Effects of diuretics on outputs and flows of urine and urinary solutes in healthy subjects

Urine output is an early and strong predictor of acute kidney injury and associated mortality: a systematic literature review of 50 clinical studies

BACKGROUND

Although the present diagnosis of acute kidney injury (AKI) involves measurement of acute increases in serum creatinine (SC) and reduced urine output (UO), measurement of UO is underutilized for diagnosis of AKI in clinical practice. The purpose of this investigation was to conduct a systematic literature review of published studies that evaluate both UO and SC in the detection of AKI to better understand incidence, healthcare resource use, and mortality in relation to these diagnostic measures and how these outcomes may vary by population subtype.

METHODS

The systematic literature review was performed following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist. Data were extracted from comparative studies focused on the diagnostic accuracy of UO and SC, relevant clinical outcomes, and resource usage. Quality and validity were assessed using the National Institute for Health and Care Excellence (NICE) single technology appraisal quality checklist for randomized controlled trials and the Newcastle-Ottawa Quality Assessment Scale for observational studies.

RESULTS

A total of 1729 publications were screened, with 50 studies eligible for inclusion. A majority of studies (76%) used the Kidney Disease: Improving Global Outcomes (KDIGO) criteria to classify AKI and focused on the comparison of UO alone versus SC alone, while few studies analyzed a diagnosis of AKI based on the presence of both UO and SC, or the presence of at least one of UO or SC indicators. Of the included studies, 33% analyzed patients treated for cardiovascular diseases and 30% analyzed patients treated in a general intensive care unit. The use of UO criteria was more often associated with increased incidence of AKI (36%), than was the application of SC criteria (21%), which was consistent across the subgroup analyses performed. Furthermore, the use of UO criteria was associated with an earlier diagnosis of AKI (2.4-46.0 h). Both diagnostic modalities accurately predicted risk of AKI-related mortality.

CONCLUSIONS

Evidence suggests that the inclusion of UO criteria provides substantial diagnostic and prognostic value to the detection of AKI.

Aqueous extract from Equisetum arvense stimulates the secretion of Tamm-Horsfall protein in human urine after oral intake

BACKGROUND

Within European traditional phytotherapy, extracts from different herbal plants are used for prevention and therapy of uncomplicated urinary tract infections and for flushing out of kidney grits. Besides increased urine flow by slight diuretic effects, also stimulation of Tamm-Horsfall protein (syn. THP, uromodulin) in the distal part of the kidney could explain reduced kidney gravel and anti-virulent activity against uropathogenic E. coli.

PURPOSES

Evaluation of THP-inducing activity of extracts from Equisetum arvense, Levisticum officinalis, Ilex paraguariensis, Juniperus communis, Urtica dioica, and Taraxacum officinale by quantification of THP in urine samples after oral application to humans.

STUDY DESIGN

7 days p.o. application of the test intervention to healthy volunteers (n = 10 per intervention group) and analysis of urine samples at day 1 (untreated control values), and days 3, 6 and 8 on THP content by validated ELISA. Antiadhesive activity of urine samples was monitored by flow cytometry using UPEC strain NU14 against human T24 bladder cells.

RESULTS

An aqueous extract from E. arvense, fully characterized by a specific LC-MS method, induced THP concentration in urine samples significantly during a 7-day p.o. application up to 300%, related to the untreated controls. Ex vivo investigation of the individual and pooled urine samples with elevated THP concentrations showed good correlation to antiadhesive effects against UPEC NU14 to T24 cells. Urine samples of the Equisetum treated volunteers had no effect on the proliferation and on biofilm formation of UPEC NU14. Silica excretion in the urine samples had no correlation to the respective THP levels. Monitoring of electrolyte content in the urine samples indicat ed diuretic effects of the intervention with Equisetum extract. Detailed phytochemical analysis of the Equisetum extract by LC-MS and LC-UV revealed an analytical protocol, which identified > 80 compounds from the extract by MS evaluations and 18 compounds by UV detection. This protocol will provide a valuable tool for future quality control of Equisetum extract.

