and P < 0.01, respectively.

 

View this table:
[in this window]
[in a new window]
  		Table 1. Chronic Effects of Salt Intakes for 10 Days on Atrial and Plasma Concentrations of ANPa
 

View this table:
[in this window]
[in a new window]
  		Table 2. Mean Values of Binding Constants for Regionally Specific Reversible Binding of 125I-ANP in the Kidney of Control and Salt Intake Ratsa
 

   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Changes in Water Intake and Renal Functions by Salt Intake.
Water intake, urine volume, and urinary excretion of sodium were increased in terms of time during 0.9% salt intake (n = 15, all P < 0.05). However, responsiveness of water intake to 2% salt intake was variable at the beginning of exposure and then tended to increase after the fifth day (n = 8, all P > 0.05). Urine volume and urinary excretion of sodium tended to increase without significance. The average of each parameter obtained form individual rat for 6 days after exposure of salt was calculated and shown in Figure 1a–dGo. With both salt intakes, marked natriuresis and diuresis appeared, as compared with control rats (Fig. 1cGo and 1dGo). However, no significant differences in water intake, water balance, urine volume, and urinary excretion of sodium were found between 0.9% and 2% salt intake rats (Fig. 1a–dGo).

Changes in Atrial and Plasma ANP Concentrations.
Atrial and plasma concentrations of ANP did not change significantly at 12 hrs or on the first, third, and sixth day of 0.9% salt intake (Fig. 1EGo). With 0.9% salt intake for 10 days, atrial and plasma concentrations of ANP also did not change (Table 1Go). With increasing salt concentration by 2% for 10 days, atrial and plasma concentrations of ANP did not change except the ratio of right atrial ANP concentration to left side (Table 1Go).

Stretch-Induced ANP Secretion from Atria of Salt Intake Rats.
Figure 2Go shows atrial volume, ANP secretion, and ECF translocation in response to elevations in intra-atrial pressure from left atria of rats receiving 0.9% salt solution (0.9% salt rats, n = 10) for 10 days. Changes in atrial volume were proportionate to intra-atrial pressure in both groups and no significant differences in atrial volume were found (Fig. 2BGo). The basal rate of ANP secretion from atria of 0.9% salt rats was constant and not significantly different from control atria (Fig. 2CGo). Increases in atrial volume caused proportional increases in ANP secretion, which tended to increase in 0.9% salt rats (Fig. 2CGo).



View larger version (25K):
[in this window]
[in a new window]
  		Figure 2. Changes in atrial pressure (A), volume (B), ANP secretion (C), and ECF translocation (D) from left atria of rats exposed to 0.9% NaCl for 10 days. Atrial volume change was increased by the elevation of intra-atrial pressure. A stepwise increase in atrial volume resulted in proportional increases in ANP secretion and concomitant ECF translocation. The stretch-activated ANP secretion tended to increase in 0.9% salt intake rats. Open column and dot indicate control rats (CONT). Closed column and dot indicate 0.9% salt intake rats (0.9% NaCl). Symbols are mean ± SEM.

 
We have previously shown that the ANP secretion is regulated by two-step sequential mechanism, the first being the release of ANP from atrial myocytes into the extracellular space, followed by the translocation of ECF with the released ANP into the atrial lumen with atrial contraction (21, 20). Therefore, the translocation of ECF using [3H]-inulin clearance was measured simultaneously with the secretion of ANP. The basal ECF translocation was not significantly different from control atria (Fig. 2DGo). Increases in atrial volume caused increases in mechanically stimulated ECF translocation in both groups, and no significant differences in ECF translocation between them were found.

