Segmental chloride and fluid handling during correction of chloride-depletion alkalosis without volume expansion in the rat

Relationship between serum chloride and prognosis in non-ischaemic dilated cardiomyopathy: a large retrospective cohort study

OBJECTIVES

Serum chloride has a unique homeostatic role in modulating neurohormonal pathways. Some studies have reported that hypochloremia has potential prognostic value in cardiovascular diseases; thus, we aimed to investigate the association of baseline serum chloride with clinical outcomes in elderly patients with non-ischaemic dilated cardiomyopathy (NIDCM).

DESIGN

Retrospective study.

SETTING AND PARTICIPANT

A total of 1088 patients (age ≥60 years) diagnosed with NIDCM were enrolled from January 2010 to December 2019.

RESULTS

Logistic regression analyses showed that serum chloride was significantly associated with in-hospital death. Receiver operating characteristic (ROC) curve analyses showed that serum chloride had excellent prognostic ability for in-hospital and long-term death (area under the curve (AUC)=0.690 and AUC=0.710, respectively). Kaplan-Meier survival analysis showed that the patients with hypochloremia had worse prognoses than those without hypochloremia (log-rank χ²=56.69, p<0.001). After adjusting for age, serum calcium, serum sodium, left ventricular ejection fraction, lg NT-proBNP and use of diuretics, serum chloride remained an independent predictor of long-term death (HR 0.934, 95% CI 0.913 to 0.954, p<0.001).

CONCLUSIONS

Serum chloride concentration was a prognostic indicator in elderly patients with NIDCM, and hypochloremia was significantly associated with both in-hospital and long-term poor outcomes.

Hemodynamic factors associated with serum chloride in ambulatory patients with advanced heart failure

BACKGROUND

Lower serum chloride (Cl) is associated with mortality in heart failure patients and may be more prognostically relevant than sodium. However, the association of hemodynamics and Cl levels is unknown.

METHODS

438 sequential patients with advanced chronic heart failure (ACHF) underwent invasive hemodynamic assessment with measured serum Cl levels during an evaluation for ACHF. Patients were followed for death, heart transplant (HT), or ventricular assist device placement (VAD). A backwards regression model determined hemodynamic predictors of Cl (removal, P<0.1) with candidate variables: Fick cardiac index (FCI), pulmonary capillary wedge pressure (PCWP), right atrial pressure (RAP), mean arterial pressure (MAP), heart rate (HR), and pulmonary artery systolic pressure (PASP). All models were also adjusted for serum sodium and bicarbonate.

RESULTS

In this cohort, the median Cl level was 102 [98-104]meq/L (range 86-113meq/L). Chloride was weakly correlated with FCI (rho 0.12, P=0.01) and MAP (rho 0.21, P<0.001); but not PCWP, RAP, HR or PASP (P>0.05 for all). In the multivariable model, FCI (beta 0.73meq/L/L/min/m², P=0.002) but not RAP (P=0.3) or MAP (P=0.2), remained associated with Cl. Lower Cl was associated with increased risk of death, HT, or VAD placement (HR 0.94/meq/L, 95% CI 0.89-0.99, P=0.01). However, this association was attenuated after additional adjustment for BUN (P=0.27) and PCWP and FCI (0.48).

CONCLUSIONS

Lower FCI, not lower MAP or higher cardiac filling pressures, was associated with lower chloride. Although lower chloride was associated with poor long-term outcomes, this risk attenuates with adjustment for more conventional clinical parameters.

Pharmacologic Approaches to Electrolyte Abnormalities in Heart Failure

Electrolyte abnormalities are common in heart failure and can arise from a variety of etiologies. Neurohormonal activation from ventricular dysfunction, renal dysfunction, and heart failure medications can perturb electrolyte homeostasis which impact both heart failure-related morbidity and mortality. These include disturbances in serum sodium, chloride, acid-base, and potassium homeostasis. Pharmacological treatments differ for each electrolyte abnormality and vary from older, established treatments like the vaptans or acetazolamide, to experimental or theoretical treatments like hypertonic saline or urea, or to newer, novel agents like the potassium binders: patiromer and zirconium cyclosilicate. Pharmacologic approaches range from limiting electrolyte intake or directly repleting the electrolyte, to blocking or promoting their resorption, and to neurohormonal antagonism. Because of the prevalence and clinical impact of electrolyte abnormalities, understanding both the older and newer therapeutic options is and will continue to be necessity for the management of heart failure.

Implications of Serum Chloride Homeostasis in Acute Heart Failure (from ROSE-AHF)

Lower serum chloride (Cl) levels are strongly associated with increased long-term mortality after admission for acute heart failure (AHF). However, the therapeutic implications of serum Cl levels during AHF are unknown. We sought to determine the short-term clinical response and postdischarge outcomes associated with serum Cl levels in AHF. Serum Cl was measured at randomization (n = 358) and during hospitalization from patients with AHF in the Renal Optimization Strategies Evaluation in Acute Heart Failure trial. Outcomes included diuretic response and renal function at 72 hours and death and rehospitalization at 60 and 180 days. Baseline Cl tertiles were 84 to 98; 99 to 102; and 103 to 117 meq/l. Baseline Cl level was associated with diuretic efficiency (p <0.001) but not change in cystatin C (p = 0.30) at 72 hours and was associated with 60-day death (hazard ratio [HR] 0.86, p = 0.029), 60-day death and rehospitalization (HR 0.90, p = 0.01), and 180-day death (HR 0.91, p = 0.049). These associations were attenuated with additional adjustment for loop diuretic dose (p >0.05). Chloride change correlated with weight change (ρ 0.18, p = 0.001), cystatin C change (ρ -0.35, p <0.001), and cumulative sodium excretion (ρ -0.21, p <0.001) but was not associated with any clinical outcomes (p >0.05 for all). In conclusion, serum Cl levels in AHF were inversely associated with loop diuretic response and were prognostic. However, changes in Cl levels were associated with parameters of decongestion but not with clinical outcomes.

It is chloride depletion alkalosis, not contraction alkalosis

Maintenance of metabolic alkalosis generated by chloride depletion is often attributed to volume contraction. In balance and clearance studies in rats and humans, we showed that chloride repletion in the face of persisting alkali loading, volume contraction, and potassium and sodium depletion completely corrects alkalosis by a renal mechanism. Nephron segment studies strongly suggest the corrective response is orchestrated in the collecting duct, which has several transporters integral to acid-base regulation, the most important of which is pendrin, a luminal Cl/HCO(3)(-) exchanger. Chloride depletion alkalosis should replace the notion of contraction alkalosis.

Molecular physiology of the renal chloride-formate exchanger

Renal apical chloride-base exchangers are essential to electrolyte and acid-base homeostasis. Different functional isoforms of apical anion exchangers have been identified in kidney proximal tubule and cortical collecting duct. Included amongst these are the following: chloride-formate, chloride-oxalate, and chloride-hydroxyl exchangers in proximal tubule; and chloride-bicarbonate exchanger in cortical collecting duct. Chloride-formate exchange, which was first identified in kidney proximal tubule, works in parallel with the apical sodium-hydrogen exchanger, and is thought to reabsorb the bulk of luminal chloride. Despite numerous studies, the molecular identities of apical chloride-base exchangers have remained unknown. Recent studies have identified a new class of anion exchangers, including pendrin (encoded by the PDS gene) and downregulated in adenoma (DRA, encoded by the DRA gene). Pendrin is expressed in the kidney, whereas DRA is not. Functional studies indicate that pendrin can function in chloride-formate and chloride-base exchange modes. It is unlikely that pendrin is the apical chloride-formate exchanger in the kidney proximal tubule. However, it is the only molecule that has been shown to mediate chloride-formate exchange. In the present review, recent studies regarding the renal distribution and membrane localization of pendrin, and its functional properties, including its roles in chloride reabsorption and base excretion, are addressed.

Bicarbonate transport in collecting duct segments during chloride-depletion alkalosis

Renal correction of chloride-depletion alkalosis (CDA) by chloride replacement results in bicarbonate secretion in the cortical collecting duct (CD) and urinary bicarbonate excretion. To assess the participation of the more distal segments of the CD, we determined net total CO2 transport in the outer medullary (OMCD), initial (IMCDi) and terminal (IMCDt) inner medullary CD segments obtained from Sprague-Dawley rats with normal acid-base balance (NML) or with CDA produced by peritoneal dialysis. Tubules were bathed and perfused with isotonic solutions containing Cl 110 mM and HCO, 25 mM. Net total CO2 transport was decreased in all segments: OMCD 22.1 +/- 4.2 to 9.2 +/- 2.0; IMCDi 38.1 +/- 4.6 to 9.3 +/- 1.7; IMCDt 6.7 +/- 1.2 to -0.5 +/- 0.4 pmol/min/mm tubule length. Perfusion rates, tubule lengths, and transepithelial voltages did not differ between groups in any segment. These data show that all CD segments beyond the cortical segment decrease bicarbonate reabsorption during CDA. This permits the bicarbonate secreted by the cortical CD to be excreted, and is likely an important mechanism for the correction of CDA.

Adaptation of the distal convoluted tubule of the rat. Structural and functional effects of dietary salt intake and chronic diuretic infusion

We studied the effects of dietary NaCl intake on the renal distal tubule by feeding rats high or low NaCl chow or by chronically infusing furosemide. Furosemide-treated animals were offered saline as drinking fluid to replace urinary losses. Effects of naCl intake were evaluated using free-flow micropuncture, in vivo microperfusion, and morphometric techniques. Dietary NaCl restriction did not affect NaCl delivery to the early distal tubule but markedly increased the capacity of the distal convoluted tubule to transport Na and Cl. Chronic furosemide infusion increased NaCl delivery to the early distal tubule and also increased the rates of Na and Cl transport above the rates observed in low NaCl diet rats. When compared with high NaCl intake alone, chronic furosemide infusion with saline ingestion increased the fractional volume of distal convoluted tubule cells by nearly 100%, whereas dietary NaCl restriction had no effect. The results are consistent with the hypotheses that (a) chronic NaCl restriction increases the transport ability of the distal convoluted tubule independent of changes in tubule structure, (b) high rates of ion delivery to the distal nephron cause tubule hypertrophy, and (c) tubule hypertrophy is associated with increases in ion transport capacity. They indicate that the distal tubule adapts functionally and structurally to perturbations in dietary Na and Cl intake.

On the mechanism by which chloride corrects metabolic alkalosis in man

To determine whether administration of chloride corrects chloride-depletion metabolic alkalosis (CDA) by correction of plasma volume contraction and restoration of glomerular filtration rate or by an independent effect of chloride repletion, CDA was produced in normal men by the administration of furosemide and maintained by restriction of dietary sodium chloride intake. Negative sodium balance (-112 +/- 16 meq) and reduced plasma volume (2.53 versus 2.93 liters, p less than 0.05) developed. The cumulative chloride deficit of 271 +/- 16 meq was then repleted by oral potassium chloride (267 +/- 19 meq) over 36 hours with continued serial measurements of glomerular filtration rate, effective renal plasma flow, plasma volume, body weight, and plasma renin and aldosterone levels. CDA was corrected, even though body weight, plasma volume, glomerular filtration rate, and renal plasma flow all remained reduced and plasma aldosterone was elevated; urinary bicarbonate excretion increased during correction. Administration of an identical potassium chloride load to similarly sodium-depleted but not chloride-depleted normal subjects produced no change in acid-base status. It is concluded that chloride repletion can correct CDA by a renal mechanism without restoring plasma volume or glomerular filtration rate or by altering sodium avidity.

Effects of chloride and extracellular fluid volume on bicarbonate reabsorption along the nephron in metabolic alkalosis in the rat. Reassessment of the classical hypothesis of the pathogenesis of metabolic alkalosis

Volume expansion has been considered essential for the correction of chloride-depletion metabolic alkalosis (CDA). To examine the predictions of this hypothesis, rats dialyzed against 0.15 M NaHCO3 to produce CDA and controls, CON, dialyzed against Ringer-HCO3 were infused with either 6% albumin (VE) or 80 mM non-sodium chloride salts (CC) added to 5% dextrose (DX) and studied by micropuncture. CDA was maintained in rats infused with DX. VE expanded plasma volume (25%), maintained glomerular filtration rate (GFR), but did not correct CDA despite increased fractional delivery of total CO2 (tCO2) out of the proximal tubule (36 +/- 2%) as compared with VE/CON (24 +/- 4%; P less than 0.05). In contrast, CC corrected CDA despite volume contraction (-16%) and lower GFR than CC/CON; proximal tCO2 delivery in CC/CDA (29 +/- 4%) did not differ from VE/CDA. CC was associated with an increment in tCO2 excretion. The data strongly suggest that maintenance and correction of CDA are primarily dependent upon total body chloride and its influences on intrarenal mechanisms and not on the demands of sodium or fluid homeostasis.

Cyclic adenosine monophosphate-stimulated anion transport in rabbit cortical collecting duct. Kinetics, stoichiometry, and conductive pathways

Cyclic AMP stimulates HCO3 secretion and Cl self-exchange in rabbit cortical collecting tubule. We found that varying peritubular [Cl] changed the Cl self-exchange rate with saturation kinetics (Km, 3-4 mM). HCO3 secretion also showed saturation kinetics as a function of mean luminal [Cl] (Km, 4-11 mM). Both Cl self-exchange and Cl-HCO3 exchange thus appear to be carrier-mediated. Addition/removal of basolateral HCO3 qualitatively changed Cl and HCO3 transport as expected for Cl-HCO3 exchange, but quantitatively changed Cl absorption more than HCO3 secretion. The diffusive Cl permeability and the transepithelial conductance in the presence of HCO3/CO2 and cAMP were higher than in their absence suggesting that HCO3/CO2 and cAMP together increase a conductive Cl pathway parallel to a 1:1 Cl-HCO3 exchanger. Thus, cAMP not only stimulates the overall process of anion exchange (probably by increasing an electroneutral exchanger and/or a series Cl conductance), but also stimulates a Cl conductance parallel to the exchange process.

Distal tubule bicarbonate accumulation in vivo. Effect of flow and transtubular bicarbonate gradients

We have performed microperfusion studies on distal tubules of normal and alkalotic rats in an attempt to demonstrate in vivo bicarbonate secretion. All perfusion solutions were free of phosphate and other nonbicarbonate buffers. In both normal and alkalotic rats, distal perfusions elicited significant tCO2 entry only at high flow (24 nl/min). Even when perfusate tCO2 concentration closely matched plasma tCO2 concentration (30 mM tCO2), significant tCO2 entry again occurred at high flow. This was associated with a rise of the perfusate tCO2 concentration, which indicated net entry of tCO2 against a concentration gradient. In this "symmetrical" perfusion situation, acetazolamide blockade prevented tCO2 entry. Accordingly: distal tubule tCO2 entry is demonstrable in both alkalotic and normal rats at high flow rates; increasing perfusate tCO2 concentration can suppress tCO2 entry; and entry can occur in the absence of a gradient and this effect can be blocked by acetazolamide.