CONCLUSION

Aqueous extract from E. arvense significantly stimulates THP secretion in urine samples after 7 days of oral intake and inhibits the interplay between UPEC and bladder host cells. This could explain the therapeutic use of this herbal material for urinary tract infections and kidney gravel. Detailed phytochemical analysis of the Equisetum extract by LC-MS and LC-UV revealed an analytical protocol, which identified > 82% of all eluted compounds. This protocol will provide a valuable tool for future quality control of Equisetum extract.

Furosemide reduces BK-αβ4-mediated K+ secretion in mice on an alkaline high-K+ diet

Special high-K diets have cardioprotective effects and are often warranted in conjunction with diuretics such as furosemide for treating hypertension. However, it is not understood how a high-K diet (HK) influences the actions of diuretics on renal K⁺ handling. Furosemide acidifies the urine by increasing acid secretion via the Na⁺-H⁺ exchanger 3 (NHE3) in TAL and vacuolar H⁺-ATPase (V-ATPase) in the distal nephron. We previously found that an alkaline urine is required for large conductance Ca²⁺-activated K⁺ (BK)-αβ4-mediated K⁺ secretion in mice on HK. We therefore hypothesized that furosemide could reduce BK-αβ4-mediated K⁺ secretion by acidifying the urine. Treating with furosemide (drinking water) for 11 days led to decreased urine pH in both wild-type (WT) and BK-β4-knockout mice (BK-β4-KO) with increased V-ATPase expression and elevated plasma aldosterone levels. However, furosemide decreased renal K⁺ clearance and elevated plasma [K⁺] in WT but not BK-β4-KO. Western blotting and immunofluorescence staining showed that furosemide treatment decreased cortical expression of BK-β4 and reduced apical localization of BK-α in connecting tubules. Addition of the carbonic anhydrase inhibitor, acetazolamide, to furosemide water restored urine pH along with renal K⁺ clearance and plasma [K⁺] to control levels. Acetazolamide plus furosemide also restored the cortical expression of BK-β4 and BK-α in connecting tubules. These results indicate that in mice adapted to HK, furosemide reduces BK-αβ4-mediated K⁺ secretion by acidifying the urine.

Effect of diuretics on renal tubular transport of calcium and magnesium

Calcium (Ca2+) and Magnesium (Mg2+) reabsorption along the renal tubule is dependent on distinct trans- and paracellular pathways. Our understanding of the molecular machinery involved is increasing. Ca2+ and Mg2+ reclamation in kidney is dependent on a diverse array of proteins, which are important for both forming divalent cation-permeable pores and channels, but also for generating the necessary driving forces for Ca2+ and Mg2+ transport. Alterations in these molecular constituents can have profound effects on tubular Ca2+ and Mg2+ handling. Diuretics are used to treat a large range of clinical conditions, but most commonly for the management of blood pressure and fluid balance. The pharmacological targets of diuretics generally directly facilitate sodium (Na+) transport, but also indirectly affect renal Ca2+ and Mg2+ handling, i.e., by establishing a prerequisite electrochemical gradient. It is therefore not surprising that substantial alterations in divalent cation handling can be observed following diuretic treatment. The effects of diuretics on renal Ca2+ and Mg2+ handling are reviewed in the context of the present understanding of basal molecular mechanisms of Ca2+ and Mg2+ transport. Acetazolamide, osmotic diuretics, Na+/H+ exchanger (NHE3) inhibitors, and antidiabetic Na+/glucose cotransporter type 2 (SGLT) blocking compounds, target the proximal tubule, where paracellular Ca2+ transport predominates. Loop diuretics and renal outer medullary K+ (ROMK) inhibitors block thick ascending limb transport, a segment with significant paracellular Ca2+ and Mg2+ transport. Thiazides target the distal convoluted tubule; however, their effect on divalent cation transport is not limited to that segment. Finally, potassium-sparing diuretics, which inhibit electrogenic Na+ transport at distal sites, can also affect divalent cation transport.