To compare the response of ANP secretion to atrial stretch between two groups, changes in ANP secretion and ECF translocation were calculated by subtracting the previous two values from the two peak values at each pressure. Figure 3A–DGo shows positive relationships among atrial volume, relative changes in ECF translocation, and ANP secretion in response to atrial stretch in 0.9% salt rats and control rats. Increases in atrial pressure caused proportional increases in the atrial volume (Fig. 3AGo), which closely correlated with mechanically stimulated ECF translocation in both groups (Fig. 3CGo). Increases in atrial volume caused proportional increases in ANP secretion (Fig. 3BGo), which closely correlated with mechanically stimulated ECF translocation (Fig. 3CGo). The positive relationship between ANP secretion and atrial volume, ANP secretion, and ECF translocation shifted leftward and upward by 0.9% salt intake (Fig. 3BGo and 3DGo). However, in 2% salt rats, no significant differences in relationships among changes in atrial volume, ANP secretion, and ECF translocation were found (Fig. 3EGo and 3FGo n, = 10). Therefore, changes in stretch-induced ANP secretion in terms of ECF translocation were markedly increased from 0.328 ± 0.029 to 0.513 ± 0.043 ng/µl (P < 0.01) by 0.9% salt intake but not in 2% salt intake (Fig. 4Go).



View larger version (26K):
[in this window]
[in a new window]
  		Figure 3. Relationships among atrial pressure, volume, ECF translocation, and ANP secretion in left atria of rats exposed to 0.9% NaCl (A–D) and 2% NaCl (E, F) for 10 days. Close correlations between those parameters were observed. The relationships between ANP secretion and atrial volume (B) and ANP secretion and ECF translocation (D) in 0.9% salt intake rats shifted to the left but not in 2% salt intake rats (F). *, significantly different from control group, P < 0.05.

 


View larger version (23K):
[in this window]
[in a new window]
  		Figure 4. Comparisons of ANP secretion in terms of ECF translocation from left atria of rats exposed to either 0.9% or 2% NaCl. Data were derived from Figure 3Go. Average of changes in stretch-activated ANP secretion in terms of ECF translocation was accentuated in 0.9% salt intake rats. **, significantly different from control group, P < 0.01.

 
Changes in Particulate GC Activity and ANP-Binding Sites in Kidney.
To evaluate changes in renal GC-A activity by salt intake for 10 days, the amounts of cGMP production by ANP were measured in tissue membranes of renal cortex and medulla (all n = 9). The basal cGMP production in renal cortical membranes was similar among three groups, but that in renal medullary membranes of 2% salt rats was lower than control rats. As shown in Figure 5Go, ANP caused increases in cGMP production in a dose-dependent manner in renal cortical membranes. The cGMP production by ANP showed accentuation in 0.9% salt rats but not in 2% salt rats, compared with the control rats. In renal medullary membranes, the cGMP production by 10-7M ANP was much higher than in renal cortical membranes of control rats (2.91 ± 0.76 vs. 1.17 ± 0.05-fold increase, P < 0.05). However, the relative increase in cGMP production by ANP in renal medullary membranes was not modified by intakes of either salt concentration.



View larger version (36K):
[in this window]
[in a new window]
  		Figure 5. The cGMP production stimulated by ANP in renal tissue membranes of rats exposed to 0.9% or 2% NaCl. ANP caused proportionate increases in cGMP production in renal cortical membranes in a dose-dependent manner, which was accentuated in 0.9% NaCl intake rats but not in 2% NaCl intake rats. The basal cGMP production in renal medullary membranes of 0.9% NaCl intake rats was lower than in control rats. The relative change in cGMP production by ANP in control rats was similar to salt intake rats. *, significantly different from control group, P < 0.05.

 
To define whether the accentuated response of cGMP production by ANP in 0.9% salt rats is due to changes in ANP receptor population or its affinity, characteristics of [125I]-ANP-binding sites in the kidney were evaluated using in vitro autoradiography. Binding sites for [125I]-ANP were distributed in both renal glomeruli and medulla of control rats (Fig. 6AGo) and completely displaced in the presence of 1µM unlabeled ANP (Fig. 6BGo). In control rats, the Kd and Bmax values for [125I]-ANP binding sites in renal glomeruli were 1.30 ± 0.08 pM and 6.43 ± 0.61 fmol/mm3 (n = 13), respectively, and those in renal medulla were 1.78 ± 0.30 pM and 1.98 ± 0.40 fmol/mm2, respectively (Table 2Go). No significant differences in Kd and Bmax values of [125I]-ANP binding to the renal glomeruli and medulla of 0.9% salt rats were found (n = 17). In 2% salt rats, however, Kd and Bmax values of [125I]-ANP binding to the renal glomeruli were decreased and Kd value [125I]-ANP binding to the renal medulla was increased (n = 8).



