Saluretic effect of the loop diuretic torasemide in chronic renal failure. Interdependence of electrolyte excretion

The influence of cardiovascular and antiinflammatory drugs on thiazide-induced hemodynamic and saluretic effects

OBJECTIVE

Thiazide diuretics are known to induce a transient fall of the glomerular filtration rate (GFR), which, in turn, reduces tubular Na(+) load. This tubuloglomerular feedback (TGF) curtails the natriuretic effect of this class of diuretics. Cardiovascular and antiinflammatory therapeutics may interfere with TGF and thereby influence the effect of thiazides once co-administration is clinically indicated.

METHODS

The effects on GFR and saluresis of hydrochlorothiazide (HCT; 25 mg) monotherapy were measured in healthy volunteers and compared to those obtained during co-administration of the thiazide and a second therapeutic.

RESULTS

In the presence of the ACE inhibitor enalapril (10 mg), the transient fall in the GFR induced by HCT was almost abolished, and Na(+) excretion increased by approximately 30 % as compared to HCT monotherapy. K(+) excretion, however, remained unchanged. Similar results were obtained with the AT II type 1 receptor antagonist candesartan (8 mg): GFR remained stable, Na(+) excretion rose by 35 % and K(+) excretion was not changed. The effect of the Ca(2+) channel blocker amlodipine (5 mg) on GFR and HCT-induced Na(+) excretion equalled that obtained with the AT(1) blocker, yet with this treatment K(+) excretion rose in proportion to Na(+) excretion. The beta-blockers propranolol (80 mg) or bisoprolol (5 mg) reduced GFR but maintained TGF. HCT-induced Na(+) excretion was significantly reduced in the presence of a beta-blocker, whereas K(+) excretion was not changed. The inhibition of cyclooxygenase by diclofenac (50 mg) or rofecoxib (25 mg) significantly reduced the diuretic/natriuretic effect of HCT, but K(+) excretion was unchanged, and TGF was still demonstrable.

CONCLUSION

In conclusion, AT(1) receptors, as well as the Ca(2+) channels in the smooth muscle cells of the afferent arteriole, are considered prerequisites for TGF function; their blockade increases the diuretic/natriuretic efficacy of thiazide diuretics. In contrast, beta-blockers and COX inhibitors do not interfere directly with TGF. These first dose effects reflect the primary response of the kidney to the drugs. They cannot, however, predict the benefits of long-term treatment.

Low-dose segmental blockade of the nephron rather than high-dose diuretic monotherapy

The specific renal effect of diuretics is due to the fact that their concentrations is almost 100-fold greater in the renal tubule than in the plasma. The function of the different segments of the nephron may be altered following changes in the effective arterial blood volume (EABV) and the extracellular fluid volume (ECFV). In diseases with reduced EABV, e.g., congestive heart failure, decompensated cirrhosis of the liver, and the nephrotic syndrome, proximal tubular hyperreabsorption of sodium occurs, leaving only a low Na+ load in the distal segments of the nephron, the site of diuretic action. Clinically, the response to diuretics is reduced or resistance to diuretics may even ensue, which can be predicted by a FENa < 0.2%. Resistance to diuretics can be overcome by short-term comedication with acetazolamide, which increases Na+ delivery to the site of action of the other diuretics used concomitantly. In states with increased ECFV, e.g. in chronic renal failure, there is distal tubular Na+ rejection, leading to a greater increase in FENa the more GFR is reduced. The remaining intact nephrons present a relatively increased response to diuretics. The efficacy of diuretic treatment in renal failure can be optimised by combining loop diuretics with thiazides. In conclusion, low-dose combination therapy, inducing "segmental blockade of the nephron", meets the functional changes along the nephron. It is therefore more effective and safer than high-dose monotherapy.