Osmotic diuresis with SGLT2 inhibition: analysis of events related to volume reduction in dapagliflozin clinical trials

OBJECTIVE

Dapagliflozin reduces hyperglycemia in type 2 diabetes mellitus (T2DM) and lowers blood pressure, at least in part, secondary to mild diuresis consequent to dapagliflozin-induced glucosuria. While blood-pressure lowering may contribute to cardiovascular risk reduction, dapagliflozin-induced diuresis may potentially contribute to adverse events (AEs) of volume reduction. The present analysis compared the frequency of AEs of volume reduction between dapagliflozin and placebo.

METHODS

Pooled data were assessed from 13 placebo-controlled dapagliflozin clinical trials ≤24 weeks in patients with T2DM, overall, and in those at risk (aged ≥65y, estimated glomerular filtration rate [eGFR] <60 mL/min/1.73 m(2), or on antihypertensive therapy). Longer-term (≤104 weeks) data were available for 9 of these trials.

RESULTS

The frequency of patients experiencing ≥1 AE of volume reduction over 24 weeks was low overall; 27/2360 (1.1%) with dapagliflozin 10 mg and 17/2295 (0.7%) with placebo; and slightly more frequent in patients ≥65 years (11/665 [1.7%] and 6/711 [0.8%], respectively) and in patients receiving loop diuretics (6/236 [2.5%] and 4/267 [1.5%], respectively). Over 104 weeks, AEs of volume reduction occurred in 38/2026 (1.9%) with dapagliflozin 10 mg and in 27/1956 (1.4%) with placebo; serious AEs of volume reduction in 4/2026 (0.2%) and 6/1956 (0.3%), respectively; and 2 patients in each group discontinued therapy due to these AEs. Dapagliflozin versus placebo incidence rate ratios did not suggest any meaningful increase in frequency of these AEs with dapagliflozin 10 mg, either overall or in those at risk. Although mean eGFR declined by 4.2 ml/min/1.73 m(2) within the first week of dapagliflozin therapy, thereafter eGFR gradually recovered to baseline levels by 104 weeks (mean change from baseline +0.02 mL/min/1.73 m(2); 95%CI: -0.9, 1.0).

CONCLUSION

No meaningful increase in frequency of AEs of volume reduction occurred with dapagliflozin 10 mg in patients with T2DM, either overall, or in those at increased risk of these events. However, caution should nevertheless be exercised when prescribing dapagliflozin to elderly patients, those with reduced eGFR, and those receiving antihypertensive medication.

An unexpected journey: conceptual evolution of mechanoregulated potassium transport in the distal nephron

Flow-induced K secretion (FIKS) in the aldosterone-sensitive distal nephron (ASDN) is mediated by large-conductance, Ca(2+)/stretch-activated BK channels composed of pore-forming α-subunits (BKα) and accessory β-subunits. This channel also plays a critical role in the renal adaptation to dietary K loading. Within the ASDN, the cortical collecting duct (CCD) is a major site for the final renal regulation of K homeostasis. Principal cells in the ASDN possess a single apical cilium whereas the surfaces of adjacent intercalated cells, devoid of cilia, are decorated with abundant microvilli and microplicae. Increases in tubular (urinary) flow rate, induced by volume expansion, diuretics, or a high K diet, subject CCD cells to hydrodynamic forces (fluid shear stress, circumferential stretch, and drag/torque on apical cilia and presumably microvilli/microplicae) that are transduced into increases in principal (PC) and intercalated (IC) cell cytoplasmic Ca(2+) concentration that activate apical voltage-, stretch- and Ca(2+)-activated BK channels, which mediate FIKS. This review summarizes studies by ourselves and others that have led to the evolving picture that the BK channel is localized in a macromolecular complex at the apical membrane, composed of mechanosensitive apical Ca(2+) channels and a variety of kinases/phosphatases as well as other signaling molecules anchored to the cytoskeleton, and that an increase in tubular fluid flow rate leads to IC- and PC-specific responses determined, in large part, by the cell-specific composition of the BK channels.