View larger version (109K):
[in this window]
[in a new window]
  		Figure 6. Dark-field photomicrograph of autoradiograms of renal cross-sections from control rats incubated in presence of 250 pM 125I-ANP (A) and adjacent sections incubated in 250 pM 125I-ANP + 1µM unlabeled ANP (B). IM, inner medulla.

 
Changes in mRNA for NPR-A and NPR-C in Kidney.
Figure 7AGo shows a representative agarose gel analysis of RT-PCR products of mRNAs obtained from the renal cortex of control, 0.9%, and 2% salt rats (all n = 6). Changes in mRNAs for NPR-A (GC-A) and NPR-C were analyzed using semiquantitative RT-PCR. The amounts of all PCR products were proportionately increased in terms of PCR cycles. Relative amounts of mRNAs for NPRs were obtained using GAPDH mRNA (Fig. 7BGo). No significant changes in mRNA levels for NPR-A and NPR-C in the renal glomeruli and medulla of salt rats were found except decreased NPR-A mRNA in renal cortex of 2% salt rats.



View larger version (77K):
[in this window]
[in a new window]
  		Figure 7. A representative agarose gel analysis of RT-PCR products of mRNAs obtained from the renal cortex of control, 0.9%, and 2% salt intake rats (A). (B) Comparisons of relative changes in mRNAs for NPR-A and NPR-C in terms of GAPDH in renal cortex and medulla of control and salt intake rats. MM, DNA molecular size marker (174 RF DNA, Hae III cut); lane 1, 4, 7, GAPDH; lane 2, 5, 8, NPR-A; lane 3, 6, 9, NPR-C. The mRNA for NPR-A in renal cortex of 2% salt intake rats was lower than in control rats. *, significantly different from control group, P < 0.05.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
We have demonstrated an accentuation of atrial ANP secretion and ANP-stimulated cGMP production in renal cortical membranes without changes in NPR-A density and expression in 0.9% salt rats. In contrast, in 2% salt rats, the NPR-A density and expression in renal cortex were decreased without changes in ANP secretion and the cGMP production by ANP. These results suggest that atrial ANP secretion as well as its binding sites in the kidney respond differently to ingested salt concentration in rats.

Chronic alterations of dietary salt and water balance have been shown to change ANP level and mRNA in atrial myocytes (11–15). Much attention has been directed toward establishment of the relationship between atrial synthesis and secretion of ANP under various experimental conditions, such as different concentrations of salt (from 0.9% to 8% salt), route of administration, and duration of exposure (from 2 days to 3 weeks).

The present study was performed to investigate the influence of salt intake (0.9% or 2%) on atrial content of ANP, its secretion, and changes in receptor density and expression. The levels of ANP in atrium and plasma were not altered significantly during acute (12 hrs and 1 and 3 days) and chronic exposures (10 days) to 0.9% salt. A 2% salt intake also did not cause any changes in the levels of ANP in atrium and plasma. Our results are consistent with the report showing no change in atrial ANP synthesis (13, 14). However, our results are not consistent with the report showing increased ANP mRNA level and decreased atrial content of ANP in animals given a high-salt diet or volume expansion (11, 12).

The decreased atrial content of ANP is explained by a result of increased ANP release to chronic volume load because of a time delay of ANP synthesis. The discrepancy may be due to different levels of water retention caused by salt intakes because hydration level is influenced by many factors, such as environmental temperature, humidity, or ventilation status, despite of the same concentration of salt intake. Another possible explanation is that the responsiveness of ANP secretion to a gradual increase in body fluid (induced by salt intake in the present study) and acute expansion (induced by saline ingestion intravenously or orally) may be different. In the former case, the atrial ANP system may be adapted slowly to changes in body fluid, and other hormonal systems regulating the body fluid may be compensating, making it difficult to detect changes in the atrial ANP system.

