Molecular Medicine |
From the Department of Molecular Cardiology, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, Ohio.
Correspondence to Kunio S. Misono, Department of Molecular Cardiology, Lerner Research Institute, NB50, Cleveland Clinic Foundation, 9500 Euclid Ave, Cleveland, OH 44195. E-mail misonok{at}ccf.org
| Abstract |
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0.6 mmol/L equivalent
chloride concentration. Competitive-binding assays at several fixed
concentrations of NaCl showed that lowering chloride concentration
caused a decrease in maximum binding but did not alter
Kd values, suggesting that a loss of
chloride turns off ANF binding rather than reducing affinity for ANF.
Saturation-binding studies showed that excess ANF cannot overcome loss
of binding caused by low chloride. Chloride-dependent ANF-receptor
binding may function as a feedback-control mechanism regulating the
ANF-receptor action and, hence, renal sodium excretion.
Key Words: atrial natriuretic factor receptors chloride sodium kidney
| Introduction |
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It is evident from these studies that there are certain experimental and disease situations in which high-plasma ANF levels do not result in renal salt excretion. The mechanism responsible for this apparent insensitivity to ANF is not known. In the present study, it was found that binding of ANF to its receptor requires the presence of chloride and occurs in a chloride concentrationdependent manner. This finding suggests that ANF-receptor binding and, hence, physiological responses to the hormone may be regulated by changes in chloride concentration at the target sites. In the kidney, this chloride-mediated control of ANF and ANF-receptor function may play a role in the regulation of renal salt excretion. The apparent insensitivity to high-plasma ANF observed in edematous states and in transgenic animals that overexpress ANF may also be explained by this chloride-mediated control mechanism.
| Materials and Methods |
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ANF-binding assay with the purified hormone-binding domain was performed by a modification of the method described previously.8 Briefly, binding was measured by incubating the hormone-binding domain (2.5 ng) with 125I-ANF (200 000 to 300 000 cpm per incubation) in 200 µL of the assay medium consisting of 20 mmol/L sodium phosphate buffer, pH 7.4, 0.05% BSA, and 0.01% bacitracin in the presence of 0.1 mol/L NaCl or other selected salt. After incubation at room temperature for 30 minutes, bound 125I-ANF was separated from unbound 125I-ANF by rapid gel filtration on a Sephadex G-75 column and counted for 125I-radioactivity. Nonspecific binding was measured by incubation in the presence of 0.1 µmol/L unlabeled ANF.
The chloride concentration dependence of ANF binding to the hormone-binding domain was measured by varying NaCl concentration in the assay medium while maintaining the total salt concentration at 0.1 mol/L by supplementing with sodium acetate. Incubation was carried out with a constant amount of 125I-ANF tracer. Competitive-binding assays were performed at several fixed concentrations of NaCl with a constant amount of 125I-ANF tracer and varying concentrations of unlabeled ANF using the purified hormone-binding domain.
Saturation-binding assays were carried out using 125I-ANF with the specific radioactivity of 0.02 µCi/pmol, prepared as described.10 The hormone-binding domain (2.5 ng) was incubated with increasing concentrations of 125I-ANF from 10 pmol to 0.1 µmol/L in 100 µL of the assay medium containing 0.1 mol/L NaCl or sodium acetate. A parallel set of control incubations was performed in the absence of the hormone-binding domain to measure background binding. Bound 125I-radioactivity values obtained in the control incubations were subtracted from those in corresponding experimental incubations.
ANF-binding assays with the bovine adrenocortical membranes were carried out according to the method described previously.9 Before the assay, the membranes were freed from chloride by dialyzing against 20 mmol/L sodium phosphate buffer, pH 7.5, containing a suspension of an anion exchange resin AG 1X8 in formate form (Bio-Rad) at 4°C overnight. The membranes (5 µg membrane protein) were incubated in the assay medium containing 0.1 mol/L NaCl or sodium acetate. After incubation at 0°C for 1 hour, membrane-bound 125ANF was separated by filtration and counted. Nonspecific binding was measured in the presence of 0.1 µmol/L unlabeled ANF.
| Results |
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Binding of ANF to Its Receptor Occurs in a Chloride
ConcentrationDependent Manner
The chloride concentration dependence of ANF binding to the
hormone-binding domain was examined by increasing the concentration of
NaCl while maintaining the total salt concentration in the assay medium
constant at 0.1 mol/L by supplementing with sodium acetate (Figure 2
). ANF binding was essentially absent at
NaCl concentrations below 0.05 mmol/L. ANF binding rose with
increasing concentrations of NaCl, reaching a maximum level above
10 mmol/L NaCl. The half-maximum binding was attained at
0.6 mmol/L NaCl.