Role of NKCC in BK channel-mediated net K⁺ secretion in the CCD

Apical SK/ROMK and BK channels mediate baseline and flow-induced K secretion (FIKS), respectively, in the cortical collecting duct (CCD). BK channels are detected in acid-base transporting intercalated (IC) and Na-absorbing principal (PC) cells. Although the density of BK channels is greater in IC than PC, Na-K-ATPase activity in IC is considered inadequate to sustain high rates of urinary K secretion. To test the hypothesis that basolateral NKCC in the CCD contributes to BK channel-mediated FIKS, we measured net K secretion (J(K)) and Na absorption (J(Na)) at slow (∼1) and fast (∼5 nl·min(-1)·mm(-1)) flow rates in rabbit CCDs microperfused in vitro in the absence and presence of bumetanide, an inhibitor of NKCC, added to the bath. Bumetanide inhibited FIKS but not basal J(K), J(Na), or the flow-induced [Ca(2+)](i) transient necessary for BK channel activation. Addition of luminal iberiotoxin, a BK channel inhibitor, to bumetanide-treated CCDs did not further reduce J(K). Basolateral Cl removal reversibly inhibited FIKS but not basal J(K) or J(Na). Quantitative PCR performed on single CCD samples using NKCC1- and 18S-specific primers and probes and the TaqMan assay confirmed the presence of the transcript in this nephron segment. To identify the specific cell type to which basolateral NKCC is localized, we exploited the ability of NKCC to accept NH(4)(+) at its K-binding site to monitor the rate of bumetanide-sensitive cytosolic acidification after NH(4)(+) addition to the bath in CCDs loaded with the pH indicator dye BCECF. Both IC and PC were found to have a basolateral bumetanide-sensitive NH(4)(+) entry step and NKCC1-specific antibodies labeled the basolateral surfaces of both cell types in CCDs. These results suggest that BK channel-mediated FIKS is dependent on a basolateral bumetanide-sensitive, Cl-dependent transport pathway, proposed to be NKCC1, in both IC and PC in the CCD.

Increased acid excretion in kidney stone formers with essential hypertension

BACKGROUND

Although several studies have reported that kidney stone disease and hypertension are associated, the link between the two conditions has not been identified. This study investigated urinary excretion of different solutes, particularly citrate and acids, in kidney stone formers and examined their association with high blood pressure.

METHODS

The retrospective study included 234 consecutive subjects, aged 47.0 +/- 15.6, attending our metabolic clinic after episodes of kidney stones. Essential hypertension was present in 82 patients (35.0%). A difference in the urinary excretion of some of the investigated components was found between subjects with normal blood pressure and those with hypertension.

RESULTS

The results showed that hypertensive subjects were older and had a higher body mass index (BMI) and serum uric acid. They had a significantly lower urinary pH (5.6 +/- 0.4 versus 6.0 +/- 0.5) and citrate (2.55 +/- 1.36 versus 2.83 +/- 1.65 mmol/24 h), higher titratable acid (38.8 +/- 19.0 versus 26.8 +/- 15.0 mEq/24 h) and ammonium (41.6 +/- 17.6 versus 34.2 +/-12.4 mmol/24 h). Logistic regression analysis with the presence of hypertension as the dependent variable produced a model with the following predictors: age (P < 0.0001), BMI (P = 0.026), titratable acid (P = 0.025) and low urinary citrate level (P = 0.033). Urinary acid excretion increased with the stage of hypertension. No difference was found in the urinary excretion of other solutes.

CONCLUSIONS

These findings suggest that essential hypertension and acid excretion are linked in stone formers.

Patients differ in their ability to self-monitor adherence to a low-sodium diet versus medication

OBJECTIVE

Poor adherence to a low-sodium diet (LSD) and prescribed medications increases rehospitalization risk in patients with heart failure (HF). Clinicians have difficulty assessing adherence objectively, so they depend on patients' self-report. The degree to which self-reported adherence reflects actual adherence is unclear. We examined patients' ability to self-monitor adherence to an LSD and medications by comparing self-reported adherence with objective evidence of adherence.

METHODS

A total of 133 patients with HF (male 71%; ejection fraction 35% +/- 14%) completed the Medical Outcomes Study Specific Adherence Scale. Adherence to the LSD and medication were assessed objectively using 24-hour urinary sodium excretion and dose counting with an electronic monitoring device, respectively. On the basis of self-report, patients were divided into adherent and non-adherent groups and evaluated for differences according to objective adherence.

RESULTS

There were no differences in urinary sodium levels between the self-reported LSD adherent and non-adherent groups (4560 mg vs. 4333 mg; P = .59). Self-reported adherent and non-adherent medication groups took 92.4% and 80.4% of prescribed doses, respectively (P < .001).

CONCLUSION

Patients were able to accurately estimate adherence to medication, but they failed to estimate LSD adherence. This finding suggests that we need to improve our means of evaluating adherence to the LSD and of educating patients more thoroughly about following the LSD. We speculated that the inability to estimate LSD adherence may be the result of gaps in patients' knowledge that preclude accurate self-assessment.

Gender differences in adherence to the sodium-restricted diet in patients with heart failure

BACKGROUND

Despite the importance of the sodium-restricted diet (SRD) to heart failure (HF) management, patient adherence is poor. Little is known about gender differences in adherence or factors that affect patients' ability to follow SRD recommendations. The purposes of this study were to determine whether there were gender differences in (1) adherence to the SRD; (2) knowledge about SRD and HF self-care; and (3) perceived barriers to following the SRD.

METHODS AND RESULTS

Forty-one men and 27 women completed the Heart Failure Attitudes and Barriers questionnaire that measured HF self-care, knowledge, and perceived barriers to follow an SRD. Diet adherence was measured by 24-hour urinary sodium excretion (UNa). Women were more adherent to the SRD than men as reflected by 24-hour urine excretion (2713 versus 3859 mg UNa, P = .01). Women recognized signs of excess sodium intake such as fluid buildup (P = .001) and edema (P = .01) more often than men and had better understanding of appropriate actions to take related to following an SRD. There were no gender differences in perceived barriers to follow an SRD.

CONCLUSIONS

Although men and women perceived similar barriers, women were more adherent to the SRD and had greater knowledge about following an SRD. Further investigation of this phenomenon is warranted to determine if better adherence contributes to improved outcomes in women.

Dose-effect relations of loop- and thiazide-diuretics on calcium homeostasis: a randomized, double-blinded Latin-square multiple cross-over study in postmenopausal osteopenic women

BACKGROUND

Thiazide diuretics (TDs) reduce whereas loop diuretics (LDs) increase urinary calcium. We studied the effects of different doses of a TD and LD on electrolytes, calcitropic hormones and biochemical bone markers.

SUBJECTS AND METHODS

In a five-period crossover study, comparing four active doses with placebo, 40 postmenopausal women with osteopenia were treated with different doses of LD bumetanide (n = 20, 0.5-2.0 mg per day) or TD bendroflumethiazide (n = 20, 2.5-10 mg per day). Each treatment period lasted 1 week.

RESULTS

Urinary calcium decreased dose-dependently in response to the bendroflumethiazide. The best hypocalciuric effect was achieved by 5 mg day-1 of bendroflumethiazide. Total plasma calcium levels increased, whereas ionised calcium at ambient pH-values decreased because of increased pH-values in response to the bendroflumethiazide. Plasma PTH levels did not change, whereas a slight dose-dependent increase occurred in plasma 1,25(OH)2D levels. As a marker of bone formation, plasma osteocalcin levels increased. Conversely, bumetanide dose-dependently increased renal calcium losses with a concomitant increase in plasma PTH and 1,25(OH)2D levels. Plasma osteocalcin levels increased and bone-specific alkaline phosphatase levels decreased dose-dependently.

CONCLUSION

Whether a LD or TD is chosen as diuretic therapy affects calcium homeostasis. The effects of LDs are potentially harmful to bone. Further studies are needed to evaluate whether long-term treatment with LDs causes osteoporosis. Until then, we suggest using, if possible, a TD rather than a LD as diuretic therapy in order not to risk deleterious effects on bone metabolism.

Clinical consequences of the biphasic elimination kinetics for the diuretic effect of furosemide and its acyl glucuronide in humans

This review discusses the possibility of whether furosemide acyl glucuronide, a metabolite of furosemide, contributes to the clinical effect of diuresis. First an analytical method (e.g. HPLC) must be available to measure both parent drug and furosemide acyl glucuronide. Then, with correctly treated plasma and urine samples (light protected, pH 5) from volunteers and furosemide-treated patients, the kinetic curves of both furosemide as well as its acyl glucuronide can be measured. The acyl glucuronide is formed in part by the kidney tubules and it is possible that the compound is pharmacologically active through inhibition of the Na+/2Cl-/K+ co-transport system; up to now the mechanism of action has been solely attributed to furosemide. The total body clearance of furosemide occurs by hepatic and renal glucuronidation (50%) and by renal excretion (50%). Enterohepatic cycling of furosemide acyl glucuronide, followed by hydrolysis, results in a second and slow elimination phase with a half-life of 20-30 h. This slow elimination phase coincides with a pharmacodynamic rebound phase of urine retention. After each dosage of furosemide, there is first a short stimulation of urine flow (4 h), which is followed by a 3-day recovery period of the body. The following clinical implications arise from study of the elimination kinetics of furosemide. Repetitive dosing must result in accumulation of the recovery period. Accumulation of furosemide and its acyl glucuronide in patients with end-stage renal failure results from infinite hepatic cycling. Impaired kidney function may result in impaired glucuronidation and diuresis. While kidney impairment normally requires a dose reduction for those compounds which are mainly eliminated by renal excretion, for diuretics, a dose increment is required in order to maintain a required level of diuresis. The full clinical impact of the accumulation of furosemide and its acyl glucuronide in patients with end-stage renal failure has to be determined.

Torsemide inhibits aldosterone secretion in vitro

Torsemide inhibited aldosterone secretion by adrenal cells from rats, cows, and guinea pigs stimulated in vitro by potassium, angiotensin, dibutyryl cyclic AMP, ACTH, or corticosterone. Inhibitory concentrations for adrenal cells (micromolar) were comparable with those reported to inhibit ion transport in isolated renal tubules. Inhibition of aldosterone secretion could reduce kaliuresis, and that may explain why torsemide causes less kaliuresis than other diuretics.

Principles of diuretic therapy

This section discusses the reasons different diuretic agents inhibit salt reabsorption at specific sites within the renal tubule. It also includes a brief review of how diuretics reach their target site of action along the nephron, together with a discussion of how disease states may affect the delivery of diuretics to those sites. When diuretics are administered to edematous patients, the natriuretic response is often blunted. In addition, increased renal tubular salt avidity is observed after administration of loop diuretics. The elements required to successfully achieve adequate natriuresis under such conditions are analyzed. Because achieving diuresis may result in significant hypokalemia, hyponatremia, metabolic alkalosis, and worsening prerenal azotemia, the prevention and management of these complications of diuretic therapy are also reviewed. A description of successful use of diuretics in specific edematous states, such as congestive heart failure, chronic renal failure, nephrotic syndrome, and liver disease, is followed by a brief discussion of the management of resistant edema and the use of diuretics in nonedematous states, including essential hypertension and other conditions.

Frusemide and its acyl glucuronide show a short and long phase in elimination kinetics and pharmacodynamic effect in man

The pharmacokinetics of 80 mg frusemide given orally were investigated in normal subjects using a direct HPLC method for parent drug and its acyl glucuronide conjugate. Two half-lives could be distinguished in the plasma elimination of both frusemide and its conjugate, with values of 1.25 +/- 0.75 and 30.4 +/- 11.5 h for frusemide and 1.31 +/- 0.60 and 33.2 +/- 28.0 h for the conjugate. The renal excretion rate-time profile showed two phases; the rapid elimination phase lasted from 0-15 h and the second and slow phase, from 15-96 h. During the first 15 h, 33.3 +/- 4.8% of the dosed frusemide was excreted; in the remaining period 15-96 h, 4.6 +/- 1.5% was excreted. In the same two periods the excretion of the glucuronide was 13.4 +/- 4.7 and 1.9 +/- 1.1%, respectively. The mean renal clearance of frusemide was 90.2 +/- 16.9 mL min-1 during the first period and 91.5 +/- 29.3 mL min-1 in the remaining period, during which the stimulation of urine production was absent. The renal clearance of the acyl glucuronide was 702 +/- 221 mL min-1 in the first period, but only 109 +/- 51.0 mL min-1 in the second period. The stimulated urine production in the first 6 h after administration amounted to 2260 +/- 755 mL (measured urine production minus baseline value of 1 mL min-1 (360 mL). During the second or rebound period (6-96 h after drug administration), the quantity of urine was 990 +/- 294 mL lower than what would have been expected from the baseline production of 5400 mL. This reduced production (0.82 mL min-1) is equivalent to an 18% reduction in the average urine flow rate of 1 mL min-1.

Diuretic drugs and the treatment of edema: from clinic to bench and back again

Despite wide variations in dietary NaCl intake, homeostatic mechanisms ensure that renal NaCl excretion matches intake at steady state. This does not imply, however, that extracellular fluid volume is maintained within narrow limits. In contrast with blood pressure, which appears to be tightly controlled, extracellular fluid volume varies significantly, even in normal individuals, when dietary NaCl intake changes. Cardiac, liver, or renal disease can perturb the relationship between NaCl intake and extracellular fluid volume and lead to symptomatic edema. All major classes of diuretic drugs in use today were developed between 1950 and 1970. These drugs were developed empirically, without knowledge of specific ion transport pathways, but experimental work during the past 15 years has shown that each major class of diuretic inhibits a specific ion transport protein in the kidney. These transport proteins have been characterized physiologically and the mechanisms by which each diuretic drug inhibits ion transport have been defined. Antibodies directed against these transport proteins have localized ion transport pathways to specific cell types along the nephron. Most recently, isoforms of each class of diuretic-sensitive Na transport pathway have been cloned. Ongoing experimental work is aimed at exploring relationships between families of transporters, determining the structural prerequisites for ion transport, and studying molecular mechanisms of transport regulation. Treatment of edema with diuretics is often straightforward, but can lead to adaptive changes in nephron structure and function. These adaptations can limit the effectiveness of diuretic drugs; maneuvers aimed at blocking these processes can be effective approaches to patients who are resistant to diuretic drugs.

Which diuretic to use?

The choice of a diuretic depends on knowledge of its efficacy in different disease states as well as the complications associated with its use. However, efficacy of the various diuretic agents as determined in healthy subjects may not always be applicable to the sick patient. The thiazide group appears to be the main offender with regard to hypokalemia and metabolic upsets; these complications seem to stem from long-term use and high dosages. Interest has been expressed on the influence of diuretic induced hypokalemia (and intracellular potassium deficiency) in inducing insulin resistance. The lipid abnormalities may arise as a result of this resistance or may be consequent on hyperinsulinemia from other as yet undetermined mechanisms. In addition, hypertension per se may be associated with insulin resistance. It is interesting to speculate that diuretics, which are commonly used in the treatment of hypertension, may, in some way, perhaps via the induction of hypokalemia, unmask this resistance.

Do diuretics cause magnesium deficiency?

1. Controlled trials, of which there are few, do not substantiate claims that diuretics play a role in causing magnesium deficiency. Consequently, the vast majority of patients taking conventional doses of thiazide diuretics (i.e. bendrofluazide 2.5 mg day-1 or equivalent) do not need magnesium supplements. On balance, potassium-sparing diuretics tend to increase serum and intracellular magnesium content; this should not be taken as evidence of prior magnesium deficiency. It remains theoretically possible that large doses of loop diuretics given more than once daily for long periods could induce negative magnesium balance and magnesium deficiency. However, it has been difficult to run appropriately controlled trials in conditions where such therapy is needed (i.e. heart failure) and until more reliable information becomes available no absolute recommendation can be made. 2. Methods for the measurement of intracellular free magnesium levels are now available and are more relevant to the assessment of magnesium deficiency than total intracellular magnesium content; the complex relationship between intracellular free and total magnesium content remains to be defined. Future work involving the effect of diuretics on intracellular free magnesium measurements should make every attempt to avoid the errors of trial design and multiple publication that litter current and past literature.

Clinicopharmacological reappraisal of the potency of diuretics

From a clinicopharmacological standpoint, the urinary excretory potency of diuretics should be assessed comparatively on the basis of the changes in 24-hour natriuresis, with respect to 24-hour natriuresis after placebo, caused by single oral doses administered to healthy adult subjects who are in habitual and steady-state external sodium balance. The potency of various formulations of loop (e.g., furosemide), of early distal tubular (e.g., the thiazides), and of potassium-retaining diuretics, as well as of several combinations of diuretics, has been evaluated in a series of studies. Two formulations of loop diuretics (muzolimine 20 mg and torasemide 2.5 mg) are definitely nondiuretic. The majority of the other formulations of loop diuretics studied are, in general, comparatively less potent than most of the common formulations of early distal tubular diuretics studied. As a general rule, most common formulations of early distal tubular diuretics are at least not less potent than the majority of common formulations of loop diuretics. Hydrochlorothiazide 25 mg and furosemide 80 mg have similar potencies. Loop diuretics increase mean renal sodium output strikingly within the first few (0-6) hours after dosing, but this forced excretion is followed by a rebound with respect to postplacebo mean urinary sodium flow; the rebound usually takes place between 6 and 24 hours after dosing. However, no rebound in mean urinary sodium flow occurs during the 24 hours following a single dose of a distal tubular diuretic; these substances increase urinary sodium excretion with lower maximal intensity but more protractedly than loop diuretics.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal excretory responses to single and repeated administration of diuretics in healthy subjects: clinical connotations

Administration of an initial oral dose of hydrochlorothiazide 25 mg to healthy subjects is followed by increased 24-hour urinary outputs of sodium, chloride, and potassium. On the fourth day of once-daily dosing with hydrochlorothiazide 25 mg, 24-hour natriuresis and chloriuresis are no longer augmented, but the elevation in 24-hour kaliuresis that follows the first dose remains unchanged. Twenty-four-hour urinary calcium output is consistently reduced during repeated once-daily administration of hydrochlorothiazide 25 mg. The first oral dose of the loop diuretic torasemide augments the average natriuresis and kaliuresis in the 6 hours immediately after dosing in healthy subjects, in a dose-dependent fashion, within the 2.5 to 10-mg range. These increased urinary outputs are followed by rebounds below postplacebo values between 6 and 24 hours after dosing. As a result of this biphasic response, torasemide 2.5 mg qualifies as a nondiuretic formulation (it does not elevate 24-hour natriuresis), whereas torasemide 5 and 10 mg qualify as diuretic formulations. After the seventh dose of torasemide 5 or 10 mg during a regimen of once-daily therapy, 24-hour urinary sodium and chloride outputs no longer differ from their postplacebo counterparts. Twenty-four-hour kaliuresis tends to increase in a dose-dependent fashion after the first dose of torasemide (torasemide 2.5 and 5 mg do not augment it significantly), but this tendency is no longer present after the seventh once-daily dose, when torasemide (2.5, 5, or 10 mg) does not elevate the mean 24-hour kaliuresis. Twenty-four-hour calciuresis tends to increase in a dose-dependent manner (torasemide 2.5 mg does not elevate it significantly) after the first dose of torasemide; this calciuretic effect does not change in intensity after 7 days of once-daily treatment. The time course of natriuresis over the 24 hours following the administration of any given formulation of a loop or of an early distal tubular diuretic to healthy subjects is alike after the first and after the nth once-daily dose; therefore, it constitutes a definite characteristic of any given oral formulation. In the case of torasemide, lower doses have more protracted effects on natriuresis, to the extent that the time course of natriuresis over the 24 hours after administration of torasemide 2.5 mg to healthy subjects resembles the time course after administration of hydrochlorothiazide 25 mg, rather than the time course after administration of the overtly diuretic formulation torasemide 10 mg.