In the next experiments, we sought to determine whether the stretch-activated ANP secretion from rat atria of salt intakes is modified despite no changes in atrial ANP content. Surprisingly, stretch-activated ANP secretion in terms of ECF translocation was accentuated in 0.9% salt intake rats but not in 2% salt intake rats. Hence, the secretion of ANP to atrial stretch responds differently in terms of salt concentrations of drinking water. Why did plasma concentration of ANP not increase in 0.9% salt group despite accentuating stretch-activated ANP secretion? One possible factor is the degradation rate of ANP, such as NPR-C density or the metabolic rate of ANP. The relative changes in mRNAs for NPR-A and NPR-C in 0.9% salt intake rats were similar to control rats. We do not know changes in metabolic rate of ANP at present. In these experimental conditions, an increased amount of body fluid (positive water balance) is also an important factor because of the dilution of secreted extra ANP.

Interestingly, the production of cGMP stimulated by ANP in renal cortical tissue membranes was accentuated in the 0.9% salt intake group. Rather, the ANP receptor density and its mRNA in renal glomeruli were decreased in 2% salt intake rats without a difference in cGMP production by ANP. We do not know the reason for the gap between receptor density (including mRNA level) and cGMP production by ANP. One possible explanation is an increased sensitivity of particulate GC to ANP. Further studies are needed to define the mechanism.

In conclusion, our data suggest that atrial ANP secretion as well as its binding site in the kidney may respond differently to ingested salt concentration. However, considering the fact that natriuresis was observed without an increase in plasma ANP level in salt-administered rats, we also suggest that other hormonal systems may also be involved in the regulation of water balance in high-salt intake rats.


   Acknowledgments
 
The authors thank Ji Sun Park for her experimental assistance and Kyong Sook Kim for her secretarial assistance.


   Footnotes
 
This work was supported by grants from Korea Science and Engineering Foundation (98-0403-10-01-5) and Korea Research Foundation (2000-015-FP0023), Republic of Korea.