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Lack of Chloride Completely Abolishes ANF-Receptor Binding
Competitive-binding assays were performed at several fixed
concentrations of NaCl by incubations with varying concentrations of
unlabeled ANF competing against a constant amount of
125I-ANF tracer (Figure 3
). The results showed that lowering
chloride concentration caused a decrease in the maximum binding but did
not alter binding affinity toward ANF (Kd
remained at
1 nmol/L).
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Excess ANF Cannot Overcome Inhibition of ANF Binding by Low
Salt
Incubation of the hormone-binding domain in the presence of 0.1
mol/L NaCl with the increasing concentrations of
125I-labeled ANF showed binding saturation at
0.1 µmol/L ligand concentration (Figure 4
). However, in the absence of chloride,
binding was undetectable, even when the concentration of the
radioligand was increased to 0.1 µmol/L, a
concentration 100-fold higher than the Kd
value. These results indicate that loss of ANF binding due to low
chloride concentration cannot be overcome by excess ANF.
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| Discussion |
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0.05 to 10 mmol/L with the
half-maximal ANF binding being attained at
0.6 mmol/L
equivalent chloride concentration. Chloride requirement for ANF binding
was also observed with the partially purified and dialyzed bovine
adrenocortical membranes, although a low level of ANF binding was still
observed when NaCl was replaced with sodium acetate. The residual
ANF-binding activity may have been due to the presence of chloride ions
bound to or trapped in membrane vesicles in the adrenocortical membrane
preparations. The chloride dependence of ANF binding to the receptor is
consistent with our recent finding of a buried, protein-bound
chloride ion in the structure of the ANF-receptor hormone-binding
domain as determined by X-ray crystallography (K.S. Misono, X. Zhang,
F. van den Akker, V.C. Yee, unpublished data, 2000). Competitive ANF-binding assays performed at several fixed concentrations of NaCl showed that lowering chloride concentration caused a decrease in maximum binding but did not alter Kd values. This indicates that a loss of chloride completely abolishes ANF binding rather than reducing binding affinity. Furthermore, saturation-binding data showed that inhibition of ANF binding by low salt could not be overcome by excess ANF. Physiologically, these results suggest that the chloride-dependent ANF binding provides an on-off mechanism for the receptor, and that loss of ANF binding on chloride deficit cannot be overcome by excess levels of ANF.
The chloride dependence of ANF binding demonstrated in this study
suggests a possibility that changes in chloride concentration at the
target sites for this hormone may modulate hormone-receptor binding and
regulate physiological responses. In the kidney,
this chloride-mediated regulation of ANF-receptor binding may function
as a feedback-control mechanism in ANF-induced natriuresis to maintain
normal salt and fluid balance. In the loop of Henle and the distal
tubule, sodium reabsorption is mediated by Na+
and Cl- cotransport systems and, hence, is
accompanied by chloride reabsorption. Thus, the chloride concentrations
at these tubular sites are tightly coupled with the state of
electrolyte and water transport. In the medullary collecting ducts
immediately downstream (the main tubular site of ANF
action11 ), the tubular chloride concentration may reach
millimolar ranges under certain physiological and
pathophysiological conditions. For example,
aldosterone treatment lowers the tubular fluid chloride
concentration to
4 mmol/L,12 13 which falls within
the range where ANF-receptor binding is chloride
concentrationdependent. Additionally, the ANF receptor is located in
both apical and basal membranes of the medullary collecting ducts
regulating sodium reabsorption and vasopressin-stimulated water
reabsorption.14 15 16 17 ANF natriuresis and diuresis
may then be regulated through control of ANF-receptor binding by the
chloride concentrations in the tubule and peritubular microcirculation.
It is probable that such a control mechanism plays an important role in
renal regulation of electrolyte and fluid homeostasis.