Renal bicarbonate reabsorption in the rat. I. Effects of hypokalemia and carbonic anhydrase

Free-flow micropuncture studies were carried out on superficial rat proximal and distal tubules to assess the participation of different nephron segments in bicarbonate transport. Particular emphasis was placed on the role of the distal tubule, and micro-calorimetric methods used to quantitate bicarbonate reabsorption. Experiments were carried out in control conditions, during dietary potassium withdrawal, and after acute intravenous infusions of carbonic anhydrase. We observed highly significant net bicarbonate reabsorption in normal acid-base conditions as evidenced by the maintenance of significant bicarbonate concentration gradients in the presence of vigorous fluid absorption. Distal bicarbonate reabsorption persisted in hypokalemic alkalosis and even steeper transepithelial concentration gradients of bicarbonate were maintained. Enhancement of net bicarbonate reabsorption followed the acute intravenous administration of carbonic anhydrase but was limited to the nephron segments between the late proximal and early distal tubule. The latter observation is consistent with a disequilibrium pH along the proximal straight tubule (S3 segment), the thick ascending limb of Henle, and/or the early distal tubule.

Atrial natriuretic factor ameliorates chronic metabolic alkalosis by increasing glomerular filtration

The kidney maintains the elevated plasma concentration of bicarbonate that occurs in chronic metabolic alkalosis. A reduction in the glomerular filtration rate (GFR) can maintain the filtered bicarbonate load at a normal level so that a normal rate of bicarbonate reabsorption suffices to prevent urinary excretion of this anion. It is also possible that bicarbonate reabsorption might increase so as to maintain the alkalosis if GFR were not reduced. To examine this latter possibility, atrial natriuretic factor was used in alkalotic rats to restore a more normal GFR and to increase the amount of bicarbonate filtered by the glomerulus. Proximal bicarbonate reabsorption remained relatively static. Higher than normal amounts of bicarbonate were then delivered out of the proximal tubule, bicarbonate appeared in the urine, and the plasma concentration of bicarbonate fell. A reduction in GFR is thus necessary for the maintenance of chronic metabolic alkalosis. Normalizing GFR induces bicarbonaturia and initiates repair of the alkalosis.

Volume-independent reductions in glomerular filtration rate in acute chloride-depletion alkalosis in the rat. Evidence for mediation by tubuloglomerular feedback

We have recently described reduced superficial nephron glomerular filtration rate (SNGFR) in chloride-depletion alkalosis (CDA) without volume depletion. To elucidate the mechanism of this phenomenon, we studied three degrees of increasing severity of CDA (groups CDA-1, 2, and 3) produced by one or two peritoneal dialyses against 0.15 M NaHCO3 and electrolyte infusions of different Cl and HCO3 content in Sprague-Dawley rats; control rats (CON) were dialyzed against and infused with Ringers-HCO3. Extracellular fluid (ECF) volume was assessed by blood pressure, hematocrit, plasma protein concentration, and 125I-albumin space; none of these variables differed among the four groups. Micropuncture of the latest proximal and earliest distal convolutions was carried out. As CDA intensified from CON to CDA-3 (plasma tCO2 25 +/- 1 to 43 +/- 1 meq/L; P less than 0.01), distally determined SNGFR declined progressively (40.9 +/- 1.7 to 28.3 +/- 1.8 nl/min; P less than 0.01), while in early distal tubule fluid, flow rate (8.6 +/- 0.7 to 3.4 +/- 0.6 nl/min) and Cl concentration (36 +/- 2 to 19 +/- 3 meq/L) decreased and osmolality (110 +/- 5 to 208 +/- 12 mosmol/kg) increased (P less than 0.01), and, in the loop segment, Cl reabsorption decreased progressively (2,009 +/- 112 to 765 +/- 128 peq/min; P less than 0.01). In early distal tubule fluid, Cl concentration correlated positively and osmolality negatively with distally determined SNGFR (P less than 0.05). Proximally determined SNGFRs did not differ among the four groups. Proximal tubule stop-flow pressure responses to increasing rates of orthograde perfusion of the loop segment from 0 to 40 nl/min did not differ between groups CON and CDA-2. We interpret these data to show that reductions in SNGFR in CDA in the rat can occur by tubuloglomerular feedback (TGF) in the absence of differences in ECF volume or of alterations in TGF sensitivity during metabolic alkalosis. Of the proposed signals for TGF sensed by the macula densa, distal tubule fluid osmolality or some related variable is the signal most compatible with our data.