Received for publication April 4, 2003. Accepted for publication July 18, 2003.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. De Bold AJ. Atrial natriuretic factor: a hormone produced by the heart. Science 230:767–770, 1987.
  2. Dietz JR. Release of natriuretic factor from rat heart-lung preparation by atrial distension. Am J Physiol 247:R1093–R1096, 1984.
  3. Lang RE, Tholken H, Ganten D, Luft FC, Ruskoaho H, Unger T. Atrial natriuretic factor: a circulating hormone stimulated by volume loading. Nature 314:254–266, 1985.
  4. Genest J, Cantin M. The atrial natriuretic factor: Its physiology and biochemistry. Rev Physiol Biochem Pharmacol 110:1–145, 1988.[Medline]
  5. Ruskoaho H. Atrial natriuretic peptide: synthesis, release, and metabolism. Pharmacol Rev 44:481–602, 1992.
  6. Smith FG, Sato T, Varille VA, Robillard JE. Atrial natriuretic factor during fetal and postnatal life: a review. J Dev Physiol 12:55–62, 1989.[Medline]
  7. Claycomb WC. Atrial natriuretic factor mRNA is developmentally regulated in heart ventricles and actively expressed in cultured ventricular cardiac muscle cells of rat and human. Biochem J 255: 617–620, 1988.[Medline]
  8. Matsubara H, Yamamoto J, Hirata Y, Mori Y, Oikawa S, Inada M. Changes of atrial natriuretic peptide and its messenger RNA with development and regression of cardiac hypertrophy in renovascular hypertensive rats. Circ Res 66:176–184, 1990.[Abstract/Free Full Text]
  9. Franch RM, Hutchinson EC, Jones AM. Ventricular atrial natriuretic factor in the cardiomyopathic hamster model of congestive heart failure. Circ Res 62:31–36, 1988.[Abstract/Free Full Text]
  10. Raine AEG, Erne P, Burgisser RE, Muller FB, Bolli P, Burkart F, Buhler FR. Atrial natriuretic peptide and atrial pressure in patients with congestive heart failure. N Engl J Med 315:533–537, 1986.[Abstract]
  11. Pollock DM, Banks RO. Influence of dietary sodium on the natriuretic activity of atrial tissue. Miner Electrolyte Metab 10:337–342, 1984.[Medline]
  12. Garcia R, Debinski W, Gutkowska J, Kuchel O, Thibault, Genest J, Cantin M. Gluco- and mineralocorticoids may regulate the natriuretic effect and the synthesis and release of atrial natriuretic factor by the rat atria in vivo. Biochem Biophys Res Commun 131:806–814, 1985.[Medline]
  13. Sugimoto T, Ishii M, Hirata Y, Matsuoka H, Miyata A, Toshimori T, Masuda H, Kangawa K, Matsuo H. Increased release of atrial natriuretic polypeptides in rats with DOCA-salt hypertension. Life Sci 38:1351–1358, 1986.[Medline]
  14. Hirata Y, Ganguli M, Tobian L, Iwai J. Dahl S rats have increased natriuretic factor in atria but are markedly hyporesponsive to it. Hypertension 6:I148–I155, 1984.
  15. Wolf K, Kurtz A. Influence of salt intake on atrial natriuretic peptide gene expression in rats. Pflugers Arch 433:809–816, 1997.[Medline]
  16. Seul KH, Cho KW, Kim SH, Hwang YH, Park CU, Koh GY. Single injection of pentobarbital induces long-lasting effects on ANP synthesis and gene expression in the rat atria. Life Sci 52:1351–1359, 1993.[Medline]
  17. Kim SH, Cho KW, Hwang YH, Oh SH, Seul KH, Koh GY, Kim SJ. Ovarian atrial natriuretic peptide secretion during the rat estrous cycle. Life Sci 51:1291–1299, 1992.[Medline]
  18. Kim SH, Cho KW, Chang SH, Kim SZ, Chae SW. Glibenclamide suppresses stretch-activated ANP secretion: involvements of K+ATP channels and L-type Ca2+ channel modulation. Pflugers Arch Eur J Physiol 434:362–372, 1997.[Medline]
  19. Cho KW, Seul KH, Kim SH, Seul KM, Ryu H, Koh GY. Reduction volume dependence of immunoreactive atrial natriuretic peptide secretion in isolated perfused rabbit atria. J Hypertens 7:371–375, 1989.[Medline]
  20. Cho KW, Kim SH, Hwang YH, Seul KH. Extracellular fluid translocation in perfused rabbit atria: implication in control of atrial natriuretic peptide secretion. J Physiol (Lond) 468:591–607, 1993.[Abstract/Free Full Text]
  21. Kim SZ, Kim SH, Park JK, Koh GY, Cho KW. Presence and biological activity of C-type natriuretic peptide-dependent guanylate cyclase-coupled receptor in the penile corpus cavernosum. J Urol 159:1741–1746, 1998.[Medline]
  22. Lee SJ, Kim SZ, Cui X, Kim SH, Lee KS, Chung YJ, Cho KW. C-type natriuretic peptide inhibits ANP secretion and atrial dynamics in perfused atria: NPR-B-cGMP signaling. Am J Physiol Heart Circ Physiol 278:H208–H221, 2000.[Abstract/Free Full Text]
  23. Kim SZ, Cho KW, Kim SH. Modulation of endocardial natriuretic peptide receptors in right ventricular hypertrophy. Am J Physiol 277:H2280–H2289, 1999.




This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Lee, K. S.
Right arrow Articles by Kim, S. H.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Lee, K. S.
Right arrow Articles by Kim, S. H.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS