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

Serum Chloride and Heart Failure

Despite significant advances in management, heart failure continues to impose a significant epidemiologic burden with high prevalence and mortality rates. For decades, sodium has been the serum electrolyte most commonly associated with outcomes; however, challenging the conventional paradigm of sodium's influence, recent studies have identified a more prominent role in serum chloride in the pathophysiology of heart failure. More specifically, hypochloremia is associated with neurohumoral activation, diuretic resistance, and a worse prognosis in patients with heart failure. This review examines basic science, translational research, and clinical studies to better characterize the role of chloride in patients with heart failure and additionally discusses potential new therapies targeting chloride homeostasis that may impact the future of heart failure care.

"I don't get no respect": the role of chloride in acute kidney injury

Acute kidney injury (AKI) is a major public health problem that complicates 10-40% of hospital admissions. Importantly, AKI is independently associated with increased risk of progression to chronic kidney disease, end-stage renal disease, cardiovascular events, and increased risk of in-hospital and long-term mortality. The chloride content of intravenous fluid has garnered much attention over the last decade, as well as its association with excess use and adverse outcomes, including AKI. Numerous studies show that changes in serum chloride concentration, independent of serum sodium and bicarbonate, are associated with increased risk of AKI, morbidity, and mortality. This comprehensive review details the complex renal physiology regarding the role of chloride in regulating renal blood flow, glomerular filtration rate, tubuloglomerular feedback, and tubular injury, as well as the findings of clinical research related to the chloride content of intravenous fluids, changes in serum chloride concentration, and AKI. Chloride is underappreciated in both physiology and pathophysiology. Although the exact mechanism is debated, avoidance of excessive chloride administration is a reasonable treatment option for all patients and especially in those at risk for AKI. Therefore, high-risk patients and those with "incipient" AKI should receive balanced solutions rather than normal saline to minimize the risk of AKI.

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.

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.

Metabolic Alkalosis

Metabolic alkalosis is a common acid-base disturbance in critically ill patients. In this review we discuss the approach to diagnosis and management of this disorder; particular emphasis is given to the causes most com monly responsible for alkalosis in critical care medicine. We present rules for (1) identifying the presence of metabolic alkalosis, ( 2 ) determining whether the disor der is simple or complicated by a second acid-base dis turbance, and (3) determining the cause: The causes are subdivided into three major groups: Chloride-respon sive, chloride-resistant, and alkali administration. The pathogenesis of each type of alkalosis is discussed sep arately, although we stress that more than one cause may be responsible in critically ill patients. The patho logical consequences of metabolic alkalosis and ap proaches to treatment are reviewed. The major issues relating to the critically ill patient are (1) identification and removal of exogenous sources of alkali, (2) iden tification and minimization of HCl losses or selective NaCl losses, and (3) maneuvers to reduce serum HCO 3 concentration without producing extracellular fluid volume overload.

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.

Cloning, renal distribution, and regulation of the rat Na+-HCO3- cotransporter

We recently reported the cloning and expression of a human kidney Na+-HCO3- cotransporter (NBC-1) (C. E. Burnham, H. Amlal, Z. Wang, G. E. Shull, and M. Soleimani. J. Biol. Chem. 272: 19111-19114, 1997). To expedite in vivo experimentation, we now report the cDNA sequence of rat kidney NBC-1. In addition, we describe both the organ and nephron segment distributions and the regulation of NBC-1 mRNA under three models of pH stress: chloride-depletion alkalosis (CDA), metabolic acidosis, and bicarbonate loading. Rat NBC-1 cDNA encodes an open reading frame of 1,035 amino acids, with 96 and 87% identity to human and salamander NBC-1, respectively. Rat NBC-1 mRNA is expressed at high levels in kidney and brain, with lower levels in colon, stomach, and heart. None appears in liver. In the kidney, NBC-1 is expressed mainly in the proximal tubule, with traces found in medullary thick ascending limb and papilla. HCO3- loading decreased NBC-1 mRNA levels, which were unchanged either by metabolic acidosis or by CDA.

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.