Constant K+/Na+ excretion ratio during peak diuresis after piretanide but insignificant K+ loss during 24 hours

The effect of piretanide on Na+ and K+ excretion and on renal haemodynamics has been studied in 14 subjects with a GFR (Inulin clearance) ranging from 140 to 2 ml.min-1. After a two day fluid and salt balance control period, oral piretanide 6 mg induced a natriuresis and kaliuresis, which was proportional to the GFR of the patients. The ratio of drug-induced K+ to Na+ excretion was always 0.13, independent of individual GFR. This was only true for the duration of the action of piretanide, tau, which was 6 h in subjects with normal GFR and 5 h in patients with impaired kidney function. Surprisingly, after tau, i.e. for 24 h after drug administration, less potassium was lost than in the pretreatment period. Neither the GFR nor the renal blood flow (PAH clearance) of the patients were affected by piretanide. In conclusion, piretanide given once a day was an effective natriuretic agent, even in end-stage renal disease, and it produced relatively little K(+)-loss when given once daily.

Pharmacodynamic and kinetic considerations on diuretics as a basis for differential therapy

Diuretics are classified according to their site of action in the nephron: loop diuretics, thiazides, and antikaliuretics. During peak diuresis the pattern of electrolyte excretion is constant and characteristic for a class of diuretics. The ratio of diuretic-induced excretion of K+ to Na+ is 0.12 for loop diuretics, 0.20 for thiazides, and -0.21 for antikaliuretics. The ratio of Ca2+ to Na+ is 0.02 for loop diuretics and 0.003 for thiazides. Mg2+ excretion follows K+ excretion in a ratio of 0.15. The natriuretic effect of a diuretic directly depends on the renal clearance of the drug and is proportionate to the number of intact nephrons. Not only loop diuretics but also thiazides and antikaliuretics were demonstrated to be effective natriuretic drugs down to end-stage renal disease. In renal failure FENa is doubled with every halfening of GFR. Loop diuretics increase FENa to a maximum of 24%, thiazides to 10-15%, and FENa is doubled by antikaliuretics. Comedication of loop diuretics with thiazides in renal failure may therefore be more effective than increasing monotherapy. In liver disease, nonrenal drug clearance is reduced the more the patient's direct bilirubin rises thus causing an increase in AUC and urinary excretion of parent drug and metabolites. Despite increased Ae, the cirrhotic patient may become resistant to diuretics as many patients with congestive heart failure or nephrotic syndrome. This is considered to be due to reduced Na+ load available at the diuretic's site of action following avid proximal Na+ reabsorption. In reduced EABV a short-term comedication of loop diuretics with carboanhydratase inhibitors is considered a more effective diuretic strategy than vigorously increasing monotherapy.

Pharmacokinetics of torasemide and its metabolites in healthy controls and in chronic renal failure

The pharmacokinetics of 20 mg torasemide i.v. has been studied in 7 healthy controls and 9 patients with varying degrees of renal impairment. Torasemide had a t1/2 of about 4h which was independent of kidney function, as the nonrenal clearance of torasemide was 3-times greater than its renal clearance. The active metabolite M 1 and the main metabolite M 5 were accumulated in chronic renal failure. In contrast to liver function, therefore, kidney failure does not have an important effect on the pharmacokinetics of torasemide.

Prediction of diuretic mobilization of cirrhotic ascites by pretreatment fractional sodium excretion

In a randomized prospective study the efficacy and side effects of xipamide versus the combination spironolactone/furosemide in the treatment of cirrhotic ascites were studied. Out of 27 patients four responded to a basic treatment consisting of salt and water restriction and one had to be excluded because of deterioration of kidney function. The remaining 22 patients were randomized to additional treatment with either 20 mg xipamide/day (group I) or 200 mg spironolactone/day combined with 40 mg of furosemide every other day (group II). A response to treatment during the first 4 days was seen in 7 of 11 patients of group I versus only 3 of 11 patients in group II. In the latter group 7 of 11 patients finally responded after 8 days of treatment. Responsiveness to either diuretic treatment strongly depended on pretreatment fractional Na excretion, FENa. The resistance to diuretic treatment can be predicted by a FENa less than 0.2%, and could be overcome by additional strategies known to reduce avid proximal Na reabsorption. Xipamide frequently induced hypokalemia, whereas hyperkalemia was seen following treatment with spironolactone/furosemide. Kidney function remained stable during either diuretic treatment.