In edematous disease states such as congestive heart failure,4 5 nephrotic syndrome,5 and liver cirrhosis,6 the plasma levels of ANF are markedly elevated, yet sodium is retained. Congestive heart failure is characterized by activation of the renin-angiotensin-aldosterone system, increased vasopressin secretion, and heightened sympathetic nervous activity. ANF, elevated because of volume expansion, is thought to be one of the principal factors opposing these activities. Although studies using the ANF-receptor antagonist HS-142-1 have demonstrated the suppressive effects of ANF on the sympathetic and renin-angiotensin-aldosterone systems as well as on sodium reabsorption,18 19 20 the high plasma levels of ANF fail to produce correspondingly enhanced natriuresis in congestive heart failure.4 5 In such an edematous state, the proposed chloride-mediated feedback mechanism for the ANF receptor may function to prevent excessive salt loss. The chronically elevated plasma ANF in this state would be expected to activate the ANF receptor and induce persistent ANF natriuresis, which would consequently reduce the tubular chloride concentration. ANF natriuresis might then be turned off by inhibition of ANF binding by insufficient chloride when the tubular chloride concentration falls below a threshold level. This chloride-mediated feedback-control mechanism may be responsible, aside from the counterregulatory action of the renin-angiotensin system, for the attenuated response to high-plasma ANF observed in the edematous states. Such a salt-conserving feedback mechanism may likewise operate in ANF-overexpressing transgenic animals, in which salt is also retained in spite of high plasma levels of ANF.
The lack of natriuretic response to high-plasma ANF observed in these states and animals is consistent with our finding in vitro that excess ANF cannot overcome the loss of ANF binding caused by low chloride. Additionally, it has been found that KCl infusion21 or acute volume expansion by isotonic saline infusion7 in ANF-overexpressing transgenic animals causes exaggerated natriuretic responses. This exaggerated natriuresis may also be explained by the chloride-mediated feedback mechanism operating in the kidney. With KCl infusion or acute volume expansion, the resultant increase in the filtered chloride concentration may unblock this chloride-mediated inhibition of ANF binding to the receptor, allowing maximal natriuresis to occur in response to high plasma levels of ANF. It has also been reported that high-dose ANF infusion in normovolemic animals induces only modest natriuresis.3 The suppressed natriuresis in response to high-dose ANF infusions may similarly have resulted from the chloride-mediated feedback mechanism, in which a decreased tubular chloride concentration after acute ANF natriuresis may have prevented further natriuresis by inhibiting binding of ANF to its receptor.
In animal models of congestive heart failure and nephrotic syndrome, ANF infusion lowers the arterial mean pressure and increases the glomerular filtration rate to levels similar to those in the respective control animals, whereas ANF natriuresis is markedly blunted.5 In addition, in ANF-overexpressing transgenic animals, high-plasma ANF is accompanied by systemic hypotension but normal salt excretion is maintained.7 These findings suggest that the chloride-mediated feedback control operates at renal tubular sites but not at the renal or systemic vascular sites.
Attenuated natriuretic response to ANF has also been observed in animals on a low-salt diet.22 The NaCl deficit in low-salt animals is associated with marked elevation in sodium reabsorption in the medullary collecting ducts. ANF infusion in these animals does not reduce this sodium reabsorption. On the other hand, the systemic hypotensive effect of ANF is still present. On the basis of these findings, it has been suggested that a salt-conserving counterregulatory mechanism operates in the kidney and overcomes ANF natriuresis under salt-deprived conditions. It was also found that the lack of responsiveness to ANF in these salt-deficient animals is reversed within 1 hour of salt repletion, suggesting that this counterregulatory mechanism is not mediated by downregulation of the ANF receptor.22 Additional studies showed that this salt-conserving mechanism operates primarily in the medullary collecting duct23 and functions independently of the sympathoadrenergic and renin-angiotensin-aldosterone systems.24 The suppressed natriuretic response to ANF could not be explained on the basis of reduced renal perfusion pressure or glomerular filtration rate.24 These findings are consistent with direct inhibition of the ANF receptor by chloride deficit as demonstrated in this study and are in agreement with the present hypothesis that chloride-mediated feedback control of the ANF receptor occurs at the renal tubular sites, regulating renal salt excretion.
In summary, binding of ANF to the ANF-receptor hormone-binding domain was found to require the presence of chloride and occur in a chloride concentrationdependent manner. It is proposed that chloride-mediated feedback control of the ANF receptor occurs in the kidney and plays a role in the regulation of ANF-mediated natriuresis and, hence, renal salt excretion. In light of this possible feedback-control mechanism in ANF natriuresis mediated by chloride, the physiological and pathophysiological roles of ANF in salt and volume homeostasis warrant additional investigation.
| Acknowledgments |
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Received April 6, 2000; accepted April 27, 2000.
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