Intrarenal hypertonic saline infusions in dogs with thoracic caval constriction

Intrarenal artery infusions of hypertonic saline can activate tubuloglomerular feedback (TGF), decreasing renal blood flow (RBF) and glomerular filtration rate (GFR). The response to infusion of hypertonic saline is enhanced by salt depletion and attenuated by salt loading, but has not previously been investigated in pathophysiological states where expanded extracellular fluid volume due to salt retention is associated with avid, renal sodium reabsorption. The renal response following intrarenal infusions of hypertonic saline was investigated in five control dogs and eleven dogs with partial constriction of the thoracic portion of their inferior vena cava, which resulted in salt retention and the formation of ascites. Intrarenal infusion of hypertonic saline induced significant reductions in RBF and GFR in both control and caval constricted dogs. The extent of these reductions were positively correlated with baseline renal function. An intravenous infusion of 50 ml/kg of 0.9% sodium chloride, which abolished the vasoconstrictor response in normal dogs, failed to abolish the decrease in RBF and GFR in response to intrarenal hypertonic saline infusion in dogs with ascites which had an initial vasoconstrictor response. We conclude that the potential for TGF is preserved in early stages of caval constriction syndrome in dogs, but that this potential activity decreases when basal renal function decreases.

Renal response to metabolic alkalosis induced by isovolemic hemofiltration in the dog

We describe a new model of chloride-depletion alkalosis (CDMA), in which the method of induction of alkalosis does not itself cause a direct alteration in sodium and fluid balance. We have used this model, which is based on hemofiltration techniques in the dog, to study the immediate response of the kidney to the induction of CDMA. Normal dogs maintained with a NaCl-free diet for several days underwent hemofiltration of 50 ml/kg over a 35 minute period. The hemofiltrate was replaced ml for ml with a solution containing sodium and potassium in the same concentrations as found in each animal's plasma water. In control animals, the replacement solution contained chloride and bicarbonate in the same ratio as in the plasma; in the experimental (CDMA) animals the replacement solution contained bicarbonate as the only anion. In the control group, the procedure of hemofiltration coupled with isovolemic replacement caused no appreciable changes in plasma composition, urinary excretion rates, GFR, or tubular handling of bicarbonate. In the CDMA group, 106 +/- 8.4 mEq of chloride were removed in exchange for bicarbonate. A marked metabolic alkalosis resulted, plasma bicarbonate concentration increasing from 21.9 +/- 0.6 to 33.3 +/- 0.6 mEq/liter. The hemofiltration procedure itself, by design, did not alter sodium or fluid balance. Nevertheless, cumulative urinary sodium excretion increased over 2.5 hours by 23.0 +/- 6.4 mEq. A natriuresis of this magnitude is equivalent to a loss of ECF volume of approximately 200 ml. GFR did not change significantly. The rate of tubular reabsorption of bicarbonate increased significantly from 1209 +/- 82 to 1559 +/- 148 mu Eq/min in CDMA animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Glomerular filtration effects of acute volume expansion: importance of chloride

The present studies were done to determine the effect on GFR of acute volume expansion (AVE) using solutions of various sodium salts and to explore if degree of tubuloglomerular feedback (TGF) activation plays a role in any GFR differences. Free-flow micropuncture and inulin clearance studies were combined to investigate anesthetized Munich-Wistar rats expanded to 10% body weight with isotonic solutions of NaCl, Ringers bicarbonate (RB), NaHCO3, Na acetate (NaAc) and Na2SO4 as well as euvolemic controls. In the clearance studies, AVE yielded per gram kidney weight GFR's greater than control (1009 +/- 51 microliter/min) in the NaCl and RB (chloride expanded) groups (1397 +/- 89 and 1389 +/- 64) microliter/min, respectively, P less than 0.05 vs. control) but not in the NaHCO3, NaAc, and Na2SO4 (non-chloride expanded) groups. Proximal minus distal single nephron GFR determinations (P-D), an estimate of the degree of TGF, were less than control 13.2 +/- 2.1 nl/min) in the NaCl and RB groups (4.1 +/- 0.7 and 5.3 +/- 1.9 nl/min, respectively, P less than 0.05 vs. control) but were not significantly different from control in any of the non-chloride expanded groups. Early distal (ED) fluid flow correlated positively with P-D in all groups. ED chloride concentration but not TCO2 nor osmolality correlated with P-D for all groups. The correlation was negative for control and chloride expanded groups and positive for non-chloride expanded groups.(ABSTRACT TRUNCATED AT 250 WORDS)

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.